Extension Floor

[ScottH]

Hi,
I am undertaking a small extension to my 1950s ex council house.
The extension is 3.5 x 4M
Foundations are in and I am looking at the floor make up at present. I have existing concrete from an old patio covering the entire space which is about 180mm below existing floor level.
The BCO has agreed that I can use this existing concrete instead of the compacted MOT but I am now looking for advise on the layers above this given the thickness limitation.
I intend to have :
sand to take out sharp points on concrete
DPL
75mm Insulation. Min specified by BCO
DPL
100mm Screed. with Reinforcing mesh.

BCO wasn't too worried about the sand. he stated that the existing concrete is a good dpl anyway.

Does this make up sound right. Any suggestions on type of insulation. Are there thermally better materials I could choose?
Is a scree mix 100mm deep directly on the second DPL OK?
Should I put any additives - fibre into the mix?
Should I go for 75mm reinforced concrete with 25 scree?

Any advise would be helpful.

 

[noseall]

1200g polythene, 80mm celotex and 100mm screed with fibres sounds good. No need for any other dpm or additives in the screed.

Your main concern is how to schedule the build so that you don't damage the screed whilst building as this is usually done in the latter stages. You however will have to do it before building up from the internal leaf dpc.

 

[ScottH]

Thanks Noseall.

By bricky wants to put this floor in after the roof to avoid damage and rain.
Do I need to join the DPC with the DPM hence installing prior to the inner leaf going up. Could I just run the DPM higher up the wall? say additional 4"