Saltpetre on a wall: what it tells you and how to treat it

White tide marks at the foot of a cellar wall, paint blistering in the hallway, plaster that sounds hollow when you tap it: in an older Brussels house, people almost always call this "saltpetre". This guide explains what the word covers, what these deposits tell you about the wall, in what order to act and what the technical sources (Buildwise, Homegrade, Energie+) say about the possible treatments. The guide is no substitute for an on-site diagnosis.

Key points

• Saltpetre is a symptom: salts dissolved in the water in the wall crystallise when that water evaporates. No water, no deposit.
• First identify and deal with the source of the damp, then let the wall dry out (months, sometimes longer), and only then treat the salts and redo the finishes.
• Not all salts behave the same way: some can be brushed off, others draw moisture from the air and call for a suitable plaster or protective layer.
• If the facade is involved, check with your municipality's planning department before changing the render or the facing.

What "saltpetre" actually means

According to Homegrade's 2024 brochure, "saltpetre" (salpêtre) is used in everyday language for any kind of efflorescence. Technically, it is potassium nitrate, found in particular in many fertilisers, in animal droppings and in combustion gases. Other salts leave similar deposits: sulphates, which occur naturally in soil and rainwater, and chlorides, which are found in de-icing products.

Buildwise (formerly CSTC), in its guide to masonry restoration published in 2003, draws a distinction that is more useful in practice, between two families:

• efflorescent salts (sulphates, carbonates), which come mostly from the building materials themselves;
• hygroscopic salts (chlorides and nitrates), which come mainly from contact, sometimes very old, between the masonry and soil, waste water, fertilisers or slurry.

The same guide explains that most nitrates and chlorides are far more hygroscopic than sulphates or carbonates: they absorb moisture from the air even when the relative humidity is normal. Its table actually lists potassium nitrate (the "saltpetre" of Homegrade's definition) among the salts that are not hygroscopic under normal conditions (equilibrium relative humidity of about 95% at 20 °C for the saturated solution), whereas calcium, magnesium and sodium nitrates clearly are. So a single word covers salts that behave quite differently.

What saltpetre tells you: there is damp, but not where it comes from

Energie+ sums up the mechanism: efflorescence is caused by salts crystallising as the water carrying them evaporates, during a drying period that follows a period of wetting. That is why it appears mostly in spring. The sheet sets three conditions: the presence of salts, damp in the brick and the recrystallisation of the dissolved salts.

The direct consequence: saltpetre proves that there is (or has been) water in the wall, but says nothing about where it comes from. Homegrade distinguishes five possible causes of damp in a home:

1. rain penetration (roof, facade);
2. capillary moisture (rising from the ground, or lateral damp in walls set against earth);
3. condensation of water vapour on a cold surface;
4. an accidental leak from a pipe;
5. residual moisture in materials after building works.

Buildwise stresses a point that makes diagnosis harder: in old buildings, damp problems often have several origins, the sources can add up, and the damage sometimes shows quite far from the source. Different causes produce similar damage (finishes coming away, efflorescence, mould). The guide therefore recommends going through every potential source, even when the cause seems obvious.

To take the diagnosis further (rising damp, condensation or rain penetration), see our guide Damp in a Brussels house.

A few clues

These pointers are taken from the sources. They are for orientation only and are not a diagnosis.

This table only covers some of the observations: the full table, which cross-references more than fifteen observations with six possible causes, is in procedure 40 of the Buildwise guide (2003).

One useful detail from the Buildwise guide: a dense cement render or a waterproof membrane on the inside or the outside hinders evaporation, and the damp can then rise higher in the wall. Covering up the bottom of a wall does not solve the problem and can make it worse.

What to do first: deal with the source, then let the wall dry

Homegrade sets out the order of work in two sentences: check whether the wall really has a damp problem (rain penetration, capillary moisture, a leak), fix that problem, then give the wall time to dry. Only if salt-related symptoms persist do you treat the salts themselves.

Drying is slow

Buildwise points out that drying out is a very slow process, whatever the intervention. As an illustration, the guide calculates that a wall 0.40 m thick, with an average moisture content of 10% by mass over a height of 1 m, holds almost 70 litres of water per linear metre. In favourable conditions, evaporation takes several months; with thick walls and poor conditions, it can take several years. The guide advises constant ventilation and a minimum of heating in cold weather.

The same guide (2003) states that, for a wall treated against rising damp, the maximum acceptable moisture content when the finishes are applied is most often between 3 and 5% by mass, depending on the substrate and the finish. This benchmark dates from 2003 and should be confirmed with a professional. The guide also explains that resistance and capacitance moisture meters give an indicative reading on a relative scale that does not match the actual moisture content: they help to locate problem areas, not to decide that a wall is "dry". To monitor drying accurately, it cites comparative weighing and the carbide method.

The salt trap during drying

Once the source of the damp has been removed, the dissolved salts travel with the water towards the surface, and the remaining water evaporates and leaves them behind. Buildwise notes that these interventions have the effect of increasing salt concentrations at the surface; Homegrade observes that the phenomenon often appears after a treatment against rising damp. So deposits appearing after works are not necessarily a sign of failure: the wall may simply be drying out.

Treating the salts: brush them off, protect the finish, or both

Efflorescent salts

According to Buildwise, the whitish efflorescence formed by sulphates or carbonates causes mainly cosmetic damage and can generally be removed easily by brushing. If these are the only salts found, light brushing or stripping the old finishes is usually enough, and the interior finishes can then be redone in the usual way.

Things change when the salts crystallise behind a finish (plaster, paint) or inside it. Buildwise explains that hydration and crystallisation of salts in the pores come with an increase in volume and with high pressures, which cause deterioration or detachment. Energie+ adds that efflorescence on the surface does not damage the material, whereas efflorescence forming inside the structure of the brick can make it deteriorate prematurely.

Hygroscopic salts

These salts capture moisture from the air and hold on to it. Homegrade notes that they can stop the wall from drying even after the source of the damp has been dealt with. Buildwise adds that they almost never form efflorescence and so cannot be removed by dry brushing, and that where their concentration is high, walls do not dry out completely, even after effective treatment of the rising damp.

Both sources then describe the same approach: rather than waiting for the salts to disappear, you limit their effect on the finish. Homegrade talks of limiting crystallisation at the surface, without this making it possible to remove the salts, and lists four types of solution:

• renovation plasters, which are permeable to water vapour and hold back the salts before they are deposited on the surface;
• waterproof cement renders finished with a facing board;
• waterproof drainage membranes;
• building a free-standing inner lining wall.

The 2003 Buildwise guide gives a similar list (studded sheets, panelling on a backing that resists moisture and salts, a new wall in front of the salt-laden masonry, mineral renovation plasters, waterproof coatings) and stresses one condition: this protection only makes sense after effective treatment of the rising damp and before the interior finish.

Can the salts be removed from the wall?

The 2003 guide states that, to the best of its authors' knowledge, there was at that time no effective, safe and economical technique for removing nitrates accumulated in masonry. The subject has been studied again since, but we cannot say that it has been settled. In 2024 Buildwise published a video that presents renovation plasters and waterproof protection as ways of limiting the impact of salts, along with desalination techniques (poultices, electrophoresis), with an emphasis on their advantages and their limits. Its DeSalt research project (2023-2024) states that the most effective and most energy-efficient way to dry out buildings is a matter of debate, and that solutions to salt problems often treat only the symptoms or are not effective enough. We have not had access to the details of these findings: if someone offers you desalination, ask which study it is based on.

Redoing the finish at the right time

Buildwise is clear about timing: do not plaster, render, paint or apply a water repellent until the substrate is dry enough, and recommends determining the salt content before finishing. If you finish too early, it becomes harder to spot hygroscopic salts in time, and the risk of damage to the new finishes is high.

To analyse the salts, Homegrade and Buildwise mention test strips, used by professionals, which reveal the most common salts and indicate how much is present. The 2003 guide acknowledges that at the time there was practically no information on acceptable concentrations, and that, from practical experience, a content below 0.5% by mass would rarely seem to cause damage. It also cites, as a guide, Austrian values from 1995 that are much lower for individual salts (for nitrates: below 0.05% no action, from 0.05 to 0.15% to be assessed case by case, above 0.15% action essential). So do not draw conclusions from any single threshold, and have the analysis interpreted by a professional.

Saltpetre and works: planning rules and renting

Facade: check before rendering or changing the facing

In Brussels, a planning permit is the rule and an exemption is the exception: the principle, the exemptions and the procedure are explained in our guide Planning permits in Brussels. Where saltpetre is concerned, Homegrade's advice page on works exempt from a permit (updated on 10-08-2026) picks out two cases:

• changing the facing material or the cement rendering of a facade that is not visible from the public space (for example, rendering a rear facade) is exempt from a permit, provided the facade is not in the protection zone of a listed building ("classé") or of a building in the process of being listed;
• for a facade on the street or visible from the public space, the page does not provide for this exemption. In a remark on insulating a visible party wall, which it describes as "open to interpretation", it states that a permit "could be required", in particular because changing the facing material of a facade visible from the public space "is not exempt from a planning permit" (our translation from the French). It also places external insulation that changes the facing materials of a street facade among the cases where differences of interpretation remain and where it advises confirming with the planning department.

Homegrade advises asking your municipality's planning department whenever there is any doubt. This page covers non-protected buildings and does not explicitly mention interior plastering (it only lists, in general terms, renovating a room among the interior works that are exempt). These exemptions do not apply if the works depart from the RRU (regional planning regulations), a municipal regulation or a subdivision permit (permis de lotir). If your house is protected, close to a protected building or in a protection zone, the answer may be different: see what heritage status changes for your works. In Uccle (Ukkel in Dutch), Forest, Ixelles or Watermael-Boitsfort, it is your municipality's planning department that decides: call them before you order anything.

Renting

The Homegrade brochure states that homes let to tenants must meet minimum standards of safety, health and equipment; if they do not, the tenant can file a complaint for non-compliance with the Direction de l'Inspection Régionale du Logement (DIRL, the regional housing inspectorate). It also points out that the tenant must heat and air the home and inform the owner of any damage. For a specific case, contact the DIRL or a legal advice service: this guide does not give legal advice.

How to move forward

1. Photograph and record: height of the deposits, which way the wall faces, the room concerned, when the deposits appeared, recent works (new screed, new windows, external render, insulation).
2. Look for the source: roof, gutters and cornices, facade pointing and render, pipes, ground moisture, ventilation and heating of the room. See the guide Damp in a Brussels house.
3. Do not cover it up: no cement render, waterproof paint or new wallpaper on a wall that has not dried.
4. Get a diagnosis if in doubt: Homegrade offers advisers and states that all its services are free; a professional can also measure the moisture and salt content.
5. Choose the treatment after drying: brushing for efflorescent salts, suitable protection (renovation plaster, inner lining wall, etc.) if the salts are hygroscopic or abundant.
6. Ask the municipality if the facade is involved.

For injections against rising damp, Homegrade recommends a specialist company, products with a technical approval (ATG) and compliance with Buildwise technical information note 252. Homegrade adds that these works are generally guaranteed for at least 20 years, and that the technique only works in a porous wall (brick or stone), not in concrete. Ask what the guarantee covers and what it excludes.

Frequently asked questions

Is saltpetre a health hazard?

Homegrade describes salt damage as generally not serious, but points out that it makes it impossible to keep an attractive finish and is likely to come back without treatment. The brochure links the health risk not to the salt deposit itself but to the damp: it states that damp, by encouraging mould and fungi, harms the occupants' health. If you have symptoms you think are linked to mould, talk to your doctor: the brochure states that CRIPI (Cellule régionale d'intervention en pollution intérieure, the regional indoor pollution unit) steps in at a doctor's request.

Can I just brush off the deposits and repaint?

According to Buildwise, that can be enough if only efflorescent salts are involved and the source of the damp has gone. If the source is still active, or if the wall contains hygroscopic salts, Homegrade warns that without treatment the damage is likely to come back. Painting too early, on a wall that is still damp, means the paint may deteriorate again.

How long should I wait before redoing the finishes?

No universal period is given: Buildwise states that evaporation takes several months in favourable conditions and can take several years for thick, poorly ventilated walls, and that the criterion is a moisture content usually between 3 and 5% by mass, measured properly. This benchmark dates from 2003: have it confirmed by a professional.

Does a renovation plaster solve the problem?

Homegrade presents it as a way of holding back the salts before they are deposited on the surface, permeable to vapour diffusion, but makes clear that the aim is to limit crystallisation at the surface, not to remove the salts. Buildwise, for its part, lists these plasters among the forms of protection to consider after effective treatment of the damp, and only if the wall is laden with salts. So they are no substitute for dealing with the source of the damp.

Do I need a permit to treat an outside wall?

For a facade not visible from the public space on a non-protected building, Homegrade states that changing the facing or the cement rendering (such as rendering a rear facade) is exempt from a permit. For a street facade, it refers you to the municipality's planning department. If in doubt, especially if the building is protected or close to a protected building, that department is where the check is made.

On 03-10-2026 the editorial team reviewed the Buildwise guide to masonry restoration (2003), the Buildwise online summaries (2024 video, Pathologies sheets, DeSalt project), the Homegrade brochure (2024), the Homegrade advice on permit exemptions (updated on 10-08-2026) and the Energie+ sheet on efflorescence; the 2003 Buildwise guide, the Homegrade advice on exemptions and the Energie+ sheet were reread on 19-10-2026. The Buildwise guide dates from 2003 and some Pathologies sheets are only available to subscribers: their full content was not consulted. Planning rules and technical practice change: check with your municipality's planning department and a qualified professional before deciding on any works.