A perimeter drain sits at the foot of the foundation, with a fall of 3 to 10 mm/m, a filter medium of increasing grading free of fines, and a rot-proof 200 g/m² geotextile, according to the building pathology sheets published by France's Agence Qualité Construction. On a below-ground basement, that drain is never enough on its own: the wall takes water pressure over its full height, so the job becomes a drain at footing level, a waterproofing layer on the wall, and mechanical protection against the backfill. The work rarely fails on principle and almost always on execution: no fall, drain slots facing the wrong way, inspection chambers buried under the finishes. Before you price it, the question to settle is not "does it need draining" but "is the water coming from the masonry or from condensation", because those two answers do not sell at the same price.
Recognising the signs of a drainage defect around the foundations
Persistent damp: rising damp, efflorescence and musty odours
On a drainage defect, your value is not in noticing the damp, the client did that before calling you. It is in qualifying it before you price it. Record the height the damp reaches on the wall face, the relative humidity of the air at the foot of the wall and higher up the room, and any salt efflorescence, which points to water that has carried salts through the masonry rather than simple surface condensation. Put those dated readings in your site report: they are what separates rising damp from lateral ingress, and therefore a drain from a wall treatment.
Marks on walls and floors: tide marks, blistering paint, blackening joints
Tide marks at the bottom of partitions, blistering paint, warping skirting, darkening joints: these point the diagnosis, they do not prove it. The test that settles it costs a few euros and forty-eight hours. Bond and seal a sheet of glass or aluminium over the damp area, then come back and read it. Condensation on the room side means it is a ventilation and thermal bridging problem, and no drain will fix it. A mark on the wall side means the water is coming through the masonry and drainage is in scope. Running that test before quoting stops you selling a trench that will not change the problem, and it is the first omission that will be held against you in a dispute.
Structural problems: cracks, subsidence and render coming away at the base
Cracks, settlement, render coming away at the foot of the facade: once structural damage is visible, the quotation changes in nature. A drain installed without prior investigation on a building that is moving exposes you on structural warranty ground, for work whose cause you never established. Two habits protect you. Write an explicit exclusion into the quotation, separating what the drain treats from what it does not. And refer the client to a geotechnical study when cracking is active, particularly on shrink-swell clay, rather than taking on a structural diagnosis that is not yours to make.
Understanding how perimeter drainage removes water and limits damp
The path of water around a foundation: run-off, water table, clay soil
Around a house, water arrives by run-off, then infiltrates along the below-ground walls. If the water table rises or the ground stays waterlogged, the pressure creates hydrostatic pressure, and the damp eventually finds a way out, often through the joints. In clay soil, water moves slowly and pools more easily. Drainage then works to divert this flow before it settles in.
The role of the drain, the gravel bed and the geotextile: avoiding clogging
The perforated drain collects water at the foot of the foundation. It's laid in a gravel bed to let water pass through freely. A filtering geotextile wraps the whole assembly. It holds back fine soil particles and limits clogging; otherwise the drain becomes a blocked pipe, and the water comes back into contact with the wall.
Discharge and outlet: soakaway, stormwater connection and the slope to maintain
The collected water needs to leave via an outlet. Depending on the site, this can be a soakaway with a pump or a connection to the stormwater network, if permitted. Plan for a regular slope, often at least 0.5%, and a discharge point lower than the drain so the flow stays natural.
The drain in six figures
Perimeter drainage rarely fails on principle, almost always on execution. The building pathology sheets published by France's Agence Qualité Construction keep finding the same causes: unsuitable local backfill, insufficient quantities, filter medium grading not respected, drain badly positioned, no slope, slots facing the wrong way. Here are the six points your job will be judged on.
| Checkpoint | What the AQC reports |
|---|---|
| Drain slope | 3 to 10 mm/m, to be rechecked after backfilling |
| Filter medium | gravel of increasing grading in the direction of flow, free of fines |
| Geotextile | rot-proof, 200 g/m² |
| Drain slots | orientation respected at installation, a classic defect |
| Inspection chambers | present and accessible for checking and jetting |
| Outfall | connection to the drainage network verified, not merely assumed |
The least known trap is what happens around the drain. The AQC lists treating the surroundings among its twelve lessons on moisture, and its recommendation runs against the usual reflex: work the natural ground slope while limiting how much you seal the soil, plant the ground nearby to slow water movement, and build a channel to carry run-off away. Concreting the perimeter to make it look tidy makes rising damp worse rather than solving it, because the soil can no longer evaporate. The same logic applies when you go on to insulate the walls: a wall that can no longer dry will degrade, as covered in our article on insulating stone walls.
Choosing the right drainage solution for the ground and the building
Preliminary diagnosis: soil type, water level, condition of the foundation and renders
Before installing drainage, look at the ground under real conditions. Clay soil holds onto water. Free-draining soil clears it better, but can become waterlogged if the water table rises. Also identify the slope, a possible outlet, and the sources of water (rain, run-off, springs). On the building side, a local survey lets you check the condition of the foundation, the renders, the cracks and the joints. If the damp mostly comes from capillary action, drainage alone won't work miracles.
Rigid or flexible drain, diameter and accessories: what changes on the job
A flexible corrugated drain installs quickly in a trench, but needs a clean gravel bed and geotextile protection. A rigid PVC drain holds the slope better and makes connections easier. On a house, DN 100 is often the starting point, with inspection chambers at corners, inspection tees, and a regular slope toward the outlet. Without these accessories, clearing blockages quickly becomes a headache.
When to combine it with waterproofing of below-ground walls: membrane, protection and upstands
As soon as there's a risk of water pressure, a habitable basement, or degraded render, combine drainage with waterproofing of the below-ground walls. A bituminous membrane or a waterproofing render is then protected by a dimpled sheet or a panel, to avoid punctures during backfilling. Take care with the upstands above the finished ground level and continuity at the tricky points (corners, penetrations).
The scope of the package is not a matter of feel: it follows from the substructure configuration, and that is what belongs on the quotation.
| Substructure configuration | What the drain alone handles | What has to go with it |
|---|---|---|
| Ground-bearing slab, no below-ground room | run-off and backfill water at the foot of the wall | separate roof-water management, natural ground fall |
| Ventilated crawl space | backfill water, keeping the ventilation openings clear of water | through-ventilation kept free after backfilling |
| Below-ground basement, not habitable | free water at footing level only | wall waterproofing, dimpled sheet |
| Below-ground basement converted to living space | nothing sufficient on its own | wall waterproofing, upstands, mechanical ventilation, low-point treatment |
| Permanent or fluctuating water table | nothing: a drain is no substitute for a tanking system | hydrogeological investigation, tanking or pumped sump |
The last line is the one that costs money when it surfaces mid-job. A drain laid below the water table fills up and feeds the masonry instead of drying it: if a water level is suspected, monitor it over a few weeks before pricing, and write the exclusion into the quotation.
Installing perimeter drainage to the rules: points to watch on site
Excavation and safety: depth, shoring, neighbours and buried services
Before opening the trench, locate the buried services (utility locate requests, marking/staking). Take neighbours and property boundaries into account. As soon as the excavation gets deep, plan for suitable shoring and safe access. Keep enough width to install the drainage, on a stable base, without undermining the footings.
Installing at the right level: at the foot of the foundation, a regular slope and inspection chambers
Drainage is installed as low as possible, at the foot of the foundations, on a gravel bed. Aim for a regular slope toward the outlet, with no reverse slope. Provide inspection and flushing chambers at changes of direction and roughly every 10 to 15 m, to check the flow and make maintenance easier.
Backfilling and finishing: filtering materials, compaction, managing roof water
Backfill with filtering materials (gravel, geotextile) to avoid clogging. Compact in layers to limit settlement, while protecting the wall's waterproofing. Manage roof water separately, with a downpipe connected to the stormwater network or a dedicated infiltration solution. Drainage isn't designed to absorb roof water.
Maintenance, checks and common mistakes to avoid in 2026
Maintenance plan: checking the inspection chambers, flushing the drain and clearing blockages if needed
Every year, open the inspection chambers to check for deposits, roots, and proper flow. After a heavy rain event, a quick check avoids bad surprises. Flush the drain with clean water from the upstream chamber toward the downstream one. If water is standing or rising back up, plan for clearing (rod, jetting) and check the outlet.
The recurring pitfalls: a drain set too high, insufficient slope, a poorly planned outlet
Drainage doesn't forgive a rough approximation. A drain set too high leaves damp in contact with the walls. An irregular slope creates pockets of sludge. And a poorly planned outlet (inaccessible, too small, without reliable discharge) turns the network into a bathtub. Key point too: keep the inspection chambers accessible, even after the finishes are done.
Special cases in 2026: whole-house renovation, energy audit and coordination with ground-floor insulation work
In a whole-house renovation, the energy audit often flags losses through the ground floor. Before insulating, secure the damp situation: drainage, crawl-space ventilation, and continuity of the drainage runs. Since drainage falls under no standardised scheme, it is priced as an ancillary trade, line by line on the quote alongside the energy measures. Good practice: coordinate the "flooring" trade and the "water" trade so an inspection chamber or outlet doesn't end up sealed under the insulation.
