Foundation build up spec

[BMcN]

I have a query on my foundations and would like some opinions.

 

We are going for a solid sub floor with strip foundations.  The architect has specced the strip foundations as I expected but for the sub floor he has specced 150mm hardcore, then 150mm reinforced concerete, followed by the insulation and UFH + screed.

 

I was not expecting the sub floor to be poured concerete, I was expecting this to be hardcore + blinding sand followed my insulation and screed.   The extra concrete is 3-4k worth extra cost, is this normal practice?  I dont see why as it is not load bearing, the strip founds are taking the load.

 

I have attached the spec and foundation plan.

[nod]

Pretty normal 

Though I went Block and beam then insulation UFH then screed

[StructuralEngineer]

Hi, and I'm glad you posted this up here.

 

Your Architect appears to be competent going by the drawing and notes, though he/she is using standard notes for the foundations and floor slab. In other words, these may or may not work for your site. As you have discovered you could be paying several thousands of pounds extra because a structural engineer hasn't been consulted (I assume).

 

Some observations. Yes, a 150mm concrete slab with a single layer of mesh is the standard way to build a ground-bearing slab. The 150mm hardcore is also a standard requirement.

 

The screed is because the architect has specified underfloor heating and insulation over the slab. It's quite common to build slabs without screed otherwise.

 

The foundation top right of your garage should be joined, not left untied like this.

 

Is there a reason for the 140mm blockwork? How many stories is your property? Any reason for the additional blockwork skin to the inside of the walls at the bottom of the plans?

 

Here are some ways you could reduce cost:

1. You can have insulation below your slab and do away with the need for screed entirely (see https://www.quinn-buildingproducts.com/resources/insulation-solutions/solid-ground-floor-insulated-below-ground-bearing-slab/ ) but you may need to forego the underfloor heating in that case.
2. Your strip footings seem to be wider than normal. For a standard 2 storey home, 450mm - 600mm wide would be more common. A structural engineer could advise and potentially save you money. You might need a trial pit to be dug.
3. Try using a suspended ground floor instead. This negates the need for compacted hardcore/stone, for sand blinding, and can be done either with cast-insitu RC, beam and block, or timber. A timber floor might be ideal as you have the insulation below it (see https://www.nu-heat.co.uk/resources/floor-constructions-library/joisted/ ).
4. Don't forget, as well as the inefficiencies highlighted above, your floor is also very deep, so to construct it your builder might need to dig down and remove spoil from site, which costs something.

Hope this helps!

[BMcN]

 

On 19/03/2018 at 13:34, StructuralEngineer said:

Hi, and I'm glad you posted this up here.

 

Your Architect appears to be competent going by the drawing and notes, though he/she is using standard notes for the foundations and floor slab. In other words, these may or may not work for your site. As you have discovered you could be paying several thousands of pounds extra because a structural engineer hasn't been consulted (I assume).

 

Some observations. Yes, a 150mm concrete slab with a single layer of mesh is the standard way to build a ground-bearing slab. The 150mm hardcore is also a standard requirement.

 

The screed is because the architect has specified underfloor heating and insulation over the slab. It's quite common to build slabs without screed otherwise.

 

The foundation top right of your garage should be joined, not left untied like this.

 

Is there a reason for the 140mm blockwork? How many stories is your property? Any reason for the additional blockwork skin to the inside of the walls at the bottom of the plans?

 

Here are some ways you could reduce cost:

1. You can have insulation below your slab and do away with the need for screed entirely (see https://www.quinn-buildingproducts.com/resources/insulation-solutions/solid-ground-floor-insulated-below-ground-bearing-slab/ ) but you may need to forego the underfloor heating in that case.
2. Your strip footings seem to be wider than normal. For a standard 2 storey home, 450mm - 600mm wide would be more common. A structural engineer could advise and potentially save you money. You might need a trial pit to be dug.
3. Try using a suspended ground floor instead. This negates the need for compacted hardcore/stone, for sand blinding, and can be done either with cast-insitu RC, beam and block, or timber. A timber floor might be ideal as you have the insulation below it (see https://www.nu-heat.co.uk/resources/floor-constructions-library/joisted/ ).
4. Don't forget, as well as the inefficiencies highlighted above, your floor is also very deep, so to construct it your builder might need to dig down and remove spoil from site, which costs something.

Hope this helps!

Thanks for the replies.

 

Yes my spec is to have UFH hence the slab.  This is also the main reason behind wanting a slab instead of timber joints, as it is more simple and efficient for the UHF.

 

I can't comment on the strip foundations. The house will be 1.75 storey.  Again, unsure on why it is 140mm block instead of 100mm, I will query.  The 220mm to the front is due to the site sloping away, so it will be about 600mm drop.

 

The 150mm concrete with screed sub floor - Does this need to be ground bearing?  Isnt all the load through the strip founds?  Isnt the only load from the screed then onto the PIR or EPS insulation? 

[StructuralEngineer]

If the concrete floor is designed "suspended" it sits on the blockwork over the strip footings, and does not need to touch the ground (although you have to shutter the underside with something). So to clarify, either you go with a groundbearing slab (compacted stone base, RC slab), or a suspended slab (shuttering, RC slab). The screed is immaterial to the strength of the slab and is only there to cover over the heating and to level the floor.