I’m building a 20 ft. extention on the back of my Brooklyn rowhouse, and my neighbor’s yards are higher than mine, putting the sill of the extention below grade. Is there any way to waterproof the side, maybe a peel-n-stick membrane running up the side, from the foundation up the wall? Or do I have to raise the mudsill above grade?
thx, Pat
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Replies
Much better to step the foundation up at least 6" above grade. This means concrete and mudsill above interior floor height, which means that you may have to thicken your framed wall to match, or frame a short wall along the concrete. Insulation, wiring, and plumbing can get complicated in this sort of situation.
I agree that you're better off building up the masonry to above-grade level. But sometimes this isn't possible. Had that problem at the back of my garage a year ago. Old sill and bottom clapboards were rotted. I solved the problem by removing the bad wood, installing a p-t bottom plate, covering the slab/sill area with peel-and-stick, then installing a wide Azek board in place of the bottom two clapboard courses. Azek can take ground contact no problem. So far this has worked out well. Good luck.
The peel 'n stick may last a while, who knows, but it's not an approved method and I would not recommend it or use it on a customer's house. Did a manufacturer give the OK for this?
anything than other foundation material covered by the soil is an invitation to rot (lots of PT is not ground contact rated), future moisture issues and an interstate for bugs & termites (if you got them where you live) to enter the house. Do what it takes to step the foudation up, poured wall or CMUs.
There are a half-dozen different things you can do, short of raising the foundation. None (other than maybe building to PWF specs) is going to be strictly code, but several should provide 50 year lifetime under average conditions. (My choice would be treated framing, cement board "sheathing", and rubber membrane, maybe two layers.)
If you do "cheat", be sure to backfill the few inches adjacent to the house with gravel, not soil.
But look at stepping up the foundation. There are ways to do it without having to affect the inside floor height.