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British Standard

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 BRITISH STANDARD BS 6399-3:
1988
Incorporating
Amendments Nos. 1
and 3 and
Implementing
Amendment No. 2

Loading for buildings —

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Part 3: Code of practice for imposed roof

loads

ICS 91.060.01; 91.080.20

 BS 6399-3:1988

Committees responsible for this

British Standard
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

The preparation of this British Standard was entrusted by the Sector Board for
Building and Civil Engineering (B/-) to Technical Committee B/525/1, upon
which the following bodies were represented:

British Constructional Steelwork Association Ltd.

British Iron and Steel Producers’ Association
British Masonry Society
Concrete Society
Department of the Environment (Building Research Establishment)
Department of the Environment (Property and Building Directorate)
Highways Agency
Institution of Civil Engineers
Institution of Structural Engineers
National House Building Council
Royal Institute of British Architects
Steel Construction Institute

This British Standard, having

been prepared under the
direction of the Civil Engineering
and Building Structures
Standards Committee, was
published under the authority of
the Board of BSI and comes
Amendments issued since publication
into effect on
31 May 1988 Amd. No. Date of issue Comments
© BSI 11-1998
6033 August 1988

The following BSI references 9187 September

relate to the work on this 1996
standard:
Committee reference CSB/54
Draft for comment 86/13528 DC 9452 May 1997 Indicated by a sideline in the margin

ISBN 0 580 16577 9

 BS 6399-3:1988

Contents

Page
Committees responsible Inside front cover
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Foreword ii
Section 1. General
1 Scope 1
2 Definitions 1
3 Symbols 1
4 Minimum impose roof loads 2
Section 2. Snow loads
5 Snow load on the roof 4
6 Snow load on the ground 4
7 Snow load shape coefficients 6
8 Snow sliding down roofs 9
Appendix A Annual probabilities of exceedance different from 0.02 21
Appendix B Snow drift load calculations 21
Appendix C Addresses of advisory offices 21
Figure 1 — Basic snow load on the ground 5
Figure 2 — Snow load shape coefficients for flat or monopitch roofs 7
Figure 3 — Snow load shape coefficients for pitched roofs 10
Figure 4 — Snow load shape coefficients for curved roofs 11
Figure 5 — Snow load shape coefficients and drift lengths for
valleys of multi-span pitched or curved roofs 13
Figure 6 — Snow load shape coefficients and drift lengths at
abrupt changes of roof height 14
Figure 7 — Snow load shape coefficients and drift lengths for single
pitch roofs abutting taller structures at 90° 16
Figure 8 — Snow load shape coefficients and drift lengths for
intersecting pitched roofs 17
Figure 9 — Snow load shape coefficients and drift lengths for
local projections and obstructions 19
Table 1 — Values of salt for corresponding values of sb 4
Publications referred to Inside back cover

© BSI 11-1998 i
 BS 6399-3:1988

Foreword

This Part of this British Standard Code of practice has been prepared under the
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direction of the Civil Engineering and Building Structures Standards Committee

as a new Part to BS 6399 (formerly CP 3: Chapter V).
Imposed roof loads were previously included in BS 6399-1. This new Part of
BS 6399 now gives more information on imposed roof loads and in particular
gives snow loading data separately, allowing account to be taken of the variation
of snow in the United Kingdom and the effect of redistribution of snow on roofs
due to wind. Use of the uniformly distributed snow loads are subject to an
overriding minimum requirement.
This code can be used for design using permissible stresses or partial factors. In
the former case the values should be used directly while in the latter case they
should be factored by an appropriate value depending upon whether an ultimate
or serviceability limit state is being considered. The exception to this is the
treatment of the load cases involving local drifting of snow, where it is
recommended that these are treated as exceptional loads and used in design with
reduced safety factors.
Section two of this Part of BS 6399 is broadly in agreement with ISO 4355-1981
“Bases for design of structures — Determination of snow loads on roofs”,
published by the International Organization for Standardization (ISO). However,
one difference is that, in general, the uniform snow load condition and the drift
snow load condition are treated as independent load cases. This is in recognition
of the United Kingdom’s maritime climate which means that for many parts of
the country the maximum snow load condition is likely to result from a single fall
of snow, rather than an accumulation over several months.
The treatment of snow drifting against obstructions in section two is similar to
that given in BRE Digest 290, issued in October 1984, but now withdrawn.
However, it should be noted that there are some differences as follows:
a) the notation has changed to conform better to ISO 3898;
b) there are increased restrictions on the amount of snow that can form in the
drift;
c) the drift loads are to be treated as exceptional loads.
The last point explains why the upper bound values for the snow load shape
coefficients have apparently increased. (Digest 290 was drafted so that the drift
loads could be treated as ultimate loads.)
The designer should be aware that the deposition and redistribution of snow on
roofs are very complex phenomena. The type and record length of the ground
snow data available and the paucity of observational data on roof snow loads
make it extremely difficult to estimate snow load distributions reliably. This Code
models the actual drift shapes and load intensities by simplified linear
distributions, based on assumptions on the amount of snow available to drift and
limitations on the drift height. Wherever possible, available observational data
have been incorporated in the development of the design models.
In this Part of BS 6399 numerical values have been given in terms of SI units,
details of which are to be found in BS 5555. Those concerned with the conversion
and renovation of existing structures or buildings designed in terms of imperial
units may find it useful to note that 1 N = 0.225 lbf and 1 kN/m2 = 20.89 lbf/ft2.
The full list of organizations that have taken part in the work of the Technical
Committee is given on the Inside front cover.
Amendment 2 has been issued to address problems encountered with use.

ii © BSI 11-1998
 BS 6399-3:1988

A British Standard does not purport to include all the necessary provisions of a
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contract. Users of British Standards are responsible for their correct application.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.

Summary of pages
This document comprises a front cover, an inside front cover, pages i to iv,
pages 1 to 22, an inside back cover and a back cover.
This standard has been updated (see copyright date) and may have had
amendments incorporated. This will be indicated in the amendment table on
the inside front cover.

© BSI 11-1998 iii

Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

iv
blank
 BS 6399-3:1988

Section 1. General

1 Scope 2.4
site snow load
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This Part of BS 6399 gives minimum imposed roof

loads for use in designing buildings and building the load intensity of undrifted snow at ground level,
components which are to be constructed and used in at the altitude of the site
the UK and the Channel Islands. It applies to: 2.5
a) new buildings and new structures; snow load shape coefficient
b) alterations and additions to existing buildings the ratio of the snow load on the roof to the undrifted
and existing structures. snow load on the ground
Caution is necessary in applying the snow load 2.6
calculations for sites at altitudes above 500 m and snow load on roof
specialist advice should be obtained in such the load intensity of the snow on the roof
situations (see appendix C).
NOTE The titles of the publications referred to in this code are
2.7
listed on the inside back cover. redistributed snow load
the snow load distribution resulting from snow
2 Definitions having been moved from one location to another
For the purposes of this Part of BS 6399 the location on a roof by the action of the wind
following definitions apply. 2.8
2.1 exceptional snow load
imposed roof load (or imposed load on roof) the load intensity resulting from a snow deposition
the load assumed to be produced by environmental pattern which has an exceptionally infrequent
effects on the roof, excluding wind loads, and by use likelihood of occurring and which is used in design
of the roof either as a floor or for access for cleaning with reduced safety factors
and maintenance 2.9
NOTE The environmental effects included in the imposed roof variably distributed load
load are those due to snow, rain, ice and temperature. Snow is
treated specifically in this code while the minimum imposed roof a vertical load on a given area in plan of varying
load value allows for loads resulting from rain, ice and local load intensity
temperature. However, for certain cases in the UK specific
consideration may have to be given to temperature effects,
e.g. movement joints; these cases are not included in this code. 3 Symbols
The minimum imposed roof load value also allows for a certain For the purposes of this Part of BS 6399 the
build-up of water on the roof due to ponding, but it does not allow
for the effect of drains becoming blocked. This can be caused by following symbols apply.
general debris or ice and consideration may need to be given in
design to what happens to the drain water if this occurs. For roofs
with no access, the minimum imposed roof load value includes an A Altitude of site in metres above mean sea
allowance for repair and maintenance work. For roofs with level;
access, consideration needs to be given to how the roof may be
used and, if necessary, an appropriate floor load as recommended bi Horizontal dimension, suffix i = 1, 2 or 3 to
in clause 5 of BS 6399-1:1996 should be used. The minimum distinguish between several horizontal
imposed roof load specified does not include an allowance for dimensions on the same diagram;
loads due to services.
Fs Force per unit width exerted by a sliding
2.2
basic snow load mass of snow in the direction of slide;

the load intensity of undrifted snow in a sheltered h Assumed maximum height of snow in a
area at an assumed ground level datum of 100 m local drift (valleys of multi-span roofs and
above mean sea level, estimated to have an annual the intersections);
probability of exceedance of 0.02 hoi Vertical height of obstruction,
2.3 suffix i = 1, 2 or 3 to distinguish between
altitude of site several vertical heights on the same
diagram;
the height above mean sea level of the site where the
building is to be located, or is already located for an lsi Horizontal length of snow drift,
existing building suffix i = 1, 2 or 3 to distinguish between
several snow drifts on the same diagram;
salt Coefficient used in correcting basic snow
load on the ground for altitude;

© BSI 11-1998 1
 BS 6399-3:1988 Section 1

sb Basic snow load (on the ground); c) a uniformly distributed load of 0.6 kN/m2
measured on plan for roof slopes of 30° or less; or
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sd Snow load on roof; a uniformly distributed load

s0 Site snow load (on the ground); of 0.6 [(60 – a)/30] kN/m2 measured on plan for
roof slopes (a) greater than 30° and less than 60°;
a Angle of pitch of roof measured from the or zero load for roof slopes equal to or greater
horizontal; than 60°; or
b Equivalent slope for a curved roof; d) a concentrated load of 0.9 kN.
d Angle between the horizontal and a These loads assume that spreader boards will be
tangent to a curved roof at the eaves; used while any cleaning or maintenance work is in
progress on fragile roofs. The recommendations of
µi Snow load shape coefficient, suffix i = 1, 2, this clause may also be used where a ladder is
etc. to distinguish between shape permanently fixed to allow access to a roof for
coefficients at different locations.
cleaning and maintenance only.
4 Minimum imposed roof loads 4.3.2 Small buildings. This subclause is an optional
alternative to 4.3.1, which means that the detailed
4.1 General calculations using snow load shape coefficients do
In 4.2 and 4.3 “access” means access in addition not have to be carried out. It applies to any building,
to that necessary for cleaning and repair and where no access is provided to the roof (other than
“no access” means access for cleaning and repair that necessary for cleaning and maintenance),
only. which has:
The effects of deflection under concentrated loads a) a roof area no larger than 200 m2 in plan; or
need only be considered when such deflection would b) a width no greater than 10 m and a pitched roof
adversely affect the finishes. with no parapet;
All roof slopes are measured from the horizontal and provided that there are no other buildings
all loads should be applied vertically. within 1.5 m of its perimeter, and provided that the
4.2 Minimum imposed load on roof with access roof configuration also meets one of the following
conditions:
Where access is provided to a roof allowance should
be made for an imposed load equal to or greater than 1) the roof has no abrupt changes of height
that which produces the worst load effect from one greater than 1 m, at which drifting could occur;
of the following: 2) the area of a lower part of the roof, on which a
a) the uniformly distributed snow load; or drift could form, is not greater than 35 m2.
b) the redistributed snow load; or For the purpose of this subclause the roof area is
defined as the total covered area, in plan, of the
c) a uniformly distributed load of 1.5 kN/m2 entire building structure. Also, chimneys and
measured on plan; or dormers whose vertical elevation area, against
d) a concentrated load of 1.8 kN. which a drift could form, is less than 1 m2 can be
Where the roof is to have access for specific usages ignored as an abrupt change of height.
the imposed loads for c) and d) above should be Providing the above conditions are met, an
replaced by the appropriate imposed floor load as allowance should be made for an imposed load equal
recommended in 5.1 of BS 6399-1:1996, including to or greater than that which produces the worst
any reduction as appropriate as recommended in 6.3 load effect from one of the following:
of BS 6399-1:1996. i) a uniformly distributed load of 1.25 times the
4.3 Minimum imposed load on roof with no site snow load s0 (see 6.2); or
access ii) a uniformly distributed load of 0.75 kN/m2; or
4.3.1 General. Where no access is provided to a roof iii) a concentrated load of 0.9 kN.
(other than that necessary for cleaning and
For roof slopes (a) larger than 30° and less than 60°
maintenance), allowance should be made for an
the values given by i) and ii) may be reduced by
imposed load equal to or greater than that which multiplying by [(60 – a)/30]. For roof slopes larger
produces the worst load effect from one of the than 60° the minimum uniformly distributed load
following:
requirement is zero.
a) the uniformly distributed snow load; or
b) the redistributed snow load; or

2 © BSI 11-1998
 Section 1 BS 6399-3:1988

4.4 Curved roofs 4.6 Roof coverings

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The minimum imposed load on a curved roof should A load of 0.9 kN on any square with a 125 mm side
be calculated in accordance with 4.3. In provides for loads incidental to maintenance on all
evaluating 4.3.1 c), the roof should be divided into self-supporting roof coverings, i.e. those not
not less than five equal segments and the mean requiring structural support over their whole area.
slope of each segment considered to be equivalent to No loads incidental to maintenance are appropriate
the roof slope, a. The snow loads should be to glazing.
determined according to clause 7.
4.5 Partial loading due to snow removal
In certain cases snow may be artificially removed
from, or redistributed on, a roof, e.g. due to excessive
heat loss through a small section of roof or manually
to maintain access to a service door. This can result
in more severe load imbalances occurring than those
resulting from clause 5 (which have been derived for
natural deposition patterns). To provide for these
situations, if they are likely to occur and if other
information is not available, a load case should be
considered comprising the minimum imposed
uniformly distributed load according to clause 4 on
any portion of the roof area and zero load on the
remainder of the area.

© BSI 11-1998 3
 BS 6399-3:1988

Section 2. Snow loads

5 Snow load on the roof 6.2 Site snow load (s0)

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The snow load on the roof sd (in kN/m ) is 2 The snow load at ground level increases as the
determined by multiplying the estimated snow load altitude of the ground level increases. As the basic
on the ground at the site location and altitude (the snow load on the ground is given for an assumed
site snow load) by a factor known as the snow load ground level altitude of 100 m, it is necessary to
shape coefficient in accordance with the following adjust the value for locations where the ground level
equation: is above 100 m. The site snow load s0 (in kN/m2)
should be calculated from the following equations:
sd = µ i s0
s0 = sb
where
for sites whose altitude is not greater than 100 m; or
s0 is the site snow load (in kN/m2) (see clause 6);
s0 = sb + salt ((A – 100)/100)
µi is the snow load shape coefficient µ1, µ2, etc. for sites whose altitude is above 100 m but not
(see clause 7). greater than 500 m
Several snow load cases may have to be considered where
in design to check adequately for the different snow
sb is the basic snow load on the ground
load patterns that can occur. Each load case may
require the use of one or more different snow load (in kN/m2) (see 6.1);
shape coefficients. Depending upon the pattern salt = 0.1sb + 0.09 (alternatively see Table 1);
being considered the snow load on the roof should be
treated either as a uniformly distributed load or as A is the altitude of the site (in metres).
a variably distributed load over all or part of the It is not necessary to make any correction for the
roof. It should be assumed to act vertically and refer height of the building. For sites whose altitude is
to a horizontal projection of the area of the roof. For above 500 m specialist advice should be sought
the redistributed snow load cases the distribution of (see clause 1 and appendix C).
the snow in the direction parallel to the obstruction NOTE For simplicity of calculation it is assumed that the same
is normally assumed to be uniform. value for the basic snow load on the ground should apply for
altitudes between 0 and 100 m. If preferred the equation for
The snow load on the roof should be considered to be altitudes greater than 100 m may be used for altitudes between 0
a medium term load for the majority of design in the and 100 m; in these cases the correction term, salt((A - 100)/100),
will automatically be negative.
UK, i.e. to have a notional duration of one month.
Table 1 — Values of salt for
6 Snow load on the ground corresponding values of sb
6.1 Basic snow load (sb) sb salt

The basic snow load on the ground has been kN/m2

assessed for the UK by statistical analysis of the 0.30 to 0.34 0.12
snow depth records kept by the Meteorological
Office and converted into a load by the use of a 0.35 to 0.44 0.13
statistically derived conversion factor. The values 0.45 to 0.54 0.14
are given as lines of equal load intensity (isopleths)
0.55 to 0.64 0.15
on the map in Figure 1. They are corrected for an
assumed ground level datum of 100 m above mean 0.65 to 0.74 0.16
sea level and have an annual probability of 0.75 to 0.84 0.17
exceedance of 0.02 (for other annual probabilities of
exceedance see appendix A). For locations between 0.85 to 0.94 0.18
the lines the load intensity should be obtained by 0.95 to 1.00 0.19
interpolation.
NOTE The sopleths in Figure 1 are derived from analysis of
data from a limited number of recording stations and therefore
unusual local effects may not be included. These include local
shelter from the wind, which may result in increased local snow
loads, and local configurations in mountainous areas, which may
funnel the snow and give increased local loading. If the designer
suspects that there may be unusual local conditions that may
need to be taken into account, then the Meteorological Office or
informed local sources should be consulted.

4 © BSI 11-1998
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI
Section 2

© BSI 11-1998
Figure 1 — Basic snow load on the ground

5
BS 6399-3:1988
 BS 6399-3:1988 Section 2

7 Snow load shape coefficients NOTE The snow load shape coefficient, being a ratio of two
loads, is non-dimensional. The equations of the form 2h0i/s0 are
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7.1 General principles correct, although apparently having the dimensions kN/m3,
because of the density correction. The correction is based on
Snow is naturally deposited in many different limited information which shows that the snow density is
patterns on a roof depending upon the wind speed, increased when the snow forms in drifts.
the wind direction, the type of snow, the external 7.2 Single span roofs
shape of the roof and the position and height of any
7.2.1 General. These are flat, monopitch, pitched or
surrounding roofs or obstructions. Therefore, it is
curved roofs of single span. The snow load shape
often necessary to consider several loading
coefficients do not include any allowances for
situations to ensure that all the critical load effects
drifting at parapets or other obstructions as these
are determined.
should be treated independently (see 7.4).
The primary loading conditions to be considered are:
7.2.2 Flat or monopitch roofs. For these roofs it is
a) that resulting from a uniformly distributed necessary to consider a single load case resulting
layer of snow over the complete roof, likely to from a uniform layer of snow over the complete roof.
occur when snow falls when there is little or no The value of the snow load shape coefficient (µi) is
wind; dependent on the angle of the pitch of the roof
b) those resulting from redistributed (or unevenly measured from the horizontal (a) and should be
deposited) snow, likely to occur in windy obtained from Figure 2. This value is assumed to be
conditions. constant over the complete roof area.
Condition b) can be caused by a redistribution of 7.2.3 Pitched roofs
snow which affects the load distribution on the 7.2.3.1 General. For this type of roof it is necessary
complete roof, e.g. snow transported from the to consider two load cases. For both cases the value
windward slope of a pitched roof to the leeward side; of the snow load shape coefficient (µi) is dependent
usually modelled as a uniformly distributed load on on the angle of pitch of the roof measured from the
the leeward side of the roof and zero load on the horizontal (a). For asymmetric pitched roofs, each
windward side. It can also be caused by side of the roof should be treated as one half of a
redistribution of snow which affects the load corresponding symmetric roof.
distribution on only a local part of the roof, e.g. snow
drifting behind a parapet; modelled as a variably 7.2.3.2 Case 1; uniform load. This results from a
distributed load. Both types of redistribution should uniform layer of snow over the complete roof. The
be considered if appropriate. For a complex roof value for the snow load shape coefficient should be
shape there may be several load cases associated obtained from Figure 3(a) ; this value is assumed to
with condition b). be constant over the complete roof area.
In general, load cases should be considered to act 7.2.3.3 Case 2; asymmetric load. This results from
individually and not together. In some transport of snow from one side of the ridge to the
circumstances more than one of the load cases will other side. This situation only needs to be
be applicable for the same location on the roof. When considered for roof slopes greater than 15°. The
this arises they should be treated as alternatives. value for the snow load shape coefficient for one
slope of the roof should be zero, i.e. no snow load.
NOTE However, where, for example, on a lower roof area
sheltered from all wind directions, there is the possibility of The value for the snow load shape coefficient for the
redistribution of snow from a higher roof to form a local drift on other slope should be obtained from Figure 3(b); this
top of a uniform snow load distribution on this lower roof, it value is assumed to be constant over the loaded
would be appropriate to consider the local drift load acting in
combination with the uniform snow load on the lower roof.
slope of the roof.
Redistribution of snow should be considered to occur
on any roof slope and at any obstruction, as it should
be assumed that the wind can blow from any
direction.
The equations given in Figure 2 to Figure 9 for
determining the snow load shape coefficients are
empirical; where they are associated with local
drifting of snow they include a correction to allow for
an increased weight density in the drift. Therefore,
when using the equations the dimensions of the
building and of the obstruction (b1, h01, ls1, b2, etc.)
should be in metres and the site snow load should be
in kN/m2.

6 © BSI 11-1998
 Section 2 BS 6399-3:1988
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Figure 2 — Snow load shape coefficients for flat or monopitch roofs

7.2.4 Curved roofs 7.2.4.3 Case 2; asymmetric load. This results from
7.2.4.1 General. For this type of roof it is necessary transport of snow from one side of the curved roof to
to consider two load cases. For both cases the value the other side. This situation only needs to be
of the snow load shape coefficient (µi) is dependent considered for equivalent roof slopes greater
on an equivalent slope for the curved roof (b). In than 15°. The value for the snow load shape
determining the equivalent slope it is necessary to coefficient for one side of the roof should be zero,
distinguish between two types of curved roofs; i.e. no snow load, while the values for the snow load
type 1, where the angle between the horizontal and shape coefficients for the other slope should be
the tangent to the curved roof at the eaves (d) is 60° obtained from Figure 4(b) . The values for the snow
or less; and type 2, where the angle is greater load shape coefficients are assumed to be constant
than 60°. For type 1 curved roofs the equivalent in the direction parallel to the eaves.
slope is the angle between the horizontal and a line 7.3 Multi-span roofs
drawn from the crown to the eaves. For type 2 This clause gives roof snow loads for multi-span
curved roofs the equivalent slope is the angle pitched, multi-span convex-curved and northlight
between the horizontal and a line drawn from the roofs.
crown to the point on the curved surface at which a
tangent to the surface makes an angle of 60° with To determine the uniform and asymmetric snow
the horizontal. load cases, these structures may be divided into the
single-span basic elements considered in 7.2. The
7.2.4.2 Case 1; uniform load. This results from a appropriate local drift loads as given 7.4 should also
uniform layer of snow over the roof. The value for be considered.
the snow load shape coefficient should be obtained
NOTE Local redistribution of snow on a multi-span roof is
from Figure 4(a). This value is constant over the roof difficult to predict. The designer should exercise care,
except for type 2 roofs where the portions of the roof particularly with a structure sensitive to asymmetric loading
where the tangents make an angle with the (e.g. arched roof), to ensure that the load cases considered
describe the critical loading conditions both for elements and for
horizontal greater than 60° are assumed to be free the structure as a whole.
of snow.

© BSI 11-1998 7
 BS 6399-3:1988 Section 2

7.4 Local drifting of snow on roofs Where simultaneous drifts in several valleys of a
multispan roof are being considered in the design of
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7.4.1 General. When considering load cases using

snow load shape coefficients obtained from this a structure as a whole, a maximum limit on the
subclause it should be assumed that they are amount of drifted snow on the roof should be
exceptional snow loads and that there is no snow applied. The total snow load per metre width in all
elsewhere on the roof. Appendix B explains the the simultaneous drifts should not exceed the
logical process behind the calculations. product of the site snow load and the length of the
building perpendicular to the valley ridges.
The snow load on the roof calculated using the
coefficients in this subclause should be assumed to 7.4.3 Roofs abutting or close to taller
be variably distributed. In the direction at 90° to the structures.
obstruction or valley it should decrease linearly to 7.4.3.1 Abrupt change of height. This subclause
zero over the length of the drift. In the direction applies where there is an abrupt change of height
parallel to the obstruction or valley it should be greater than 1 m, except that relatively slender
uniform and assumed to extend along the complete obstructions (e.g. chimneys) exceeding 1 m in height
length of the obstruction or valley, except where but less than 2 m wide and door canopies projecting
stated otherwise. not more than 5 m from the building should be
considered as local projections and obstructions
In some circumstances more than one local drift
with local drifting determined according to 7.4.5.
load case may be applicable for the same location on
For parapets, see 7.4.3.3.
a roof in which case they should be treated as
alternatives. The appropriate drift length and snow load shape
NOTE In determining the upper bound values for these drift coefficient for an abrupt change of height should be
loads account has been taken of known cases of excessive, drifting obtained from Figure 6 or from the following in
of snow in the UK. However, it is recommended that they are which all parameters are as defined in Figure 6.
treated as exceptional snow loads because of the rarity with
which they are expected to occur. For design, it is suggested that Drift length ls1 is the least value of 5h01, b1
these local drift loads are assigned a partial factor γf = 1.05. and 15 m.
7.4.2 Valleys of multi-span roofs. The appropriate Snow load shape coefficient m1 is the lesser of:
snow load shape coefficients and drift lengths for (2h01)/s0 and (2b)/ls1
local drifting of snow in valleys should be obtained
from Figure 5 or the following: where b is the larger value of b1 and b2
Drift length: with the restriction: m1 # 8
lsi = bi The snow load patterns implied in Figure 6 are also
Snow load shape coefficient: applicable for roofs close to, but not abutting, taller
m1 is the lesser of 2h/s0 and 2b3/(ls1 + ls2) buildings, with the exception that it is only
necessary to consider the load actually on the roof of
with the restriction m1 # 5 interest, i.e. the load implied between the two
and where all parameters are as defined in Figure 5 buildings can be ignored.
and below. NOTE The effect of structures close to, but not abutting the roof
under consideration will depend partly on the roof areas
For roofs of more than two spans with available from which snow can be blown into the drift and the
approximately symmetrical and uniform geometry, difference in levels. However, as an approximate rule, it is only
necessary to consider nearby structures when they are less
b3 should be taken as the horizontal dimension of than 1.5 m away.
three roof slopes (i.e. span × 1.5) and this snow load
7.4.3.2 Single pitched roof with ridge at 90° to a
distribution should be considered applicable to
taller structure. For this case, the local drift
every valley, although not necessarily
from 7.4.3.1 should be modified according to
simultaneously, (see below).
Figure 7, which implies a non-uniform variation in
NOTE If the structure is susceptible to asymmetric loading, the
designer should also consider the possibility of drifts of differing
the direction parallel to the obstruction.
severity in the valleys. 7.4.3.3 Parapets. Local drifting against parapets
For roofs with non-uniform geometry, significant should be determined in accordance with Figure 6 or
differences in ridge height and/or span may act as from the following in which all parameters are as
obstructions to the free movement of snow across defined in Figure 6.
the roof and influence the amount of snow Drift length ls1 is the least value of 5h01, b1
theoretically available to form the drift. Care should and 15 m.
be taken in the selection of b3 (the greater length of
Snow load shape coefficient µ1 is the lesser of:
building from which snow is available to be blown
into the drift). (2h01)/s0 and (2b)/ls1

8 © BSI 11-1998
 Section 2 BS 6399-3:1988

where b is the larger value of b1 and b2 Drift length ls1 is the lesser value of 5h01 and b1.
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

with the restriction: µ1 ≤ 8 Snow load shape coefficient µ1 is the lesser of:
For drifting in a valley behind a parapet at a gable (2h01)/s0 and 5.
end the snow load at the face of the parapet should In addition, for door canopies projecting not more
be assumed to decrease linearly from its maximum than 5 m from the building, the value of snow load
value in the valley to zero at the adjacent ridges, shape coefficient should not exceed:
providing the parapet does not project much higher (2b)/ls1
than the ridge.
where b is the larger of b1 and b2 (see Figure 9).
NOTE For the purpose of this subclause, when considering a
parapet across the end of a valley the snow load at the ridge can
be assumed to be zero providing that the parapet does not project 8 Snow sliding down roofs
more than 300 mm above the ridge.
Under certain conditions snow may slide down a
7.4.4 Tee intersections. For intersecting pitched pitched or curved roof. The force Fs (in kN per metre
roofs the snow load shape coefficients and the drift width) exerted by a sliding mass of snow in the
lengths should be obtained from Figure 8. For this direction of slide is calculated from the following
case the variation in the direction parallel to the equation:
obstruction is non-uniform.
Fs = sdb sina
7.4.5 Local projections and obstructions. The effect
of drifting can be ignored if the vertical elevation where
area against which the drift could form is not
greater than 1 m2. The drifts which occur at local sd is the snow load on the roof (in kN/m2 );
projections and obstructions affect a relatively small b is the distance on plan from the gutter to the
area of roof only. Included in this category is drifting ridge (in metres);
against local obstructions not exceeding 1 m in
height and also drifting on canopies (projecting not a is the angle of pitch of the roof measured from
more than 5 m from the face of the building) over the horizontal.
doors and over loading bays, irrespective of the
height of the obstruction formed. A relatively tall, The appropriate value for sd is obtained from
slender obstruction over 1 m high but not more clause 5. It should be the most onerous value arising
than 2 m wide, may also be considered as a local from uniformly distributed snow on the roof slope
projection. For that specific case, the height against under consideration. It may result from either the
which the drift may form, h0i may be taken as the uniform load case or the asymmetric load case.
lesser of the projection width and the projection This force should be taken into account in the design
height. For parapets, see 7.4.3.3. of snowguards or snowfences if snow is likely to slide
The appropriate snow load shape coefficient at the off the roof endangering people or property below. It
face of the obstruction and the drift length should be should also be taken into account in the design of
obtained from Figure 9 or the following in which all any obstruction on a roof which may prevent snow
parameters are as defined in Figure 9. sliding off the roof.

© BSI 11-1998 9
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

10
BS 6399-3:1988

Figure 3 — Snow load shape coefficients for pitched roofs

Section 2

© BSI 11-1998
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI
Section 2

© BSI 11-1998
Figure 4 — Snow load shape coefficients for curved roofs

11
BS 6399-3:1988
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

12
BS 6399-3:1988

Figure 4 — Snow load shape coefficients for curved roofs (concluded)

Section 2

© BSI 11-1998
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI
Section 2

© BSI 11-1998
Figure 5 — Valleys of multi-span pitched or convex curved roofs

13
BS 6399-3:1988
 BS 6399-3:1988 Section 2
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 6 — Snow load shape coefficients and drift lengths at abrupt changes
in roof height and parapets

14 © BSI 11-1998
 Section 2 BS 6399-3:1988
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 6 — Snow load shape coefficients and drift lengths at abrupt changes
in roof height and parapets (concluded)

© BSI 11-1998 15
 BS 6399-3:1988 Section 2
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 7 — Snow load shape coefficients and drift lengths for single pitch roofs
abutting taller structures at 90°

16 © BSI 11-1998
 Section 2 BS 6399-3:1988
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 8 — Snow load shape coefficients and drift lengths for intersecting pitched roofs

© BSI 11-1998 17
 BS 6399-3:1988 Section 2
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 8 — Snow load shape coefficients and drift lengths for intersecting pitched
roofs (concluded)

18 © BSI 11-1998
 Section 2 BS 6399-3:1988
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 9 — Snow load shape coefficients and drift lengths for local
projections and obstructions

© BSI 11-1998 19
 BS 6399-3:1988 Section 2
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

Figure 9 — Snow load shape coefficients and drift lengths for local
projections and obstructions (concluded)

20 © BSI 11-1998
 BS 6399-3:1988

Appendix A Annual probabilities of c) Finally an over-riding maximum value for the

amount of snow is applied by arbitrarily
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

exceedance different from 0.02

assuming that the local load cannot be larger
Site snow loads with an annual probability of than the snow load on the ground increased by a
exceedance of 0.02 (as given by 6.2) will be certain multiplicative factor. The factor is
appropriate for the design of most structures. usually 5 or 8 depending upon the severity of
However, a lower probability of exceedance should drifting likely to occur for the design
be considered for the design of: configuration being considered.
— a structure where greater than normal Appendix C Addresses of advisory
reliability is required;
offices
— a structure where damage due to snow loading
would result in great financial loss or The addresses of advisory offices of the
unacceptable impaired usage of the building. Meteorological Office are as follows.
Where site snow loads with a probability of For England and Wales;
exceedance different from 0.02 are considered Meteorological Office,
appropriate, the designer should consult the Met 0 3,
Advisory Offices listed in appendix C for guidance.
London Road,
Appendix B Snow drift load BRACKNELL,
calculations Berkshire,
This appendix briefly describes the general process RG12 2SZ.
adopted for determining the maximum local load Tel: 01344 420242 Extn. 2299
intensity resulting from snow in a drift at a change
in roof height. It is included for designers who wish For Scotland;
to understand the parameters which influence the Meteorological Office.
magnitude of the snow loads associated with 231 Corstorphine Road,
drifting of snow. It is not intended for use in
EDINBURGH,
calculating snow load shape coefficients.
EH12 7BB.
Up to three basic checks are made as follows.
Tel: 0131 344 9721 Extn. 524
a) It is first assumed that the drift forms to the
top of the obstruction and the density of the snow For Northern Ireland;
increases from that of the fallen snow on the
ground. This check takes the general form of Meteorological Office,
limiting the snow load shape coefficient to: Progressive House,
rhoi/so 1 College Square East,
where BELFAST,
r is the weight density of the snow in the BT 1 6BQ.
drift (2 kN/m3 assumed); Tel: Belfast 28457
hoi is the height of the obstruction (in metres); The address of the advisory service of the Building
Research Establishment is as follows.
so is the site snow load (in kN/m2). Building Research Advisory Service,
b) In some cases, a check is then made to ensure Building Research Establishment,
that there is sufficient snow available on the roof
Building Research Station,
to form a drift to the top of the obstruction. This
check takes the general form of limiting the snow Garston, WATFORD,
load shape coefficient to: WD2 7JR.
2bi/lsi Tel: 01923 664664
where

bi is the length of the building from which

snow will be blown into the drift
(in metres);
lsi is the length of the drift (in metres).

© BSI 11-1998 21
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

22
blank
 BS 6399-3:1988

Publications referred to

BS 5555, Specification for SI units and recommendations for the use of their multiples and of certain other
units1).
Licensed Copy: bat.79ee6000390b0300 bat.79ee6000390b0300, University of Bath, 16 February 2005, Uncontrolled Copy, (c) BSI

BS 6399, Loading for buildings.

BS 6399-1,Code of practice for dead and imposed loads.
BS 6399-2, Wind loads.
CP 3, Code of basic data for the design of buildings1).
CP 3: Chapter V, Loading.
ISO 3898, Bases for design of structures — Notations — General symbols1).
ISO 4355, Bases for design of structures — Determination of snow loads on roofs1).
BRE Digest 290, Loads on roofs from snow drifting against vertical obstructions and in valleys2).

1)
Referred to in the foreword only.
2)
Withdrawn.

© BSI 11-1998
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