Scribd
Upload
1K views1 page

NHBC DPM

This document provides guidance on edge details for raft foundations. It addresses whether a raft foundation requires a "toe" or stepped edge to support an external cavity wall and provide weatherproofing at the floor/wall junction. While NHBC standards recommend a stepped edge, raft foundations can be accepted without one if adequate weatherproofing is provided, such as using a combination of a stepped cavity tray, damp proof membrane (DPM), pre-formed corner trays, and sealed lap joints as shown in the detail diagram. The key is preventing moisture passage into the building.

Uploaded by

rbashir6
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
1K views1 page

NHBC DPM

This document provides guidance on edge details for raft foundations. It addresses whether a raft foundation requires a "toe" or stepped edge to support an external cavity wall and provide weatherproofing at the floor/wall junction. While NHBC standards recommend a stepped edge, raft foundations can be accepted without one if adequate weatherproofing is provided, such as using a combination of a stepped cavity tray, damp proof membrane (DPM), pre-formed corner trays, and sealed lap joints as shown in the detail diagram. The key is preventing moisture passage into the building.

Uploaded by

rbashir6
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
You are on page 1/ 1

4.

5/01

Technical Guidance
Raft foundations - edge detail

Question
Does a raft foundation slab require a ‘toe’ for support of the outer leaf of a cavity wall and to provide
weather-proofing at the floor/wall junction (as shown in NHBC Standards clause 4.5 – D12)?

Considerations
NHBC Standards clause 4.5 – D12 states ‘the foundation design shall prevent the passage of moisture to
the inside of the dwelling’ and shows how this can be achieved with a stepped edge on a raft foundation.
NHBC Standards clause 4.5 – D12(a) refers to dpc cavity trays not being an acceptable method of
weather-proofing the edges of specialist foundations such as rafts and ground beams. This is to avoid
details where a failure of the stepped tray would lead to a direct leakage of water into the building.

Answer
Raft foundations can be accepted without a toe provided it can be shown that there is adequate
weather-proofing at the junction of the wall and the raft.
The detail shown below is one acceptable method of achieving this. It uses a combination of a stepped
cavity tray and dpm together with pre-formed corner trays and sealed lap joints.

dpm taken up edges of floor, turned


225mm high stepped cavity tray into wall and lapped with stepped
with fully sealed joints and cavity tray. All joints in DPM to be
pre-formed corners fully sealed.

dpc

Weepholes 150mm min


at 900mm centres

Ground
Level

raft foundation

SECTION THROUGH EDGE OF FLOOR SLAB AND EXTERNAL WALL

Issued: December 2008


Supersedes: n/a - first issue
Page 1/1

You might also like