Foundation Drainage: How to Keep Water Away From Your House

Most damp basements are a drainage problem, not a waterproofing problem. Here is how the layers actually work.

A foundation drain is a perforated pipe laid alongside the footing, below the level of the basement floor, surrounded by washed drainage gravel and wrapped in geotextile. Its job is to collect groundwater before it can build up against the wall and route it to a soakaway or storm connection. The pipe is the cheapest part of the system; the gravel envelope around it is what actually does the work, and it is where most failed installations went wrong. Plan on 150 to 200 mm of clean gravel under and around the pipe, never sand and never the soil you just dug out.

Why Fabric Placement Decides the Lifespan

Geotextile separates the gravel from the surrounding soil so fine particles cannot migrate in and silt up the voids. Wrap the fabric around the whole gravel envelope, not tightly around the pipe itself. Fabric hugging the pipe clogs at the perforations within a few years and the drain stops draining while looking perfectly intact from the outside. Overlap seams by at least 300 mm, because a gap in the fabric is a direct path for silt.

Fall matters as much as materials. Aim for a consistent 1:200 slope toward the outlet, which is 5 mm per metre. That is shallow enough to be easy to miss with the eye, so set it with a laser or a line level rather than by judgement. A drain with a dip in it holds standing water, and standing water in cold ground is how pipes crack.

The Part Above the Drain

Even a perfect drain fails if the surface above sheds water toward the house. Grade the finished ground away from the wall at roughly 1:20 for the first two metres, and check that downpipes discharge into the drain or well clear of the foundation rather than onto the soil beside it. A single downpipe emptying next to a wall can deliver more water in one storm than the surrounding ground receives in a month.

Summary

Foundation drainage is layered: gravel envelope, fabric around the gravel, consistent fall, and surface grading that sends water away. Use our Gravel Calculator to work out the volume for your trench length, and order 10 per cent extra, because settling gravel always takes more than the arithmetic suggests.