Extension dpc height

[23vc]

Hi
Pouring concrete for my extension footings soon and plan to pour 6 or 7 courses of bricks below dpc.
I can't have a screed as I'm keeping a very thick old layer of concrete so only got room for insulation and slab.
Existing house, dpc is about 2inch below finished floor level, but if I go with finished floor level at same height as dpc ( as per loads of great advice on this forum) it means extension dpc is a but higher than existing house, which I was hoping to just link vertically when doing the brickwork up to damp.
If there's any chance anyone fancies sanity checking the above, it would be much appreciated as always!

 

[vinn]

I doubt that anyone on here has got away with advising an OP to have a FFL "at the same height as a DPC"?
Sometimes its unavoidable when dealing with existing circumstances but its not advisable to build it in.
The best practice is for the DPC to be 150mm below the DPC.

Why dont you scan a rough sketch of a section view of whats existing & what you propose? Include the external ground level.

 

[23vc]

Thanks vinn see pic

 

[vinn]

Thanks for the sketch.
So the existing concrete slab surface is level with the house FFL?
This "old layer of concrete" will then have been bridging the house DPC - level means bridging.
Presumably the slab doesn't have an existing membrane (DPM) below the concrete?
And the plan is to dig out and build foundations around the footprint of the slab?

 

[23vc]

No, the old concrete is 200mm below existing floor level (and new floor level)
I should have included it on the sketch sorry.
It's where your hardcore would normally be.
Bco has ok'd dpm and insulation straight over it. Then new slab on top.

 

[^woody^]

It sounds like your house has a timber floor, where it is common for the DPC to be lower than it.

And new dpc should be at the same level as the new floor, and lapped over the existing.

Btw, you would be better off adding extra insulation and then screeding on top, or having a layer of concrete on your slab, then insulation and screed.

 

[23vc]

Thanks Woody
When you say dpc lapped over the existing do you mean run it up the wall and chop it in to the existing ? (only about an inch or 2 different between new dpc and old)
I would prefer a screed but I only have 200mm to spare above that existing concrete so I only have room for insulation and slab.

 

[^woody^]

How can you be running DPC "up the wall"? It will run along the wall and then down 50mm and then over the existing DPC.

200mm would be 120 or 130 of Celotex and then 80 or 70 screed.

 

[23vc]

Thanks Woody that's answered the question I was trying to ask. Sorry I am not great at explaining things on here.
I still need to pour a new slab, I can't just insulate and screed it. So it's 100mm insulation and 100mm slab.

 

[^woody^]

You said that you are going over an existing concrete layer. If that is a slab, you can just insulated and screed.

 

[23vc]

Maybe slab is an exaggeration, it's an existing garage base with different sections and some if it needs blinding/levelling a bit, then round the back there is going to be hardcore instead, so I still need a new slab throughout

 

[oldbutnotdead]

Maybe slab is an exaggeration, it's an existing garage base with different sections and some if it needs blinding/levelling a bit, then round the back there is going to be hardcore instead, so I still need a new slab throughout

No you don't- hardcore, old concrete, whatever- long as it is all sound and you have a good blinding level (50mm is the book figure) then you can stick your polythene and Celotex straight on the sand

 

[23vc]

No you don't- hardcore, old concrete, whatever- long as it is all sound and you have a good blinding level (50mm is the book figure) then you can stick your polythene and Celotex straight on the sand

I still need a slab, surely. Where it's just hardcore it needs an oversite not just insulation and screed, and it needs to be a continuous oversite throughout

 

[oldbutnotdead]

Can't remember what problem we're solving now but yes the ADs imply you do need a slab but doesn't have to be 100mm (if you do your floor to one of the guides in the Approved Docs then it WILL comply. Other methods may comply, talk to your BCO). You can chuck a blinding layer above your random hardcore/old concrete/various things, then insulate, then 80mm slab then screed above that to FFL. Had a similar sketch converting the coalhole to a shower room- couldn't get deep enough for any of the floor layouts in the Approved Documents due to very shallow footings. BCO comes along, looks at it, says 'Well in an ideal world you'd do that but in the real world stick 80mm Celotex and 80mm concrete down & I'll be happy.

 

[^woody^]

It comes down to what is reasonable, cost effective and easy to do.

With a slab you'll need to power float it, or use self levelling afterward's. And is the finish going to be acceptable?

Or a screed or reinforced screed may be better?

The point is, don't constrain yourself to just one method, when there are options.