How to waterproof a basement wall or foundation

A buried wall that lets damp through doesn't always have badly applied waterproofing: sometimes the problem is that it lacks the drainage that should go with it. They are two different systems that work together, and one doesn't replace the other.

Bituminous membrane: the direct barrier against the wall

The bituminous membrane is bonded directly to the wall before backfilling with soil, protecting it from the permanent damp of the ground. Check the junctions with the foundation and the corners carefully before backfilling: once the ground is filled in, any fault at those specific points is very expensive to locate and repair afterwards.

Drainage membrane: it complements, it doesn't replace

The drainage (dimpled) membrane creates an air and drainage cavity against the buried wall, before backfilling with soil. It doesn't replace the wall's waterproofing, it complements it: it stops the hydrostatic pressure of the ground pushing water directly against the waterproofing layer, greatly extending its life. Both are bought with a 10% margin for overlaps.

Cementitious waterproofing mortar: for drinking-water tanks

Cementitious waterproofing mortar is applied directly onto concrete or masonry, and is more rigid than a flexible membrane. There are specific formulations approved for drinking-water tanks — if it's for a tank of water for human consumption, check that the product is certified for that specific use; not all cementitious waterproofing mortars are, even if the appearance and application are the same.

Drain pipe: it needs a fall, not just gravel

The perimeter drain pipe collects the seeped water at the base of the wall and carries it to a discharge point, stopping it building up against the foundation. Always lay it with a surrounding layer of gravel and some fall towards the collection point — without a fall, the water stagnates inside the pipe itself instead of draining away, and the system doesn't do its job even if it looks properly installed.

Frequently asked questions

Can I fit only drainage membrane without waterproofing the wall? It isn't enough: the drainage membrane complements the waterproofing by reducing water pressure, but doesn't replace the wall's waterproof barrier.

Will any waterproofing mortar do for a drinking-water tank? No: you need a product certified specifically for that use, even if the appearance and application are the same as a standard waterproofing mortar.

Why does my drain pipe not drain the water even though it's installed? Most likely it lacks a fall towards the collection point — without one, the water stagnates inside the pipe itself.

Conclusion

A well-done basement wall waterproofing combines the direct barrier (bituminous membrane or cementitious mortar), pressure relief (drainage membrane) and drainage (drain pipe with the right fall). Checking the junctions with the foundation before backfilling is the step which, if skipped, is much more expensive to put right afterwards.