Water infiltration in a wall
or cellar: what to do?

An infiltration is recognised by its link with the weather. The stain appears or spreads during driving rain, or in the days that follow, then fades when the weather turns dry again. It often has a sharp outline, a ring, and a position that gives away its origin: top of a wall, underside of a window sill, corner of the ceiling, foot of a downpipe. The paint blisters, the plaster turns powdery, the interior render flakes. In a cellar, water beads or runs down the face of the buried wall, and a puddle forms at the foot after heavy rain or snowmelt.

It becomes a real problem when the stain returns with every spell of rain, when it gains ground from one season to the next, when water runs, or when timber is affected: lintel, joist, flooring laid against the wall. An isolated mark after an exceptional storm can be monitored; a stain that grows cannot.

Two other kinds of damp look like it and are treated quite differently. Condensation comes from the air in the house: mist on windows, black spots in cold corners, worse when the heating is low or ventilation poor, and unrelated to rain — see condensation on windows and mould on walls. Rising damp comes up from the ground through the foot of the wall: a fairly even horizontal band, white salts blooming, and damp that stays after weeks without rain — that is the subject of our article on saltpetre. If you are torn between the three, our guide damp walls: what to do covers the overall diagnosis.

Note the dates. Over a few weeks, record when the stain appears and when it dries, alongside the rainy days. If it only reacts to wind-driven rain, look at the exposed facade. If it reacts to all rain, even gentle, think of a gutter, a box gutter or the roof. If it follows snowmelt or long wet periods on a partly buried wall, the water is coming from the ground. And if it does not change in dry weather, it is probably not an infiltration: read the water meter in the evening and then in the morning, without having drawn any water in between; if it has turned, a pipe is leaking.

Read the position of the stain. High on the wall or at the corner of the ceiling, under a roof or a terrace: water is coming down from above (an overflowing gutter, a perforated box gutter, a flashing coming away at the foot of a chimney or along a wall). Around a window: the sill, the joint between the frame and the masonry, the reveal. In the middle of a wall, on the rain side: a crack, a loose render, hollowed stone joints. At the foot of a wall or in the cellar: earth against the wall, ground sloping down towards the house, a downpipe emptying right at the facade.

Walk round the house in the rain, with an umbrella rather than a ladder. You will see what a visit in fine weather never shows: the box gutter overflowing, the disjointed downpipe spraying the wall, water standing against the plinth, the sill without a groove sending water back onto the render. In dry weather, look for cracks and tap the render with the flat of your hand or with a handle: an area that sounds hollow has come away, and water is running behind it.

The water test confirms a suspicion. In dry weather, hose one area at a time with a garden hose, at low flow, starting from the bottom, and watch the inside before moving on to the next: the window sill, then the reveal, then the wall above. For a gutter, pour a bucket of water into the box gutter. No pressure washer: it pushes water where rain does not go and skews the result.

Painting over it. A paint, even one sold as “anti-damp”, hides the stain for a few months. The water keeps coming in, makes the new coat blister or comes out a little further along.

Waterproofing the room side of a wall that is wet from outside. The trapped water stays in the masonry, spreads upwards or sideways, and the wall no longer dries on either side.

A cement render on an old wall. On stone or rammed earth, cement ends up cracking: rain gets in through these cracks, then cannot get out again. These walls need a lime render, which lets water vapour through.

Relying on a dehumidifier. It dries the air in a room, not a wall that is receiving water: useful to finish drying after the repair, with no effect as long as the path stays open.

Lining or insulating a wall that is taking in water. The lining hides the problem and deteriorates in contact with it. The order is always the same: stop the water, let it dry, then insulate.

This is the simplest cause to treat, and one of the most common. Within your reach: clearing gutters and gutter guards of leaves and needles, in autumn and after windy spells, if they can be reached safely; refitting a downpipe; making sure it empties into an inspection chamber or away from the wall, never at its foot.

A zinc roofwork system that is perforated, poorly sloped or too small for the roof area needs replacing. On our 2024-2026 quotes in Savoie and Haute-Savoie, the complete zinc roofwork for a house (zinc gutters, downpipes, cladding of the fascias) most often comes to between €7,000 and €8,000 excl. VAT; the line-by-line breakdown is in our guide to zinc gutter prices.

If the stain is under the roof while gutters and box gutters are sound, the roof covering itself is at fault: a broken or slipped tile, a flashing coming away, a worn chimney flashing. A spot repair is often enough; when water ingress multiplies on a worn roof, our guide to re-roofing helps you decide when repairing is no longer enough. In Savoie, do not let the winter go by: water that has entered a crack freezes, thaws and widens it with every cycle.

Fine surface crazing in the render lets very little water through. An open crack, running right through the wall or stepping along the joints, lets driving rain in: before filling it, you need to know whether the wall is still moving. A sealant applied over an active crack reopens by the next season.

Around windows, you can redo the exterior joint between the frame and the masonry yourself when it is cracked or loose, clear the small drainage slots at the bottom of the frames and clean a dirty sill. A sill must direct water away from the wall: if it has no groove under its nose (the drip) or if it slopes towards the facade, the water runs down the render and then gets under the window frame.

The work: hacking off the loose render, treating the cracks, redoing the render. On an old stone wall, our masons redo a lime render, priced at €140 excl. VAT per m² on our recent quotes (two projects). When the whole facade is concerned, see our guides to the price of facade rendering and to facade renovation. A facade water repellent only comes after the repair: applied over a cracked render, it closes nothing.

A buried wall receives water from the ground over its full height. The more the ground slopes down towards the house, the more the earth holds water, and the harder it pushes against the masonry; if nothing collects it, it ends up getting through. Many old houses in our villages were built with no protection or drain at all against the cellar walls.

Within your reach: removing the earth, wood or rubble piled against the facade, re-grading the ground so that it slopes away from the wall, connecting downpipes so that they no longer empty at the foot of the house, leaving the air vents open so that the cellar can breathe, and storing nothing directly against a damp wall.

The work treats the wall from the outside. Our masons open a trench along the wall down to its base, brush and clean the face, apply a bituminous emulsion in two cross-applied coats, fit a dimpled membrane over it (Delta MS type) whose top is protected by a flashing, then a drain at the foot of the wall, surrounded by gravel and geotextile, which carries the water to an inspection chamber or an outfall. Backfilling is then done with a draining material or with the site's own soil.

Our prices, taken from our 2024-2026 quotes in Savoie and Haute-Savoie. The protection (bituminous emulsion and membrane) is priced at €30 excl. VAT per m² on seven of our nine quotes, and €103 excl. VAT per m² on two 2024 quotes covering a small area of 18 m²: from €33 to €113 incl. VAT with 10% VAT. The drain ranges from €32 excl. VAT per metre (drainage sock, fine gravel, backfill with the site's own soil) to €85 excl. VAT per metre (trench, drain, geotextile, filtering gravel and removal of spoil), on two projects; the flashing that protects the top of the membrane, €21 excl. VAT per metre. Earthworks are priced separately, by the volume of earth: their prices are in our guide to the price of earthworks.

Three limits, for completeness. Our quotes specify that this protection is aimed at walls that do not face habitable rooms: a cellar converted into a living space calls for waterproofing at another level, decided on the basis of a study. When the wall cannot be exposed (party wall, under a terrace or a structure), there are treatments from the inside; we have not priced any in our recent quotes and therefore give no price. Finally, the water collected has to go somewhere: article 640 of the Civil Code forbids making the flow towards the lower land worse. On sloping land held by a wall, drainage and outfall are designed together with the structure, as explained in our guide to the retaining wall.

An infiltration is always treated in the same order: close the water's path, let it dry, then repair the inside. A single company saves weeks here: our salaried craftsmen, each in their own specialty, carry out these stages one after the other on the same site. Our masons open up, protect, drain and redo the renders; our roofers deal with gutters, box gutters and the roof covering; our joiners repair or replace a window that lets water through; our plasterboard fitters and our painters redo plasters, linings and paintwork.

Between the two, patience is needed. A thick wall takes several weeks to start drying, sometimes longer: we check before closing up, rather than fitting a new lining on masonry that is still wet.

Do not wait if water is running, if a stain is spreading even though the gutters have been put right, if a structural timber is affected, if a crack opens or runs through the wall, or if the cellar takes in water with every heavy rain. Get help too when the origin cannot be found: water can travel within a wall and come out several metres from where it got in.

A good quote begins with a written cause: where the water gets in, and why the proposed repair stops it. It then separates the lines: earthworks (length, depth, soil kept or removed), preparation of the wall, protection (product, number of coats, membrane, protection of the top), drain and the point where the water is discharged, backfill, zinc roofwork or render, then interior repairs. Be wary of a quote that deals only with the inside and provides nothing outside. VAT is 10% for improvement work on a home completed more than two years ago.

On insurance, the official sheet Water damage insurance on service-public.fr states that infiltrations through the roof, walls or windows may be covered if they result from a sudden and accidental event, and that they are generally excluded when due to poor maintenance or wear and tear. The contract may also exclude the repair of whatever caused the damage: the insurer then compensates the damaged paintwork and coverings, not the gutter or the render. Notify it as soon as you notice the loss, within the period set by the contract, which cannot be less than 5 working days, and read your general terms and conditions, in particular what they say about facades. If the house is less than ten years old, an infiltration that makes a room unfit for its use falls under the builder's 10-year structural guarantee (garantie décennale) (articles 1792 et seq. of the Civil Code).