How to fix rising damp: every method compared, with costs

It is tempting to start with the treatment. The most thorough Australian guidance starts somewhere else. 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, answers a reader asking which damp-proofing treatment to use with two words: “Wrong question.” Its method is a sequence of seven steps, and a new damp-proof course is step six. This page goes through them in order, with what each costs where a price has been published.

The seven steps, in order

The guide is explicit that the decision about inserting a damp-proof course, and what form(s) that should take, are decisions for later in the process (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 10). Not every step is needed: if step one finds a broken downpipe and step two fixes it, there may be nothing left to do. Its warning about who you ask matters as much as the order: many damp-proofing contractors specialise in one treatment method only, so seek independent advice (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 2.1 and s 11.1).

Step 1: diagnosis first

Everything after this depends on knowing which damp it is — rising, falling from a gutter or downpipe, penetrating from soil against the wall, or condensation — and whether an existing damp-proof course has been bridged. Moisture meters map the damp but salt distorts them, so they are never proof on their own, and a salt test shows how much of the job is salt removal. Rising damp sets out what a proper inspection covers and who should do it. One Brisbane referral service publishes an on-site assessment and written quote at $450 + GST for up to two hours; ask whether the drainage and bridging checks are included, in writing.

Step 2: good housekeeping — often the cheapest fix

This step can make the rest unnecessary, and much of it costs nothing:

• Unbridge the damp-proof course. Lower garden beds and paving so the DPC is exposed; cut back render that runs over it. The building code requires a new DPC to sit at least 150 mm above the adjacent ground level (ABCB Housing Provisions Standard 2025, 5.7.4(2)); the guide suggests 250 mm or more where rain splashes off hard paving.
• Fix the water. Gutters, downpipes, stormwater drains, leaking taps against the wall. The guide’s advice is to go out in heavy rain and look.
• Ventilate under the floor. Clear blocked vents and weep holes; NSW Health makes the same point about sub-floor ventilation (NSW Health, Mould factsheet). A published Sydney price list puts a subfloor ventilation system at $600 to $2,500.
• Don’t seal the ground. One of the worst mistakes made by renovators is to remove a ventilated timber floor and replace it with a concrete slab poured on sand or other fill. (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 12.4) The slab stops the ground evaporating, so the moisture goes up the walls instead; the guide says the same is why external paving should be permeable.

Steps 3 to 5: manage the salt, then stop and look

Sacrificial treatment. Deliberately weak lime mortars and renders take the salt attack instead of the bricks. Combined with good housekeeping, the guide says they may make mild salt damp a manageable problem without the need for the expensive insertion of a DPC (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 13). What the guide calls the wrong response is repointing in hard cement mortar, which drives the damp higher and makes the bricks decay instead of the joints.

Removing salt. Surface salt is vacuumed off with an industrial vacuum and brush head, which captures it rather than washing it back into the soil. Deeper salt is drawn out with poultices of absorbent clay or diatomaceous earth, left on for about 2–3 weeks until dry, or by captive-head washing (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 14). The Sydney price list publishes salt-retardant replastering at $80 to $150 per linear metre.

Review. The guide puts a deliberate pause here: has the damp stress dropped enough that the wall is relatively dry and decay is minimal? If so, periodic desalination and renewal of sacrificial mortar may be enough, and it avoids the more expensive and invasive work that follows.

Step 6: the four ways to put in a new damp-proof course

The guide calls chemical impregnation the most frequently used remedial treatment in Australia, and notes that it will not stop water under pressure, so it cannot be used where there is a hydrostatic head, such as tanking a cellar (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). The published per-metre prices for it differ widely: $55 to $160 (stated as typical by a Brisbane contractor), $150 to $400 (a Sydney contractor) and about $200 to $400 (a Brisbane referral service). Ask what is included: the injection alone, or plaster removal, salt treatment and replastering as well.

Step 7: desalinate, because a new DPC is not the end

For older walls, which the guide says in Australia will almost certainly contain salt, a new damp-proof course leaves the salt already in the wall above it. That salt keeps attacking plaster and paint as the wall dries, which is why the guide treats desalination as a step of its own and says slot sawing in particular must be combined with it 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 16.2). Where interior plaster is replaced, the guide says to use lime or lime-cement plaster where walls may stay damp, not gypsum, which it notes is itself a slightly soluble salt.

The treatments to avoid

The guide’s list of old treatments ends bluntly: “These methods should no longer be considered.” (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 24). They are:

• Hard cement render along the base of walls. It stops evaporation, so the damp rises behind it and comes out above it.
• Damp-proof mortar additives as the only damp-proofing.
• Knapen tubes (atmospheric syphons). Tests showed an empty hole is just as effective at drying the wall around it.
• Passive electro-osmosis — copper strips earthed to the ground. The guide says there is no scientific basis to it, and attributes most claimed successes to other work done at the same time, such as fixing gutters and drainage.

The guide’s dos and don’ts add one more: don’t seal walls with water-repellent coatings. Rising damp and salt damp covers the causes, the bridging checks and the full table of published prices.

Common questions