Efflorescence on brick and fretting mortar: what salt means

A white powder on the face of a brick wall and a mortar joint crumbling to sand look like two different problems. They are usually the same one: salt, carried in by water and left behind when the water evaporates. What decides whether it is cosmetic or serious is where in the wall the salt crystallises — on the surface, or inside the pores. This page uses the 2008 technical guide Salt attack and rising damp, written for the Heritage Council of NSW, Heritage Victoria, the South Australian heritage department and Adelaide City Council.

Efflorescence and subflorescence: the same salt, two places

That is why the same wall can show both: a powder where the surface stays damp and crumbling where it dries fast (D. Young, Salt attack and rising damp: a guide to salt damp in historic and older buildings (Heritage Council of NSW, Heritage Victoria, SA Department for Environment and Heritage, Adelaide City Council, November 2008), s 6). The white powder is the visible part. The damage, when there is damage, happens behind the face.

Where the salt comes from

The guide lists the sources of salt in walls: saline soils and groundwater, sea spray near the coast, airborne salts even inland, air pollutants, inorganic garden fertilisers, pigeon droppings and leaking sewers, salt in the brick, stone or mortar sand itself, salty water used in making them, and cleaning compounds (D. Young, Salt attack and rising damp: a guide to salt damp in historic and older buildings (Heritage Council of NSW, Heritage Victoria, SA Department for Environment and Heritage, Adelaide City Council, November 2008), s 6.1). Sodium chloride, sodium sulphate and calcium sulphate are the common culprits, and sodium sulphate is particularly damaging. A high level of nitrate salts can point to a leaking sewer.

For salt attack to happen, the guide says four things must all be present: permeable masonry, available moisture, available soluble salts and evaporation (D. Young, Salt attack and rising damp: a guide to salt damp in historic and older buildings (Heritage Council of NSW, Heritage Victoria, SA Department for Environment and Heritage, Adelaide City Council, November 2008), s 9.1). None can be eliminated completely, but the one owners have most control over is moisture — gutters, downpipes, drainage, garden watering, and a damp-proof course that is working and not bridged.

New brickwork or an old wall?

New brickwork. A white bloom on a newly built wall is the case the guide describes as relatively harmless, and the guide lists salt naturally present in the brick, stone or mortar sand among the sources. Brush it off dry and watch whether it keeps coming back.

An older wall. Salt that keeps returning near the base of an old wall, especially with damp patches, bubbling paint inside, or crumbling joints, says moisture is still moving through the masonry. That is a damp question first: rising damp from the ground, falling damp from a gutter or downpipe, or penetrating damp from soil or paving against the wall. Rising damp covers how to tell which and what an inspection should check.

Fretting: when the mortar starts to go

Fretting is the grain-by-grain loosening the guide attributes to salt crystallising inside the pores, and soft mortars are the first to go. Consumer Affairs Victoria’s checklist for buyers says fretting - where the mortar between the brickwork falls out - can indicate major structural problems (Consumer Affairs Victoria, Inspect properties before you buy).

In a traditional brick wall with lime mortar, the mortar wearing first is partly by design: it is weaker than the bricks and takes the salt attack instead of them. That buys time but not indefinitely. The guide’s decay curve for a hundred-year-old house shows a long period of little decay while salt builds up in the pores, then rapid acceleration — in only ten more years the decay will be twice as bad as it is now (D. Young, Salt attack and rising damp: a guide to salt damp in historic and older buildings (Heritage Council of NSW, Heritage Victoria, SA Department for Environment and Heritage, Adelaide City Council, November 2008), s 9.4). Its conclusion is that procrastination is not an option: enough mortar loss risks local collapse of the brickwork, while putting mortar back and taking salt out resets the clock.

Testing for salt yourself

The guide gives a do-it-yourself method. Drill samples from known depths in the wall — 0 to 10, 10 to 20 and 20 to 40 mm are commonly tested — crush them lightly, and: “Weigh out 5 grams of each sample and add to 50 ml of deionised water.” Measure the electrical conductivity of the solution, convert it to parts per million, multiply by 10 for the dilution and divide by 10,000 to get a percentage (D. Young, Salt attack and rising damp: a guide to salt damp in historic and older buildings (Heritage Council of NSW, Heritage Victoria, SA Department for Environment and Heritage, Adelaide City Council, November 2008), Box 4). The guide’s rule of thumb is that more than about 0.5% salt by weight is cause for concern and a reason to consider removing it. It suggests the NSW Department of Primary Industries’ ‘Salt Bag’ agricultural kit as a convenient source of the meter and calibration solution.

What to do, and what not to do

• Remove surface salt dry. The guide recommends an industrial vacuum with a brush head, because it captures the salt rather than returning it to the soil. Washing it down risks dissolving it back into the wall or the soil.
• Fix the moisture. Gutters, downpipes, drainage, sprinklers against walls, and any bridge over the damp-proof course.
• Repoint in lime, not cement. Hard cement mortar in old brickwork drives the damp higher and moves the salt attack into the bricks; the guide’s sacrificial approach uses deliberately weak mortars and renders.
• Don’t seal the bricks. The guide says water-repellent coatings have led to many failures, trapping moisture and salt behind them.

Where a lot of salt is in the wall, the guide’s repair sequence adds poultices and desalination, and only later a new damp-proof course; How to fix rising damp sets that out with the published prices. No published Australian price for repointing could be read from its source for this page.

Buying or insuring. A pre-purchase building report should flag significant defects such as rising damp, but the NSW Government says an inspector would not normally check concealed damp-proofing (NSW Government, Pre-purchase inspection reports when buying a home). Salt attack develops over years, and standard home cover takes wear and tear back out (Insurance Contracts Regulations 2017 (Cth) reg 19(2)(b)), so treat it as maintenance rather than a claim.

Common questions