Measurement Sketch Calculation for roof members

(rid
ge
) record all dimesions in the sketch
r
RL
W RL
W
(rid
(they will be needed for the
fte ge selection of roof members
f ra )
no fte
r)
spa in) (ra from the tables)
url
derp spa
n
(un spa
W
RL no
f ra
Ex fte
r RL
po W

length
(wa
ll) ter
) sed ll)
(wa raf raf
n(
RL
W spa ter

len
lenght

gth
o/h
o/h

Coupled roof Cathedral roof

rafter run (½ width) rafter run (½ width)

width of building between wallplates

Rafter run = external width between the wall plates divided by two.
Rafter span = rafter run divided by cos°.
Overhang = eaves width divided by cos° (add dimensions for brick veneer).
Ridge strut = rafter run x tan°.
Decide whether an underpurlin is needed; if it is place it at mid-span.
New rafter span = rafter span found in 2) divided by two.
Vertical strut to underpurlin = ridge strut length divided by 2 (if u/p positioned at midspan).
Strut perpendicular to rafter = rafter span x tan° (if u/p positioned at midspan).
Roof load width (RLW) = rafter span (if placed at midspan) otherwise ½ span1 + ½ span2.

Underpurlin & Fan strut example

length for roof area supported (RAS)

spread of fan strut spread of fan strut

reduced span 1 reduced span 2

position of strutting beam wall

wall
(or wall)

span of u/p 1 span of u/p 2 (worst case)

worst case wall dimension (centre to centre)

Determine the position of struts (usuallly on supporting walls).

If the distance between supporting walls is excessive a strutting beam may be needed.
Span of underpurlin can also be reduced if fan-strut is used.
Determine the length of the strut (vertical or perpendicular to rafter)
and the dimensions between the struts (or fan-struts).
Find the worst case and calculate the span of the underpurlin.
Roof load area = RLW × ½ u/p span left + ½ u/p span right from strut (or fan strut).
Hanging beams are required if ceiling joist span is excessive.
Place hanging beams in center of room or if needed divide room length/width by 3 (4)
and space them equally.

Timber Framing Design Karl Boeing 1 of 2

 spread of fan strut
2 x length of strut
(vert. or perp.)

= =

le
ng
=
th
underpurlin

of
fa
wall plate

n
st
length of strut (vert. or perp.)

ru
t
position of strutting beam
(or wall)
length of fan strut = strut (vert. or perp.) x square root 2

Calculation (show all numbers):

To avoid selecting an incorrect dimension place all you calculated figures in the sketch

Rafterspan = =
cos °
Rafter span (cont.) = /2=

Overhang = =
cos °
Ridge strut = x tan ° =
Vertical strut (mid span) = /2=
Fan strut length (vert) = x 2 =
Strut perpendicular to rafter = x tan °=
Fan strut length (perp) = x 2 =
Roof load width (RLW) Underpurlin =
Roof load width (RLW) Wall = + =
Roof load width (RLW) Ridge beam =
Strutting beam 1
Roof area supported = x =
Strutting beam 2
Roof area supported = x =
Strutting beam 3
Roof area supported = x =
Timber Framing Design Karl Boeing 2 of 2