Foundation help required..Piccy!!

[ali68]

Could someone cast there eyes over this drawing and tell me if I am close to the mark or not!

It's for an extension to my house and I cannot get a builder to start it this year in my area.

The brown circle is an existing drain and drg states to provide 150mm concrete surround where drain is under extension floor.

All pointers mucho appreciated.

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[ColJack]

"concrete slab" would be the screeded floor not poured concrete?

get the under floor heating in it while building.. even if you don't intend to use it .. you might decide to later on and it's a lot harder to do then...

 

[Robert too]

I always prefer drains not to be encased now.

I suggest they should be on a pea shingle bed and then the trench backfilled with pea shingle to the underside of the slab .

The slab is looking a bit thin and you havent shown a dpm or a screed.

But then again , wherever I can , I spec an insitu conc suspended slab bearing onto the inner skin of the foundation walls .

 

[ali68]

Floors tend not to be screeded here, usually poured concrete and then a powered concrete finisher to get the finish.

Omitted DPM and DPC, it's coming in another drawing

Point taken about encasing drain will speak to BCO.

Underfloor heating is still under consideration, more than likely it will be going in. If oil doesnt get any dearer.

 

[ali68]

But then again , wherever I can , I spec an insitu conc suspended slab bearing onto the inner skin of the foundation walls .

Is this the general idea you mean Robert, thanks for your help so far..

 

[Robert too]

That is the general idea , you wouldnt need to increase the slab width at the edges though.

A suspended slab does need mesh reinforcement ( normally 2 sheets ) the type depends on the span and what edge support you can get

 

[^woody^]

A suspended slab can cause problems if the foundations either settle or subsequently move, and the floor goes with them.

It is a solution for certain circumstances, but is not standard practice.

In that latest detail, you have created a thermal bridge, and also a route for damp into the floor slab.

Other points .. plastic drains do not require encasing. Cavity fill should be to 225mm below DPC level, and sloped to external with weep holes.

Dust is not an adequate blinding.

Check you floor insulation, as 70mm is not adequate unless more insulation is specified elseware and calcs done to prove overall thermal efficiency.

Are the foundations deep enough?

Edit

Also you need to properly detail the connection between the timber frame and the floor - in particular with regards to DPC, fixing and internal drip off from the breather membrane/sheathing

 

[Robert too]

A suspended slab can cause problems if the foundations either settle or subsequently move, and the floor goes with them.

It is a solution for certain circumstances, but is not standard practice.

I draw it for every domestic extension now, It certainly wont cause a foundation to fail , and it gives me a bit of security from a poor ground bearing slab

A number of insurance companies have started to decline claims where a slab alone has dropped without the foundations moving .

 

[^woody^]

No it won't cause foundations to move, but a building should remain flexible and will move naturally and seasonally, or more unaturally if there are local gound problems.

By introducing a rigid floorslab, there is potential for the inflexibility to be lost and any future movement of the structure will cause additional issues with the floorslab.

Ok, it may never happen, but IMO, it is not a detail I would specify unless it was needed for a specific purpose.

A properly prepared independant floorslab should not move.

With regards to insurance cover, unless movement of the floor is specifically excluded, then they can not refuse a claim. All loss adjusters I deal with tend not to have a clue about the cause of damage and just try and not pay out. But they can not argue with the facts

 

[garyo]

what's the definition of a floating slab? Does it need to be separated by the surrounding walls by, say, 50mm of insulation, or does it class as floating if it has the insulation below and DPM up the sides?

Gary

 

[^woody^]

'Floating slab' is not a term used in my part of the world. Its either a floor or a suspended floor (ie built off the walls). The insulation under and around the floor does not class it as anything other than an 'insulated floor'

A 'floating floor' is a term for a system where a functional floor is built on top of a structual floor, and is normally in multi-storey flats and designed for sound proofing