Extension foundation plans are through, floor slab / underpinning query

[KayleyH]

Further to my previous query we have had our foundation plans through for our extension (yay) and it looks like we will need to make a decision about an instruction on it.  I will of course chat with the buildings control officer, architect, ground works people and structural engineer about the most suitable option, but that will take a little time no doubt as everyone is very busy ... so thought I'd get some advice from here in the meantime. 

 

Our house is very old, around 400 years old, timber frame.  It had a small existing sunlounge which was added in the 70's.  We have taken this down in preparation for ground work as and when.  There is an instruction on the foundation drawings saying to check the existing foundation under original house and if less than 600 mm (which I'm sure it will be) then we need to either 'underpin the adjoining wall, use a suspended timber construction or use a ground bearing floor slab (with BCOs approval)'. 

 

To my (uneducated about buildings) eye it feels like the ground bearing floor slab would be the least invasive for our poor old house.  Can anyone explain the pros and cons of these options.  When we took the original sunlounge down I suspect it was on a ground bearing floor slab, as there was about a foot of concrete under the whole floor area.  Very grateful for any information anyone has.  Many thanks.

[jayc89]

You might be surprised, if it's not shown any signs of movement after all this time. Ours is (only) 150ish years old and when we poured our new floor slab, we found the foundations to be well below 700mm. 

 

A ground bearing floor slab would almost certainly be the cheapest option too - MOT 1, sand blinding (some use a thin layer of EPS for this...), insulation, run your underfloor heating pipes (if you're having it), then pour your concrete, saves on using an additional screed too. The majority is also DIYable, if you fancied it.

[George]

Was that from the structural engineer? 

 

I'd not want to be underpinning an old timber frame. The dwarf walls are quite short and would not react well to being underpinned. 

 

The 'load bearing ground bearing slab' sounds like a raft foundation - a reasonable way to build it but perhaps over the top. So long as the geometry works, a normal strip or trench foundation can work against old buildings - just need precautions regarding not leaving it open for too long and possibly stepping it up to match existing footing depth immediately adjacent.