Extension foundations

[ey143]

Curious as to how my extension will be built but when constructing a single storey extension say 5m by 8m, is the whole thing dug down to foundation level of 1.2m or whatever is required or just the perimeter where the walls go up?

If the latter then how much of the middle parts are dug?

 

[noseall]

If any internal dividing walls are load bearing i.e. those that carry floor joists or roof load etc, then these will require a foundation also.

 

[ey143]

OK thanks. What about the rest of the areas,how much would be dug there?

 

[noseall]

OK thanks. What about the rest of the areas,how much would be dug there?

Typically 1m below ground level or 1.2m below your DPC.

We also remove the oversite material plus a perimeter 'path' of 1.2m is removed from around the building (where applicable) which ends up being stoned as to provide a clean working area. This further confuses as to where actual ground level is, so measuring down say 1.2m from DPC is fairly reliable.

If the ground still looks dodgy at this depth, then go deeper. An extension we did in the spring of this year required foundations that went down 2.4m below ground. 'Twas a tough old dig, let me tell you.

 

[ey143]

So it's almost at the same depth of the foundations of a perimeter then (in normal depths).

Was I wrong to either read somewhere or be told (maybe I misheard/misunderstood) that only the perimeter need be dig to foundation depth and the middle areas are superficially dug to allow for joists etc?

 

[Alastairreid]

The oversite does not go to the same depth as the strip foundations, the depth has to be dug to accommodate your hardcore, dpm, and oversite concrete ,then allow a minimum 150mm clearance from the bottom of the joists to the top of the concrete oversite.

 

[ey143]

So typically that would be how deep in total?

 

[Alastairreid]

100mm hardcore, 25mm sand blinding, dpm, oversite concrete 50mm, 150mm clearance from underside of joists= 325mm

 

[noseall]

100mm hardcore, 25mm sand blinding, dpm, oversite concrete 50mm, 150mm clearance from underside of joists= 325mm

Is the OP having a suspended wooden floor then...?

 

[ey143]

Good point. What do you recommend for an extension ? If we want underfloor heating then I guess concrete floor right?

 

[jeds]

I tend to follow the original house. If the house is solid floor - I go with solid floor, if timber = timber. etc. Except a solid floor can only be placed on a maximum 600mm backfill, without having to use engineered fill. So for a solid floor house where backfill is more than 600mm I'd go with a beam and block floor as the closest match.

 

[noseall]

Good point. What do you recommend for an extension ? If we want underfloor heating then I guess concrete floor right?

The simplest and most economical way is a solid floor.

A typical UFH spec would be...

100mm - 150mm compacted stone hard core.

25mm - 50mm sand blinding layer (also compcted).

DPM.

100mm concrete slab.

80mm+ Celotex, Kingspan or similar.

Tape joints or add another DPM layer

70mm (mim) screed.

The UFH would be fitted on top of the insulation layer and under the screed.

 

[ey143]

With the concrete floor acting as a large heat sink, does the heat not ask also get absorbed downwards thereby wasting energy too?

How would I then convert the rest of the ground-floor if it's not also concrete but wooden floor joists etc

 

[Alastairreid]

100mm hardcore, 25mm sand blinding, dpm, oversite concrete 50mm, 150mm clearance from underside of joists= 325mm

Is the OP having a suspended wooden floor then...?

Yeah assumed he was as he mentioned joists.

 

[noseall]

With the concrete floor acting as a large heat sink, does the heat not ask also get absorbed downwards thereby wasting energy too?c

No. That's what the 80mm of insulation is preventing.

How would I then convert the rest of the ground-floor if it's not also concrete but wooden floor joists etc

Heating mat