Rising damp and salt damp: causes, diagnosis and repair

Rising damp is ground water travelling up a masonry wall, and it often brings salt with it. A damp-proof course is the barrier meant to stop it, built into the wall a few courses above the ground. The heritage guidance says rising damp is often caused not by a missing damp-proof course but by one that has been bypassed — and the fix starts with finding the bypass, not with drilling the wall.

What it is, and how it differs from other damp

NSW Health describes rising damp as ground moisture rising up a brick or stone wall, made worse by poor sub-floor ventilation or moisture under the floor (NSW Health, Mould factsheet). 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, sets out the first diagnostic step as working out which kind of damp it is:

By itself, the guide says, rising damp causes wet walls and musty smells but limited decay; it is when salt is present in the soil that it combines with rising damp to cause substantial decay of brick, stone and mortar. Damp brick walls are one of the signs Consumer Affairs Victoria tells buyers to look for, as an indicator of rising damp or salt damp (Consumer Affairs Victoria, Inspect properties before you buy).

The usual cause: a bridged damp-proof course

The heritage guide is direct: rising damp is often caused by bridging the damp-proof course — a moisture pathway around it (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 7.2). The bridges it lists are ordinary things done to houses over the years:

• Garden beds and paving built up against the wall. The guide calls this a common cause, and notes garden beds bring both water and salt from fertiliser.
• Render or plaster carried down over the DPC, inside or out.
• A new concrete floor or path poured against the DPC without vertical damp-proofing.
• Pointing over the face of the DPC, or a DPC that never crossed the whole wall thickness.

For new masonry, the ABCB Housing Provisions require the damp-proof course to be at least 150 mm above the adjacent ground level, or 75 mm above paving that slopes away from the wall, continuous, and visible from the outside (ABCB Housing Provisions Standard 2025, 5.7.4(2)). The heritage guide, written for older buildings, says a DPC needs to remain about 200 mm above ground or paving level to be effective. So the first check costs nothing: walk around the house, find the DPC line, and measure down to the soil or paving. Where the ground has crept up to it, that is the likely cause. While you are there, check the weep holes are clear, which NSW Health and the QBCC both ask owners to do.

Why a moisture meter reading is not a diagnosis

For anyone being quoted for a damp treatment, this is the point to know. Hand-held moisture meters measure electrical properties, and salt changes those as much as water does. So meters cannot distinguish between a relatively dry but salty wall and a wet one, and readings above 100% are common in salty walls. The guide’s conclusion: “Moisture meters should never be used as the sole basis for diagnosing a damp problem.” (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 11.2).

That does not make meters useless. The guide says they are good for mapping the extent of a damp patch, for checking high on a wall for another source such as a leaking downpipe, and for watching change over time with the same meter. What they cannot do on their own is prove a wall needs a chemical damp course. Ask any contractor who recommends one what else they checked: the ground level against the DPC, the drainage, the downpipes, and whether the reading pattern looks like rising, falling or penetrating damp.

Repair, in the right order

1. Remove the bridge. Lower the garden bed or paving below the DPC, cut back render that crosses it, and make sure ground and paving fall away from the wall.
2. Fix the water. The guide’s first “do” is to go out in heavy rain and check gutters and downpipes for leaks and overflows, and the base of the building for water lying against walls. The QBCC tells owners on reactive clay to repair leaks in plumbing, drainage and stormwater immediately (QBCC, A guide to preventing structural damage).
3. Let it dry, and watch. Meters are useful here, used the same way over time.
4. Only then, a new DPC if needed. The guide says chemical impregnation is now the most frequently used treatment for remedial dampcoursing in Australia, and that older walls, which in Australia will almost certainly contain salt, need salt removal or sacrificial treatments as well for a new DPC to succeed (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 16.3).

And what not to do, from the same guide: don’t use hard cement mortar to repoint failed lime mortar joints, which drives the damp further up; don’t seal the wall with water-repellent coatings; and don’t render the base of the wall in hard cement, which it calls a very common but poor treatment — the damp rises behind the render and comes out above it (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 2.3). No published Australian price for rising damp treatment could be read from its source for this page.

Buying, renting and insurance

Buying. The NSW Government says a pre-purchase report will include significant defects such as rising damp, but a building inspector would not normally check concealed damp-proofing (NSW Government, Pre-purchase inspection reports when buying a home). If a report or your own look finds damp low on brick walls, a specialist’s opinion before exchange is cheaper than after.

Renting. In NSW, premises must have floors, walls and ceilings not subject to significant dampness (Residential Tenancies Act 2010 (NSW) s 52(1B)(b)); South Australia’s minimum standard requires premises reasonably free from mould or other irritants, structural … damp (Housing Improvement Regulations 2017 (SA) reg 15(j)). Rising damp is a building problem, so it is the landlord’s to fix. Mould in a rental property covers each state.

Insurance. Rising damp is gradual, and standard home cover takes wear and tear, rust and corrosion back out (Insurance Contracts Regulations 2017 (Cth) reg 19(2)(b)); it is not one of the listed events. Where a sudden event — a burst pipe under the floor, say — has wet a wall, that is a different claim. Subsidence and cracked walls covers the other way water in the ground damages a house.

Common questions