Damp proof course on garage extension

22 Jan 2021
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United Kingdom
Looking for a little bit of advice with regards to damp proofing a potential garage extension... First of all a bit of background.

Yesterday morning I opened my garage to be greeted with an unexpected indoor swimming pool. On investigation water was pouring over the DPC, the previous owners had "built" a patio against the garage wall and the concrete was one block above the DPC. Living near the woods at the bottom of a hill the sheer volume of rainfall recently had clearly raised the water table and the water took the easiest path, straight over the DPC into my garage.

I had to break the concrete and dig a channel to allow the water a path to escape, (it's still tricking out 8 hours later) slot should be okay now and the water stopped coming in and I've been able to drain the pool.

My question is this... I am planning on extending my garage, because it's only small and the land next to it is basically dead land and I end up just dumping stuff there. How would I go about damp proofing any slab that I put on that land? As in how to join the two damp proof layers so that there is a seal? The current slab is up to 18 inches above the DPC, that will be taken down to the level of the garage slab, that was always the plan, but I am now concerned about water ingress from an area that is clearly prone to holding water. The plan wasn't to take down that wall, but to leave it there and just create an internal doorway, using the existing small garage as a storage area and the new garage as a garage.

Also, would anyone recommend putting drainage into that area to drain water away from that land when the water table rises like that, and how would I do so?

Sorry to be long winded, I wasn't planning on taking this on this year, but now that I've destroyed the slab I may as well crack on.


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