Saltpetre on a wall:
causes and treatment.

Masonry — stone, brick, mortar, earth — contains mineral salts, and the ground it touches brings in others. When water moves through the wall, it dissolves them and carries them towards the surface. There, it evaporates and the salts stay behind: they crystallise into a white fluff. The word originally referred to potassium nitrate; today it is used for all these efflorescences at the foot of a wall.

This mechanism changes everything: saltpetre does not come from the air in the room, it comes from the wall. It appears exactly where the water comes out. Its position, its height and the date it appeared tell you where the water is coming from. So observe it before brushing it off.

The crystals do not stay politely on the surface. As they form in the pores of the render and mortar, they gradually make them burst: the render comes away, the joint hollows out, the soft stone crumbles. A wall that has had saltpetre for years loses material.

Three signs, often found together:

A white, cottony or powdery deposit, at the foot of a wall or at the bottom of a cellar wall. It crunches under the fingernail, dissolves in water and comes back after being brushed off.

Paint that blisters or wallpaper that peels away in a horizontal band at the bottom of the wall, sometimes with white crystals under the blister.

Render that flakes: it sounds hollow, comes off in patches, and a small heap of sand or powder builds up at the foot of the wall or behind the skirting board.

Often there is also a cellar smell, a wooden skirting board that is turning black and a lower wall that feels cold to the touch.

Saltpetre, mould or condensation?

Saltpetre is mineral: white, dry, crystalline. Mould is alive: black, green or grey patches, often fluffy, that stain the finger and tend to appear in corners, behind furniture or on the ceiling; our article on mould on a wall covers that case. Condensation shows as droplets on windows and cold surfaces, especially on winter mornings: see our guide to condensation on windows. All three can coexist in an old house; our guide damp walls: what to do helps you sort them out.

Brushing off the deposit pleases the eye for a while. But the water keeps passing through the wall and leaving its salts there: the fluff comes back, in the same place. Washing with plenty of water makes it worse, since you are putting back into the masonry the water that dissolves the salts.

Applying an “anti-damp” paint, putting on a cement render or fixing a waterproof lining against the wall means closing off the exit. The water does not disappear for all that: it rises higher and comes out above the treated area, on the other face of the wall or in the floor that rests on it. The Parc naturel régional du Gâtinais français information sheet on water in old buildings is blunt about it: blocked, the water comes out elsewhere, more concentrated, and waterproof coverings slowly damage the stones.

The only lasting answer comes in two stages: cut off the water supply, then give the wall back its ability to dry. Brushing, products and paint come afterwards, or are of no use.

Four causes come up again and again on old houses in Savoie, often several at once. Before any work, note where the saltpetre is, at what height, on which face of the wall and since when.

Water rising from the ground

Old walls were built directly on the ground, with no waterproof break at their base. Stone and lime mortar draw up water from the soil like a wick: this is rising damp. In a healthy house, this water evaporates as it goes through both faces of the wall. The typical sign: a band of saltpetre at a fairly even height along the whole length of the wall, inside as well as outside, more marked where the ground is damp.

A cement render or a waterproof paint on an old wall

This is what we see most often on barns, farmhouses and village houses: a stone facade covered in cement during an earlier refurbishment, an interior wall base painted with oil-based gloss paint, floor tiles or a cement screed laid over the old beaten-earth floor. The wall still receives water from the ground but can only release it through the areas left open: the saltpetre gathers just above the cement, or on the inside. One clue: the problem appeared, or got worse, after building work.

Outside ground level too high

Over the years, the courtyard has been topped up, asphalt or a concrete terrace has been poured against the facade, a flower bed has been built up against the wall. The foot of the masonry ends up buried: it takes in rain and run-off instead of letting them drain away. The sign: the saltpetre is higher or more marked on the facade concerned, and the outside ground reaches the level of the interior floor, sometimes above it.

A gutter that overflows or a downpipe that empties at the foot of the wall

A blocked gutter, or a broken downpipe that discharges onto the ground instead of into an inspection chamber, concentrates roof water onto one metre of facade. The saltpetre is then localised, under the leak, and gets worse after heavy rain. It is also the cheapest cause to remove. If the water enters instead through the facade itself — a crack, an open joint, a window sill —, it is an infiltration, described in our article on water infiltration in a wall.

The case of the cellar

In a cellar, the walls are buried over their full height: they receive water from the earth at the side, not just from below. A little saltpetre is common there as long as the cellar stays ventilated and nothing fragile is stored against the walls. It becomes a problem when it reaches the ground floor, when you want to convert the cellar, or when a concrete slab or a waterproof render prevents it from drying. Treatment then goes through the outside and through ventilation, never through a waterproof covering on the inside.

Measuring? With caution

A pin-type moisture meter gives an indication, but salts conduct current and distort the reading: a wall with saltpetre often seems damper than it is. Reading the building — ground level, condition of the gutters, type of renders and floors, date of appearance — remains the best tool. In a difficult case, a sample-based measurement, where a piece of wall is weighed before and after drying, settles the matter.

The logic does not change: remove the cause, let it dry, rebuild a wall that lets vapour through, then restore healthy air. Rendering before the water has been cut off means paying twice.

1. What you can do right away

Clean the gutters and check that each downpipe ends in an inspection chamber or is extended away from the wall. Clear the foot of the facade: earth, mulch, wood piles and planters that touch the masonry. Move furniture away from walls affected by saltpetre, remove vinyl wallpaper and waterproof skirting boards that block evaporation. Brush the deposit off dry and vacuum it up, without washing. Air the rooms: the ARS Auvergne-Rhône-Alpes advises opening the windows of each room wide for at least 10 minutes a day, summer and winter, and never blocking air inlets or extraction vents.

These actions do not cure a wall that draws water up from the ground, but they remove the simple causes and show, after a few months, what is left to treat.

2. Cutting off the water from the outside

This is the most effective part, and the one most often forgotten. Depending on the cause: lowering the ground against the facade to expose the foot of the wall, laying a perimeter drain that collects the water from the ground and carries it away, protecting the buried walls once the trench is open, repairing gutters and downpipes. Here is what these items come to in a survey of our 2024-2026 quotes in Savoie and Haute-Savoie, excluding VAT:

Complete perimeter drain: €85 excl. VAT per linear metre, including trench, drain, geotextile, filtering gravel backfill and removal of spoil. Only one quote itemises it this way (2025, farmhouse, 94 metres around the house and its extension).

Drain laid in a trench that is already open: €32 excl. VAT per linear metre for a 110 mm drainage sock surrounded by fine gravel and backfilled with the site's own soil (one 2025 quote, barn renovation).

Protection of buried walls: bituminous emulsion in two cross-applied coats, dimpled drainage membrane and a protective profile at the top, at €30 excl. VAT/m² on seven projects in 2025-2026, up to €103 excl. VAT/m² on two 2024 quotes covering small areas (18 m²). The finishing strip at the top of the membrane adds €21 excl. VAT per metre (two projects). These lines most often appear on foundations for extensions or new houses; on an existing wall, the principle is the same once the wall has been exposed, but the earthworks along the facade come on top.

Lowering the ground: our quotes contain no standard line for it. The price depends on the volume of earth, whether a digger can get access and the removal of spoil; it is priced after a visit. Our guide to the price of earthworks details these items, and the one on the price of a zinc gutter covers the handling of roof water.

In a home completed more than two years ago, this work generally falls under 10% VAT: a drain at €85 excl. VAT per metre comes to €93.50 incl. VAT. The rules are set out in our guide to VAT on renovation work.

3. Hacking off the render and redoing it in lime

Once the water has been cut off, everything that holds it in is removed: cement render, plaster saturated with salts, waterproof paint. We hack back to the masonry, and beyond the visibly affected area, because the salts have migrated higher than the deposit. Hollowed-out joints are raked out and redone. The wall then dries in the open air, for several weeks at least, longer if it is thick: this is the time to close nothing up.

Next comes the lime render, permeable to water vapour: it lets the wall release its moisture instead of trapping it, and it is the render, rather than the stone, that takes up the residual salts over the years. For the finish, we stay with products that let vapour through — limewash or lime paint, mineral paint — rather than an acrylic paint, an oil-based gloss paint or a vinyl wallpaper.

On our quotes, a lime render suited to old walls comes to €140 excl. VAT/m² (two 2025 quotes, on a facade, for a barn and a farmhouse), or €154 incl. VAT/m² with 10% VAT. Hacking off the old render is priced separately, depending on its thickness and hardness. Our quotes do not yet contain a line for interior-only lime render: it is priced after a visit. For a whole facade, our guide to facade renovation details the other items, and our article on stone walls and lime renders covers the techniques.

What about injections against rising damp?

The technique exists: holes are drilled in the bottom of the wall and a product, resin or water repellent, is injected, supposed to form a horizontal barrier against water rising from the ground. It is a job for specialist companies and does not appear in our quotes. If it is offered to you, ask three questions. Was the cause established by a diagnosis, or is the injection proposed straight away? Will the cement render and the external causes be dealt with as well? What written guarantee comes with the result? A barrier is useless if the water arrives through a gutter or through ground that is too high.

4. Restoring healthy air and ventilating

A wall that is drying gives water vapour back to the room: it has to be removed. In an old house whose windows have been replaced, the air is no longer renewed through the building's defects, and mechanical ventilation often becomes necessary. On our quotes, a humidity-controlled single-flow mechanical ventilation (VMC) fitted in a house costs from €1,600 to €2,700 excl. VAT, most often €2,100 excl. VAT, with 5.5% VAT; the details are in our guide to the price of a VMC. A dehumidifier relieves a cellar or a room while it dries, but treats no cause.

Saltpetre indicates a wall full of water, and it is that damp which weighs on health. Above 80% relative humidity, mould starts to develop, as the Gâtinais Park information sheet points out, which puts healthy air at between 40 and 60%. The ARS Auvergne-Rhône-Alpes lists among the proven effects of mould the development and worsening of asthma in children, as well as allergic rhinitis; children from birth, and people who are asthmatic, allergic or immunocompromised are the most exposed.

For the building, the risk is more direct: renders that fall away, joints that hollow out, soft stones that crumble, and timber in contact with damp masonry — joists, beams, skirting boards, flooring — that ends up rotting. If a piece of timber sounds hollow, flakes or has a fungus on it, have it diagnosed before closing anything up.

In a rented home, it is not just a question of comfort. Among the decent housing criteria set out by service-public.fr, the structural work must protect the premises against run-off water and rising water, and the ventilation must allow humidity to be removed as needed.

At Atelier NovAlp, a wall with saltpetre starts with a visit. The architect and one of our masons look at the ground around the house, the gutters, the type of renders and floors and the ventilation, then write down the probable cause before putting a price on anything.

The work is carried out by our salaried craftsmen, each in their own specialty. Our masons clear the foot of the facade, lay the drain and the protection of the buried walls, hack off the renders and rebuild the walls in lime; our plumber-heating installers install the ventilation; our painters finish with vapour-permeable products. Our plasterboard fitters only come in once the wall is dry, with insulation compatible with old masonry: our guide to insulating a wall from the inside explains this choice. On a whole house, this work fits into the order of works described in our guide to renovating an old house. To find out who does what, see our teams.

Call someone in when the saltpetre comes back despite the simple actions, when the render falls off in patches, when timber in contact with the wall is affected, before insulating or redoing a room against an old wall, and before buying a house whose wall bases have just been repainted.

A serious quote for a wall with saltpetre answers five questions:

1. What is the cause? It must be written down, with what justifies it. A treatment proposed without a named cause commits to nothing.

2. What is being treated outside? Drain, ground level, protection of buried walls, gutters: each item priced separately, in metres or square metres, earthworks and removal of spoil included.

3. What area is hacked off, and how far? The height of the hacking-off above the affected area, and what is removed: render, plaster, paint.

4. What render and what finish? The type of lime and the finish, which must stay permeable to water vapour. Be wary of the words “water-repellent” or “waterproof” applied to the inside of an old wall.

5. What drying time is planned? A schedule that runs hacking-off and rendering together with no pause leaves nothing time to dry.

The visit and quote are free: request a visit.