Subfloor below existing DPC

[bandejapaisa]

My new extension subfloor is lower than my existing house DPC. See on the photo where my block and beam sits and where the existing DPC is. I'll be having an internal step down to the new bedroom.

So I've sat the beams on the inner skin DPC, and i'll slot them into the existing wall on a DPC too, but the top of the beam is 180mm below the old house DPC (it's slate by the way). I'll have 120mm insulation, followed by 65 screed.

I'm not 100% sure what to do, but my initial thought would be to cut into the mortar of the existing DPC, and then get a 400mm wide DPC and slot that in and drape it down the side of the existing wall as an barrier between the wall and the new insulation and screed that will sit on top of my beams. When i concrete infill the bit of beam which is parallel to the existing wall, I don't want to bridge it, so my draped DPC will prevent that. Nor my screed, which would bridge the DPC too if I don't have the vertical DPC draping.

I know I have the DPM on top of my floor insulation, that I can lap up the walls, but that isn't prevent my beams from getting damp - which was the whole purpose of sitting them on the DPC.

What's the standard procedure here and products used? Can I find this info in building regs / NHBC?

Thanks

 

[Mr Chibs]

Could you not lap a wide dpc up, from under the beams, tight up the side of the B&B's and into DPC into the outer skin.

Are you using coursing slips from blocks to engineers?

 

[bandejapaisa]

The beams will be slotted into pockets cut into existing house. You can see one already cut out on the photo. so can’t easily just put a dpm under the beams and lap up. Or maybe I can ‍

Existing house is solid wall. There isn’t an outer skin.

Yes. I’m using coursing blocks to fill in the heights under my blocks around the edges. Supplied by seam company.