Underpinning & damp!

[JDLDIY]

Go for it JDLDIY - there appears to be some misconceptions that underpinning is a dark art performed by highly skilled craftsman / artisans. It is nothing more than digging a hole and filling it with (correctly mixed) concrete according to instructions. It is not for the builder/contractors to have their work insured, but the SE upon who's calculations and advice all work is carried out. My neighbours' property was recently underpinned and excavated by such "highly skilled" craftsmen working for a professional building company. They managed to get that bit right as it is hard to mess up (luckily concrete was bought in). Everything else they touched was a complete disaster, including the extension - laying the most uneven and twisting foundations known to man, and the to add to the woes the building company went bust. Eventually it was all rectified at time and cost, but in all, the SE and BC documents are what matters. The builder's insurance won't even deal with the more superficial damage they caused to both our properties during their activities. I'm in the process of doing something similar, and on the advice of numerous people I know in the building trade, I'm going to tackle it myself in pretty much the same way you're going about it, which appears to be sensible and methodical

Wow, thanks for the encouraging words, really appreciated.

And regarding damp coursing, here are a couple of possibilities. I had been considering laying in membrane sheet in the hole for underpinning (SE doesn't see a problem with it, other than fiddly, but putting pressure on concrete will make the membrane take shape of the hole). Alternatively, as with my neighbours, no additional DPC was added in the wall or foundation, but instead a wall membrane was added, complete with drainage channel at wall/floor intersection. Water that gets trapped at the membrane eventually drains off down the channels, which gets piped off and runs out into the garden.

I can't see how to best attack the DPC when underpinning I must admit. Presumably when you've dug a hole under a section of your wall, you shutter it with boards so that you can be sure to keep the top level and know then that you can pour concrete up to the underneath of the bricks/blocks....can't see where/how to put in a DPC easily so that it can be continued along the wall. But then I suppose a DPC shouldn't be going in at this level anyway. That sounds complicated what your neighbour has had done - I can't picture how that works, I'll have to look up wall membranes.

Again, many thanks, let me know how you get on!

 

[moreteavicar]

Hi, sorry, only just got round to replying! I meant to say, although you probably already know - concrete shrinks as it sets, so for this reason traditionally you leave a gap of around 50mm when underpinning, and then you dry pack it - the SE should specify this. There is another method called "overfilling" where you fill up wider and higher than the original foundation, so that as the concrete shrinks it doesn't withdraw under the foundation. The latter technique is used a lot more in the US than over here, and there is literature around supporting the technique as being just as good, even a university project here: https://www.astm.org/studentmember/PDFS/Anthony-Lemus.pdf

Regarding DPC wall membranes, you've probably googled and seen most by now - this company has some good info: https://www.deltamembranes.com/
Also an installation manual http://www.triton-chemicals.com/pdf/Triton_Cavity_Drain_Membrane_Installation_Manual.pdf
All ideas for myself too, so I can have a more meaningful discussion with the SE when the time comes

 

[JDLDIY]

Thanks so much for your reply, that's given me loads to go on. Much appreciated.