Timber Frame Extension Pier Foundation

12 Apr 2009
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United Kingdom
I have removed a conservatory due to subsidence and plan to build a single room extension in the same place. (Despite a double skin dwarf wall, the conservatory did not have any foundations to speak of and was not even built on a proper raft!) The site is within a few metres of a pond. The house has deep conventional strip foundations and is fine.

My idea is to construct a single skin timer frame affair, 5m x 2.5m. The long wall is parallel to the house. I am keen not to over engineer the foundations on such a lightweight structure. A friend suggested 3 concrete/block piers on concrete pads with lintels bridging them and then lintels set into the existing house walls to carry the timber frame.

Will this work OK and what are the pitfalls?

All help appreciated,
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There's nothing wrong at all with that method, providing that the bases are onto good ground and the beams are strong enough to span between them.
You're on the wrong chart, you should be looking at these.

Assuming that you're going to put a concrete floor in, spanning parallel to the house (with three rows of lintels out from the house at 2.5m centres), then the heaviest load will be on the middle bit that's only taking floor load. Type R11 will be good enough for all locations. 100 bearing is fine into existing walls.
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Thanks for looking at that.
I was going to cast a solid floor level with the top of the lintel, stick foam sheet insulation over that, then 12mm plank flooring directly over the top. Does that sound ok?
Is there a rule of thumb for choosing lintels?


If you go casting concrete between the lintels, you will end up with something that won't move (lintel) and something that may well (floor). Best to cast a reinforced concrete slab over the lintels, which is what I assumed you would be doing in coming up with that lintel size.

No rule of thumb - other than to ask a SE ;)
You're absolutely right. But then the lintels, piers and pads all have to be much bigger. Floors like this are quite common aren't they?
Not much difference. With the added advantage that it - as in the floor - won't move; it depends what the ground is there that requires foundations at depth.

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