Depth for oversite on my extension

[Craig600]

Hello all i run a small building company and up until now have only ventured as far as loft conversions bathroom kitchens etc, i have decided to use my own house for guinea pig for my first extension but would like some advice on the depth for the oversite.

At present i have the brickwork upto dpc level but obviously the floor for the extension is still mud.

My understanding is that i need 100mm of type 1 then 25mm of sand (builders sand?) then dpm then 100mm of concrete, 65mm of celetex and 65mm of screed.

Are these depths ok?
Should the celetex go under or over the concrete?
In relation to the dpc where does the ffl end up?

 

[simonjay]

Hello all i run a small building company and up until now have only ventured as far as loft conversions bathroom kitchens etc, i have decided to use my own house for guinea pig for my first extension but would like some advice on the depth for the oversite.

At present i have the brickwork upto dpc level but obviously the floor for the extension is still mud.

My understanding is that i need 100mm of type 1 then 25mm of sand (builders sand?) then dpm then 100mm of concrete, 65mm of celetex and 65mm of screed.

Are these depths ok?
Should the celetex go under or over the concrete?
In relation to the dpc where does the ffl end up?

I used 75mm celotex. Best check the current building regs U-value required for the foor.
If using underfloor heating, celotex must be above slab so heat is not lost downwards, in which case the screed will envelope the heating pipes / wires and may need fibres in it for strength.

If no underfloor heating, slab can be above the insulation - what I did. This gave me a solid concrete base for the cement mixer, bricks etc. In this case you can do the floor in one go without a screed, if you have the skill / equipment to achieve a satisfactory finish.

100mm slab is really the minimum I would say.

If you pour concrete onto celotex, a barrier is required (ordinary polythene will do). I think this was to stop some reaction between the cement and the foil coating, but it also stops concrete getting between or under the boards and them possibly "floating".

The finished floor should be about level with the DPC on the inner leaf.

Your brickwork up to DPC ? Its easiest to brick up to the required slab height, since you can then use the walls as a level when tamping the concrete. For me, this was one course below DPC, since I was using 65mm screed on top.

Hope that helps,
Simon.

 

[live4weekends]

100 hardcore, 50mm builder's sand to blind, 1200 gauge DPM, 70mm insulation (Celotex, Kingspan, etc..), then 500 gauge plastic (or just use the leftover 1200 gauge DPM) then 100mm concrete slab

top of my concrete slab was about 50mm below DPC, I finished with battens and floorboards

celotex goes UNDER concrete

 

[noseall]

With regards the insulation position, a lot of folks are installing under floor heating and it is becoming commonplace to situate the insulation on top of the slab, i.e. directly beneath the screed.

I personally do not like this scenario and would not feel comfortable doing it this way. In fact there is argument to suggest heating the slab may be beneficial.

 

[Craig600]

So my finished floor level is level with dpc?
Also ive read about welding the dpm to the dpc how is this acheived or is it even necessary?

Yes i have built upto dpc level because the bo requested this before his next inspection surely this wont be a problem the concrete does not have to be purfectly level if i am screeding does it? Surely if my ffl is the same as dpc then i can use the level i have at the moment to screed from?

 

[Deluks]

Agree with previous, bin the screed and use more insulation under slab. If using electric mat heating than just put down the 10mm thermal boards beforehand.

 

[noseall]

So my finished floor level is level with dpc?
Also ive read about welding the dpm to the dpc how is this acheived or is it even necessary?

Yes i have built upto dpc level because the bo requested this before his next inspection surely this wont be a problem the concrete does not have to be purfectly level if i am screeding does it? Surely if my ffl is the same as dpc then i can use the level i have at the moment to screed from?

Finished screed is at the dpc or thereabouts, so your sub-slab will need to be 75mm down from this. You end up creating a kind of trough that fills with water when it rains!

There is no need for any welding just lap the dpm onto the dpc.