Conservatory DPM to DPC

[Tristar]

I have a large conservatory about to be constructed. I discussed with the builder the issue of overlapping the existing DPC with the 1200 gauge DPM. He says that to do so properly along the 6 meter length of wall will involve cutting out the cement above the DPC which will, almost certainly, damage it. I agree with him that this is quite likely. I know that all the regulations (for brick extensions) require such an overlap, I am equally certain that in many cases this does not happen for the reason above.

Is there a reasonable, tanking type seal, which could be used to solve the problem, and to avoid damaging the existing DPC. The builder just intends to lap the membrane up the wall, but clearly water pressures will eventually force damp up into the skirting board/plasterboard.

Thanks in anticipation

 

[^woody^]

To do it 'properly' would require a lap of the thickness of the wall, not just a token scraping out and this is unfeasible and unnecessary.

Just running the DPM up the wall is what is commonly done, and I've never known of a problem because of this.

I'm not sure what water pressures you are thinking of. Do you live in a very high water table area?

 

[Tristar]

It was referred to on one of the regulatory sites I was looking at, that the weight of the structure on the ground creates pressure, and any liquid will look for an escape, i.e. all around the edge of the structure. I guess the building acts as a piston trying to displace whatever is below.
Thanks anyway Woody.I assume you don't bother with any sealant.

 

[^woody^]

I don't know what site you were looking at, but it seems to infer that next to every building will be a little fountain of water! I wonder whats next to Canary Wharf?

Water finds its own level and is not "sqeezed" up by the buildings weight.

If you were in a high water table area where the ground water was just below the floor slab, then you could take extra precautions with sealant and coatings etc. But for many/most buildings the ground under the building will be drier than that around it.

 

[Shytalkz]

Water finds its own level and is not "sqeezed" up by the buildings weight..

Woody, Woody, Woody, please stop making glib statements like this! Increase in pressure on the ground above a water table can cause the hydrostatic head to rise in some circumstances. As can removal of overburden (boiling).

But yes, wherever that info came from was overstating the case, somewhat.

 

[^woody^]

As Mr Geko said "Glib is good"

It would be some peculiar and specific ground conditions to cause the water table to rise, and in the context of a little extension then I'd glibly guess at it being insignificant to the OPs situation

 

[Shytalkz]

Agreed, but surcharging ground with a high water table can cause it to rise, believe it or not - I'm guessing the two weeks of structures didn't extend to flow nets . Doesn't have to be any particularly obtuse ground conditions either. But in this specific instance, it isn't relevant.

 

[Tristar]

Thanks very much guys.

From what you are saying a small conservatory extension is unlikely to cause a great increase in the likelihood of water being forced up between the slab and existing house wall.
The regulations, in any event do not apply to my conservatory, but I am obviously interested in seeing that the best job is done. The conservatory is relatively large at 25 Sq.m., but is mainly glass, with a 450mm dwarf wall.

Thanks again.