We have underpinned the lower ground floor (LG floor = common construction practice in Victorian London, whereby the floor is not technically a full basement.) of a mid-terrace house. Luckily, our neighbour on one side did the same thing at the same time.

The new concrete that forms the pin on the other side (i.e. the wall with the neighbour whose LG floor was not underpinned, and remains at its original slightly higher level) is therefore subject to ingress of water from the soil on the other side.

We have stripped our internal walls back to brick.

It just so happens we have also dug a basement at the rear, which requires a waterproofing system (cavity drain membrane and sump pump).

My question

Should we apply the same cavity drain membrane system to the existing LG now that we have underpinned? We can get water that drains from the LG floor out to the new basement and use its sump pump.

What are the pros and cons of doing this?

I guess the alternative might be to "tank" the wall with a waterproof render.

1 Answer 1


The only effective way to waterproof a masonry wall is from the outside - so if your alternatives are to coat the inside of it or run the water elsewhere, run the water elsewhere, as coating the inside is doomed to failure.

If the coating is on the exterior and the water is trying to get in, the water pressure holds the coating in place. If the coating is on the inside, the water pressure will pop it off, sooner or later. If the waterproofing was not put in place as the wall was built, on the dirt-facing side, it's probably never going to work - or it won't work for long.

  • Thanks for your answer. When you say "the water pressure will pop it off", I'm a bit confused. As I understand, the cavity membrane system bolts into the masonry with special waterproof plugs every 600mm, and an internal frame wall is built up against the membrane. So I don't think it will pop off. It's not a flat membrane, but rather an egg-shell formation which allows water to drain down to the bottom, pool on the floor, then find its way to the lowest point i.e. the sump.
    – hazymat
    Jan 16, 2017 at 1:50
  • The cavity drain and directing the water to a sump should work. The "waterproof render" will fail, in my opinion/experience. Since the cavity drain is the method whereby you'd run the water elsewhere, that is what I suggest doing.
    – Ecnerwal
    Jan 16, 2017 at 1:53

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