Offset foundations: For when you're tight to the boundary
The typical low rise domestic foundation detail to the left might be familiar to you. It's taken from the Building Construction Handbook 10th Edition, written by Roy Chudley & Roger Greeno.
First published in 1988 it's been one of the most popular reference books for anyone attending college and university construction courses.
It lays out quite clearly how foundations work where the load is applied centrally onto a minimum 150mm thick concrete strip foundation.
A 600mm wide foundation gives a 150mm projection either side of the wall (equal to the depth of concrete) and this allows the load to be distributed at 45 degrees across the entire width of the foundation into the supporting strata below.
But what can you do when you can't achieve the 600mm spread because you can't trespass across a boundary or if there is already another foundation next door?
In certain situations it may not be possible for the builder to construct a foundation with a typical projection. In these circumstances an offset foundation may be considered (subject to design). The details below provide a typical example which should be adequate for most single storey domestic extensions.
The offset foundation
Its 750mm wide and 450mm deep with a layer of A193 mesh placed in the bottom of the concrete with at least 50mm cover to the steel.
This detail will usually be suitable for single storey domestic extensions but will depend on the type of ground encountered.
If the allowable bearing pressure of existing subsoils is less than 100 kN/m2 at the formation level, you will need a structural engineer to design it for you or will have to change to another foundation type such as a raft or piled foundation.
Article updated 28 November 2017.
Read more about working with foundations
- Underpinning existing foundations
- How to get it right: Building a wall on foundations
- Foundations on clay soil
- Trees and foundations (foundation depth calculator)
- When and how to cut stepped foundations on a sloping site
Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.
Submitted 1 year 8 months ago
does this mean that the 450 width can be measured from the external face of the wall?
Submitted 1 year 8 months ago
Thanks very much for your comment.
We've altered the wording of the article and hope that the information is now easier to follow!
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