A perimeter drain sits at the foot of the foundation, its invert below the lowest level of the slab or room it protects and never below the footing's bearing level, 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. The full measurement protocol is set out in our article on rising damp.
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. Stormwater management on the plot, and what it imposes on discharge, is covered in our article on ground sealing. 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.
Perimeter drain depth and slope: the levels to hold
The drain invert (the inside bottom of the pipe) sits below the lowest level of what you are protecting, at the foot of the footing but never below its bearing level, with a continuous fall of 0.5 to 1% towards the outfall.
Where to set the invert: below the lowest level, above the bearing level
Two levels frame the depth. At the top, a drain set above the lowest level collects the water after it has already wetted the masonry. At the bottom, a trench deeper than the footing loosens the soil that carries it and draws fines out from under the foundation. NF DTU 20.1, which governs below-ground masonry walls, treats drainage as a complement to treating the wall, never as a substitute. So find the bearing level with a trial pit before pricing the trench depth: on an old house, it is rarely where you expect.
| Configuration | Reference for the invert | Typical trench depth (m) |
|---|---|---|
| Single-storey house on a slab | below the underside of the slab, at the foot of the footing | 0.6 to 1.2 |
| Crawl space | below the crawl-space floor | 0.8 to 1.5 |
| Below-ground basement | at footing level, below the basement floor | 2 to 3 |
| Shallow old foundations | above the bearing level, even if the drain stays above the slab | after a trial pit |
On a house founded at 40 cm, holding both levels becomes impossible: the bearing level wins, and the quote states what the drain will not deal with. On a crawl space, the drain also protects the ventilation openings, a condition for insulating the floor afterwards (see our article on the crawl space).
Drain slope: aim for 0.5 to 1%, checked after backfilling
The AQC reports falls of 3 to 10 mm/m. Aim for 5 to 10 mm/m (0.5 to 1%), as the bottom of the range leaves no margin for any settlement of the bedding. A 22 m run therefore drops 11 to 22 cm between the upstream corner and the collection chamber: that difference sets the trench level at the low point, and it belongs in the take-off. For a full loop, run two branches from the high point rather than a single loop, which would force a very deep dig at the end. Check the fall with a laser level at installation, then again after backfilling, because compaction is what creates back-falls.
Foundation drain build-up, layer by layer
A foundation drain is not a pipe in a hole: it is a filter medium in which the pipe is only the visible part. In installation order, from the bottom of the trench:
- Trench bottom trimmed to the fall and cleared of mud: it gives the slope, not the gravel.
- Non-woven geotextile, rot-proof, 200 g/m² according to the AQC, laid with enough overlap to fold back over the medium.
- Bedding of 5 to 10 cm of washed gravel.
- Corrugated drain in PVC or HDPE, DN 100 for a house, slots oriented as the manufacturer specifies.
- Surround of washed gravel free of fines (10/20 or 20/40 type), at least ten centimetres or so above the drain.
- Geotextile closed with an overlap over the medium.
- Backfill, free-draining against the wall in the lower part, then soil compacted in layers, with finished ground falling away from the wall.
Add an inspection chamber at the head of each branch and at every change of direction, a collection chamber at the low point, a solid collector pipe (unperforated) to the outfall, and a non-return valve if the discharge can back up.
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.
House drainage on clay soil: when the drain worsens shrink-swell
On clay ground, a perimeter drain is still useful against run-off, but badly designed it worsens clay shrink-swell (RGA in France). Hard against the footing, it dries out the soil carrying the foundation in summer. Badly connected, it concentrates water at one point and makes the clay swell locally. Both produce the same defect: differential movement under the house.
Why a drain can crack a house on clay
What cracks a house on clay is not the ground movement itself, it is the difference in movement from one point of the foundation to another. A drain against the footing creates exactly that difference, drying out a strip of soil under the facades while the middle of the house stays damp. French public guidance on preventing shrink-swell damage accordingly recommends keeping drainage away from the foundations, in the order of two metres, rather than against the wall. Conversely, an outlet that leaks, a soakaway dug three metres from the facade or a downpipe connected into the drain put water back where it is not wanted. The mechanism, hazard zones and heavy remedies are covered in our article on clay shrink-swell.
What changes in the design on clay ground
| Design point | Low-sensitivity soil | Clay soil, medium or high hazard |
|---|---|---|
| Drain position | at the foot of the footing | set away from the foundations, without going below the bearing level |
| Outfall | soakaway possible if the soil infiltrates | discharge to the network or well away from the house, never infiltration at the foot of a facade |
| Collector | standard fittings | sealed collector to the outfall, joints checked at handover |
| Surroundings | permeable ground, ground slope | protection against evaporation (paved apron or geomembrane) that stabilises moisture content |
| Prior survey | depending on the defects found | geotechnical survey recommended as soon as a crack is active |
The surroundings row seems to contradict the AQC, which advises limiting ground sealing: one targets damp in the walls, the other moisture content under the foundation. In a high-hazard zone, stability comes first, and the choice is written into the quote. Check the zoning on Géorisques before the visit. The geotechnical survey required by France's Élan law covers the sale of building plots and new builds, not drainage in renovation, which does not stop you recommending one when the house is cracking.
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.
Perimeter drain cost per linear metre
As a rough guide, allow 60 to 120 € excluding VAT per linear metre installed for a house without a basement, trench included, and two to three times that once the dig goes below 2 m or the wall has to be waterproofed. These ranges are market benchmarks, not a price list: depth, machine access and the outfall account for most of the spread.
| Item | Unit | Order of magnitude (€ excl. VAT) |
|---|---|---|
| Materials only: DN 100 drain, geotextile, gravel | lm | 10 to 25 |
| Drain installed, house without basement, trench up to 1.2 m | lm | 60 to 120 |
| Drain plus wall waterproofing with dimpled membrane | lm | 120 to 250 |
| Below-ground basement, 2 to 3 m dig with shoring | lm | 200 to 400 |
| Inspection chamber installed | unit | 80 to 200 |
| Pumping station when the outfall is higher than the drain | unit | 800 to 2,000 |
What drives the price and what belongs in the quote
The length is measured at the foot of the facades, plus the collector. The price rarely moves because of the drain itself: hand digging for lack of access, a patio to lift and relay, rocky ground, spoil removal. For a 40 m perimeter at 90 €/lm, the drainage package alone comes to around 3,600 € excluding VAT before chambers and outfall. That is the calculation a client compares with a cheaper bid that counted neither the geotextile nor the chambers.
In the quote, itemise line by line: trench length and depth, invert level and fall, drain type and diameter, gravel grading and volume, geotextile weight, number of chambers, type of outfall, spoil removal and reinstatement of the surroundings. On a home completed more than two years ago, French VAT is generally 10%. That detail protects you if the client later disputes the depth or the fall.
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.
