New extension on an old building

[JohnArmer]

I'm replacing a small lean-to porch at the back of our house this summer.

The main house is sandstone and lime pointed; the floor of the existing porch is currently a concrete slab with no DPC that simply butts up against the old house.

When adding a new extension to an old house, how is the DPC of the extension connected to a house that has no DPC but relies on equilibrium to keep it dry?

 

[TheSpid]

put in limecrete instead of a new DPC

 

[Flyfisher]

. . . or perhaps a band of limecrete against the house, say 500-1000mm, with a concrete slab + DPC covering the remainer of the extension (obviously bringing the DPC up against the limecrete rather than extending underneath it)?

 

[Westholme]

If you are replacing the porch with another one, my understanding is that the new porch does not require building regs approval, assuming it's neither huge nor heated/air-conditioned, therefore a DPC is not mandatory.

 

[Flyfisher]

I thought it was only planning consent that had been relaxed for certain extension works. I would expect building control to still be required though. After all there's a difference between building something with/without consent and building something safe/unsafe or compliant/non-compliant.

https://www.bbc.co.uk/news/uk-england-48405569 (would still need LBC if the house was listed of course)

On the other hand, who would ever know if the extension was already there when the house was purchased - except any neighbouring curtain-twitchers? :wink:

 

[MikeG]

If you are replacing the porch with another one, my understanding is that the new porch does not require building regs approval, assuming it's neither huge nor heated/air-conditioned, therefore a DPC is not mandatory.

Correct, so long as the (inner) door remains in place. Nonetheless, a DPC and DPM are a good idea. I would turn the DPC down to isolate the new plinth from the existing, and I would either tuck the DPM in under the existing building if foundations just happened to work like that, or turn it up to isolate the new concrete slab from the existing building, and cut it off at floor level afterwards.

 

[MikeG]

put in limecrete instead of a new DPC

You could do, but the OP doesn't say what the construction of the porch is going to be. If it is impermeable to the passage of water vapour, then it will need to be properly ventilated otherwise all that limecrete does is ensure that the space will be damp. It is imperative that people use materials in the full knowledge of how they work, and limecrete, fashionable (and useful) as it is, isn't the answer in all circumstances. Limecrete, BTW, doesn't take the place of a DPC so much as render a DPM redundant.