Crumbling bricks and mortar you can scrape out with a screwdriver

Why the mortar goes first

In a traditional lime-mortared wall the mortar is deliberately the weakest thing in it. Weak lime is more porous and more soluble than the bricks it beds, so the salt concentrates in the joints and destroys them in preference to the units around them. That is not a defect. It is the joint working as designed — a sacrificial layer that protects the brickwork by decaying instead of it.

Which is why fretting mortar is the first visible sign, and why grit at the base of the wall arrives before any brick damage does. If the joints are going and the brick faces are still sound, the wall is telling you the sacrificial system is still holding.

The corollary is the most expensive single mistake made on walls like this. If the mortar is dense and impermeable while the bricks are porous, the salt has nowhere to go except into the bricks. That is exactly what hard cement pointing does to a soft old wall — and it is also why sampling for salt analysis is normally taken from mortar in preference to face brick, unless the mortar is much less permeable than the units, in which case the salt has concentrated in the units and those are what must be sampled.

What is actually breaking the brick

Efflorescence — the white bloom you can brush off dry — is salt crystallising in open air on the face of the wall. Nothing is confined, so nothing is stressed. It is ugly and essentially harmless.

Sub-florescence, also called crypto-florescence, is the same salt crystallising inside the pore network, because the drying front has retreated into the material rather than sitting at its surface. The crystal now grows in a confined space, and the pressure that generates is enough to disrupt even sound, strong masonry.

What it does, in sequence: it lifts the fireskin — the harder, denser outer surface a brick develops during firing — off in flakes, after which the softer body underneath decays much faster. On sandstone it sheds the naturally case-hardened surface, exposing stone that can be surprisingly weak. In mortar it works grain by grain until the joint is loose sand.

Two things follow that are worth holding on to. A wall with no visible salt can be in worse condition than one covered in it, because on an exposed face rain washes the surface evidence away while the damage continues behind. And the species matters: sound bricks splitting and delaminating, rather than merely fretting at the surface, points at sodium sulfate, which swaps between anhydrous thenardite and ten-water mirabilite with humidity and temperature and generates its pressure through that volume change. A sulfate result is also a reason to look for a cement source, since Portland cement carries gypsum and alkali sulfates of its own.

The wet-dry cycle is the engine

A single crystallisation event does very little. The damage is cumulative and it scales with the number of cycles, because each cycle enlarges the pore slightly and the next one therefore does a little more.

Salts are hygroscopic. Above a critical relative humidity they pull water vapour out of the air and dissolve; below it they crystallise again. Every crossing of that threshold is one cycle. Which salt is in your wall decides how often that happens.

Why the wall can keep decaying after the damp is fixed

Mixtures of salts go wet at a lower critical humidity than either constituent alone, and real walls always contain mixtures — whatever groundwater, cement, fertiliser, sea spray, sewage and cleaning chemicals have delivered over a century. So the single-salt figures above understate how much of the year a real wall spends actively cycling.

That is also why fretting continues on walls where the damp problem was genuinely cured years ago. There is no liquid water arriving at all. The salt is harvesting humidity out of the air, dissolving on humid days, re-crystallising on dry ones, and doing incremental damage on every crossing. Walls also breathe: as masonry warms it exhales air from its pores and as it cools it inhales, so humid air is drawn in and, where hygroscopic salts are present, retained.

This is exactly the case that a moisture meter cannot see and that gravimetric testing settles. Total moisture content minus hygroscopic moisture content at 75% relative humidity gives free capillary water. High salt, negligible free water, and continuing decay means the answer is salt removal, not another damp course.

Why it concentrates below the damp course and just above the ground

Two separate things stack in that band, which is why the damage is so reliably found there.

First, salt is deposited where evaporation happens. Water rises through the wall carrying dissolved salt, evaporates at the face, and leaves the salt behind, concentrating it in a band at and just below the top of the wetted zone. In Australian exterior conditions the wetted zone typically tops out at 1.0 to 1.5 m, the active evaporative zone sits between about 0.5 and 1.2 m, and there is little evaporation below 0.3 m because the air near the ground is more humid and slower moving. That band is where the crystallisation load concentrates, and it is why the visible line usually sits slightly below the true damp front and survives long after a wall has been dried.

Second, on a wall that has a working damp-proof course, everything below that course sits outside the barrier by design. A DPC stops capillary water rising past its own line. It does nothing for the brickwork underneath it, which remains connected to the ground and goes on cycling. That zone will always need periodic attention on an old wall — a maintenance item rather than evidence of a failure.

Ground geometry decides how much of the wall is in that band. NCC Housing Provisions put a damp-proof course not less than 150 mm above adjacent ground, 75 mm above finished paving that slopes away, and 50 mm where protected by a carport or verandah. Where soil, mulch or paving has crept above that line, the barrier is bridged and the wetted zone has moved up with it.

One important lookalike. On slab-on-ground construction the same-looking damage below the damp course often reads differently: persistent damp on the exposed slab or footing edge, efflorescence and drummy render below the DPC, external paint blistering below the DPC, damp carpet edges, tile bond failure near external walls and rust staining on metal at the slab edge. That signature set is slab-edge dampness. It is a drainage, membrane-termination and concrete-quality problem, and injecting brickwork does not address it.

Eighty years of nothing, then a decade that matters

Salt accumulates for decades with almost no visible damage while it slowly fills the available pore space. Once the pores are loaded, decay accelerates sharply.

The notional curve in the Australian heritage guide runs roughly 80 years of near-nothing followed by rapid acceleration, such that a building 100 years old may be twice as damaged after only another 10. Which means the sentence people most often use about these walls — it has looked like that as long as we have owned it — is a poor basis for leaving it alone. The wall was in the flat part of the curve. Visible fretting is the signal that it no longer is.

The optimistic half of the same model is real and worth stating: reverse the decay, remove the cause and remove the salt, and you effectively reset the clock back down the curve. This is not a ratchet.

Grading what you have, before anyone quotes

Everything in this list is free, and doing it before you call anybody changes the conversation entirely.

1. Probe the joints and record depthsA screwdriver or blunt awl, light hand pressure only, at several heights and along the length of the wall. Write down how deep it goes at each point in millimetres. You are building a map, not proving a point — the pattern across the wall is more diagnostic than any single hole.
2. Measure the height of the affected bandMark the top of the damage and measure it off the ground. A continuous band under about 1.2 m with a fairly level upper edge is consistent with rising damp. Worst high on the wall, blotchy with no line, or following a pipe or a corner, is not.
3. Measure the ground against the damp courseFind the DPC — often a dark line in a mortar joint low in the wall — and measure down from it to soil, mulch and paving. If you cannot find it, dig a small inspection pit at the wall face. It is very often simply buried, which means the barrier is intact and something is bridging it.
4. Look up, in the rainGutters, downpipes, rainwater heads, flashings. Watch the wall during heavy rain. Fretting and salt appear where water evaporates, which may be several metres from where it entered.
5. Then, and only then, have the wall drilledSamples at known heights and depths — commonly 0-10, 10-20 and 20-40 mm — sealed immediately, weighed, oven-dried and reweighed for total moisture content, then re-equilibrated at 75% relative humidity for hygroscopic moisture content. Free water is what remains when you subtract one from the other. Salt content by weight decides whether desalination belongs in the scope, with about 0.5% as the working threshold. Record sample locations precisely so nearby re-samples can verify progress later.

What actually stops it

Salt attack requires four things at once: permeable masonry, available moisture, available soluble salts, and evaporation. Remove any one and decay stops. In practice none can be fully removed — you cannot make masonry impermeable without trapping moisture behind the coating, you cannot eliminate atmospheric moisture, salts are ubiquitous, and preventing evaporation entirely means keeping the wall permanently saturated. Every real remediation therefore reduces several factors rather than eliminating one, and the order decides whether it works.

• Roof plumbing and drainage first. Gutters, downpipes, rainwater heads, flashings, and stormwater discharged well clear of the building.
• Ground levels second. Re-expose the damp course — 150 to 250 mm of clearance is the remedial target, with 200 mm the ideal. Pull garden beds back to leave a sterile zone at least 300 mm wide, over 500 mm preferred, surfaced in coarse gravel so rain can soak in and soil moisture can still evaporate out. Grade the first metre to fall about 25 mm away with the low point 1.5 to 2.0 m out. Replace sprinklers with drippers kept at least 500 mm off the wall.
• Then wait and watch. The government guide is explicit: do not undertake insertion of any form of damp-proof course until the basic housekeeping measures have been completed and their effectiveness assessed over a period of at least a year. Time lag is real — a change made now can take one to three years to fully express itself.
• Remove salt-contaminated render and plaster where the analysis warrants it. Hard cement render and gypsum plaster on a salt-loaded wall are both a moisture trap and a salt reservoir, and much of it is usually already partly detached with salt crystallised at the interface.
• Only then a damp-proof course, and only if the testing still shows free capillary water rising.
• And desalination — poulticing, captive-head washing, sacrificial mortars — because no damp course removes the salt already above it. Desalination can and should begin before injection, which protects the wall above from the flush of saline moisture that injection can drive upward.

Repointing and brick replacement: the part we do not do

Badly fretted mortar has to be raked out and repointed. Badly fretted bricks have to be cut out and replaced, or turned, or dressed off and rebuilt. That is masonry work. We do not do it, we do not subcontract it, and we are not builders.

Stating that is not modesty. It is the same disclosure we would want from anyone quoting on the strength of their own diagnosis, ourselves included: the party who assesses how bad your joints are should not be the party whose income depends on the answer.

If you are engaging someone for the masonry, these are the points worth holding them to.

• Weak lime and sand, matched to the wall. Never hard cement into failed lime joints.
• Sacrificial by design. A weak lime mix with porous particulates gives the salt somewhere to go that is cheap to renew, and re-application on a maintenance cycle is the intended behaviour rather than a defect.
• Never gypsum plaster anywhere dampness may continue. Gypsum is calcium sulfate, itself a slightly soluble salt, so residual moisture triggers salt attack inside the new plaster — which is why so many freshly plastered walls fail within a year.
• Match the render stiffness to the wall. Hard brittle cement render on a soft flexible wall fails regardless of the damp, and strongly hydrophobic render coats simply prevent evaporation from any continuing rising damp, so the damp reappears above the render in twenty or thirty years.
• Sequence it against any chemical damp course. Extensive repointing can bridge a treated course, so the treated zone may need re-treatment once the new mortar has cured, using alkaline-stable dampcourse fluids.
• Where bricks are sound and only the mortar has failed, they can often be dressed off, desalinated in successive baths of fresh water without drying between baths — conductivity on the wash water shows when the salt has bottomed out — and rebuilt.
• If masonry replacement is the answer on a solid wall, the entire thickness has to be rebuilt. Partial undersetting is bad practice, because damp simply continues to rise through the untreated portion.

Spalling concrete is a different problem with a similar look

The vocabulary overlaps here and the treatments do not. Fretting is grain-by-grain loss of a masonry surface driven by salt crystallising just beneath it. Spalling on concrete — the face coming away in sheets or lumps, often with rust staining and sometimes exposed reinforcement — is usually concrete cancer: carbonation or chloride reaching the embedded steel, the steel corroding, and the corrosion product expanding and pushing the cover off.

Same appearance at ten paces, different mechanism, different scope. If what is failing is a concrete slab edge, balcony soffit, beam or column rather than brick and mortar, it should be assessed as concrete repair, and the diagnosis turns on cover depth, carbonation and chloride rather than on salt in a mortar joint.

Our part of it

We perform exactly two services. On external brickwork we inject a chemical damp-proof course into the mortar bed: a silane or siloxane delivered into holes drilled along a mortar joint at roughly 120 mm centres, 10 to 15 mm in diameter, to within about 30 mm of the far face, so the active diffuses through the pore structure and lines the pores with a water-repellent resin. It does not block the pores; the wall still breathes and still dries. Internally, we grind back and prepare concrete slabs and apply a Hydropoxy negative epoxy moisture barrier over the top of the slab before new flooring is laid.

Two limits matter specifically on a fretting wall. Injection should not be attempted where the mortar or masonry is weak and crumbling — if the course we would drill has already gone to sand, repointing has to happen first, by someone else, and cure before we can work. And on a 230 mm one-brick wall the header course should be drilled by preference; where a stretcher course is drilled instead, the stretchers on the other side must also be treated, either from the far face or by re-drilling the same holes in a second phase. Both of those cost money, which is why a rate well under about $100 per linear metre is worth asking hard questions about.

What we will not do is quote a damp course off a meter reading and a lap of the outside of the house. If nobody drilled the wall, nobody diagnosed anything — and on a fretting wall the more likely honest answer is that the roof plumbing, the ground levels and a year of monitoring come first, and that the masonry repair matters more than anything we sell.

What it costs

Questions we actually get asked

My mortar has turned to sand and I can scrape it out with a screwdriver. How serious is that?

Serious, and it does not stay stable. Salt-attack decay follows an accelerating curve: salts accumulate for decades with little visible damage while they fill the pore space, then decay speeds up sharply — the Australian guide's model has a building that has looked the same for 100 years being twice as damaged after another 10. Mortar loss of around 50 mm across 5 to 10 courses puts the brickwork at risk of local collapse. That warrants assessment on site now rather than a plan for next year. The encouraging half is that reversing the decay and removing the cause resets the clock back down the curve.

Why is it only happening at the bottom of the wall?

Because that is where evaporation concentrates the salt. Water rises through the masonry carrying dissolved salt, evaporates at the face, and leaves the salt in a band at and just below the top of the wetted zone. In Australian exterior conditions the active evaporative zone typically sits between about 0.5 and 1.2 m, with little evaporation below 0.3 m because the air near the ground is more humid and slower moving. On a wall with a damp course, everything below that course also sits outside the barrier by design and stays connected to the ground, so the two effects stack in the same band.

The bricks below the damp course are the worst. Doesn't that prove the damp course has failed?

No — that is the normal condition of the zone below a working damp course. A DPC stops capillary water rising past its own line and does nothing for the brickwork underneath, which remains in contact with ground moisture and keeps cycling. Damage below the line is a maintenance zone on any old wall. What would suggest a problem with the barrier is damage above it: a continuous band with a level upper edge climbing to a metre or more, or ground, mulch or paving that has crept above the DPC and bridged it.

I had a damp course injected two years ago and the wall is still crumbling. Was I ripped off?

Quite possibly not, and this is one of the most predictable outcomes in the trade — usually a scoping failure rather than a product failure. A chemical damp course stops water rising. It does nothing about the salt already in the wall above it, and those salts go on drawing moisture out of the air, dissolving in humid weather and re-crystallising in dry weather, damaging the masonry on every cycle. The Australian heritage guide illustrates brickwork continuing to decay above an injected damp course for exactly this reason. Before spending more, have the wall tested: if free water is now negligible and salt content is high, the damp course is doing its job and salt is the remaining problem.

Can I just render over it and be done?

Rendering makes it worse, and the reason is arithmetic rather than opinion. The height a damp front reaches varies inversely with the square root of the evaporation rate, so reducing evaporation makes the front climb until it finds enough wetted surface to shed the same inflow. Modelling of a 215 mm wall rendered to 1.25 m has the damp re-stabilising above the top of the render in roughly 425 days. That lag is precisely why the treatment looks like a success first and fails in year two or three, with salt crystallising at the render interface and taking the coating off.

What does brick fretting repair cost?

We cannot quote it, because repointing and brick replacement are masonry work we do not perform. What we can say is what drives it: linear metres of joint to be raked and repointed, how many units need cutting out and replacing, access and scaffold, mortar matching, and whether structural support is required where mortar loss is severe. Get that priced separately from any damp work, and get the damp diagnosis from someone who is not quoting the masonry, so that neither number is influencing the other.

Is fretting the same as spalling?

Not quite, and the difference points at a different trade. Fretting is grain-by-grain crumbling of a masonry surface caused by salt crystallising in the pores just beneath it, and it typically starts in the mortar. Spalling on concrete is the face coming away in sheets or lumps, usually because carbonation or chloride has reached the reinforcing steel and the corrosion product is expanding and pushing the cover off. Similar appearance, different mechanism. Brick and mortar decay is a salt and moisture question; failing concrete is a cover, carbonation and chloride question.