Paint bubbling and render blowing off near the floor

Tap it. That is the first test and it costs nothing

Run a knuckle or the edge of a coin along the render, starting at the skirting boards and working upward in a grid. Sound render on a sound substrate gives a dull, solid thud. Render that has debonded gives a hollow, drummy ring, and once you have heard the difference you cannot unhear it.

Mark the drummy zone with low-tack tape and note where it stops. Two things about that boundary are diagnostic. It is usually roughly level, which puts it in the same family as the tide mark on the wall next door. And when you get inside the drummy area later, you will very often find salt already crystallised at the interface between render and masonry, which is what broke the bond in the first place. Expect much of it to be partially detached before anyone touches it.

One boundary is worth checking specifically. Drummy or delaminating render below the damp-proof course line, together with efflorescence below the course, external paint blistering below the course, damp carpet edges and rust staining on metal near the slab edge, is the signature set for slab edge dampness rather than damp rising through brickwork. That is a drainage, detailing and concrete-quality problem, and injecting brickwork does not touch it.

What is actually pushing the coating off

Water rising through masonry carries dissolved salt with it. The water evaporates; the salt cannot, so it concentrates and stays. Over years, a wall moving roughly 0.88 litres per day per metre of length, around 320 litres per year per metre, delivers a great deal of salt into the same band. Ten metres of affected wall moves on the order of 3,000 litres a year, and every litre leaves its load behind.

Where the salt crystallises decides whether it is ugly or destructive. If evaporation happens at the surface, crystals grow in the open air as a white bloom. That is efflorescence, and on its own it is cosmetic. If evaporation is faster, the drying front sits inside the wall and salt crystallises within the pores. That is sub-florescence, and it is what makes bricks fret, sandstone shed its hardened skin, and mortar turn to sand you can rub out with a thumb.

A coating forces the second case. Put a low-permeability film over a wall that is still moving moisture, and the evaporation front retreats behind it. Salt then crystallises at the coating interface, and the coating comes off. Some salts make it considerably worse: sodium sulfate cycles between anhydrous thenardite and the ten-water hydrate mirabilite with humidity and temperature, and the hydrate occupies far more volume than the anhydrous form. That volume change is capable of splitting sound bricks, not merely blistering paint.

Seal the face and the damp does not stop, it climbs

The steady height of rise varies as one over the square root of the evaporation rate. Reduce evaporation and the wall has to find more wetted surface area to shed the same inflow, so the front rises. This is not an opinion about coatings. It falls directly out of the model published in Proceedings of the Royal Society A in 2007.

Hall and Hoff worked the case explicitly. A 215 mm wall with effective sorptivity 0.3 mm per root minute and an evaporation rate of 1 by 10 to the minus 4 mm per minute sits at a steady 695 mm. Render it to 1.25 metres and cut the evaporation rate tenfold, and the damp re-stabilises well above the top of the render. Because the adjustment is slow, they calculate roughly 425 days for the wet front to climb and reappear above the render coat.

That lag is the entire commercial story of hard cement render, damp-proof paint and waterproof coatings on damp walls. They are sold, they appear to work for a year or two, and then they fail. The customer, quite reasonably, concludes the render was not thick enough or did not go high enough, and buys more of it. The Australian heritage guide's instruction is not hedged: do not even think about sealing walls with water-repellent coatings.

There is a second cost. As a wall warms it exhales air from its pores and as it cools it inhales. If the incoming air is humid and the masonry drops below dew point, water condenses inside the pores, and hygroscopic salts capture and hold it. Any coating that stops a wall breathing shortens its life independently of what is rising underneath it.

Why a damp seal paint fails, specifically

A coating sold to seal moisture out of a wall does three things to a wall that is already damp, and all three are bad. It reduces the evaporation rate, which raises the height of rise. It traps salt and moisture behind the film, where crystallisation forces the film off. And on case-hardened sandstone it can take the stone's naturally toughened outer surface with it when it delaminates, exposing material underneath that is often surprisingly weak.

Where a coating is unavoidable, breathability is the governing property and the ranking is settled. Acrylic water-based paints are more vapour-permeable than alkyd oil-based paints, and oil-based paints should not be used on walls that remain damp or are still drying. Even acrylics can be too impermeable for old masonry. Cement-based paints are more permeable again, and limewash is the most permeable of all.

This is the exact inverse of the instinct. On a wall that might stay damp, you want the most breathable finish available, not the least.

When bubbling paint at the floor is not rising damp

Blistering low on a wall is a symptom with a long suspect list, and rising damp is not usually the one that did it. The cheap answers are also the common ones, so work through these before anyone quotes on a damp course.

Never gypsum plaster on a wall that might stay damp

Gypsum plaster is calcium sulfate. It is itself a slightly soluble salt, so any residual moisture triggers salt attack inside the new plaster and the fresh work fails from within, often inside a year. On any wall that may remain damp, the specification is lime or lime-cement plaster, with low-alkali cements where added salts need minimising.

There are two very different philosophies sold under the heading of remedial render, and they get conflated constantly. A sacrificial system is a deliberately weak, porous lime-and-sand mix with porous particulates included to give salt somewhere to go. It draws salt out of the valuable brick or stone into a layer that is cheap to renew, and it is meant to decay. That is the point, not a defect. It is the traditional and conservation-preferred approach and it is one of the seven key steps in the Australian guide.

A salt-retardant or waterproof-additive render is a rich cement-sand coat intended to keep salt and moisture away from new paint. The heritage guide is critical of the strongly hydrophobic end of that range: it prevents evaporation from any continuing rising damp and can be used as a belt-and-braces cover for an inadequate damp course, and in twenty or thirty years the damp will have risen above the hard render and will break out again. Less strongly hydrophobic salt retarders let the wall breathe and dry, and the guide calls them at least more honest, because a failed damp course becomes apparent quickly instead of being hidden for decades.

One more trap on the mortar side. Do not repoint failed lime joints in hard cement. It drives the damp further up the wall and can damage the bricks. And be aware that extensive repointing after a chemical damp course has been installed can bridge the treated zone, so that course may need re-treating once the new mortar has cured.

When you can actually repaint

Months, not weeks. After replastering, thorough drying of both construction water and residual deep dampness typically takes three to six months, and up to twelve months for wet thick walls in cooler damp climates. The widely quoted trade rule of roughly one month per 25 mm of wall thickness comes from BRE Digest 163 and was originally framed around drying construction moisture from masonry under reasonable conditions. Treat it as an order-of-magnitude expectation, not a specification. A salt-loaded wall recovering from years of rising damp is frequently slower.

Do not wallpaper for at least twelve months, longer on thick walls, and leave several weeks after replastering before decorating, with good natural ventilation throughout.

Resist the urge to force it. Drying is limited by how fast water can travel through the wall to the surface, so beyond a point extra dehumidification buys very little. More importantly, increasing the drying rate increases the rate of salt crystallisation inside the masonry, and therefore the rate of decay. Steady moderate ventilation beats aggressive forced drying. For the same reason, keep air-conditioning outdoor units away from vulnerable old walls: warm air blown at masonry concentrates salt damage directly behind the unit.

Finally, do not use a meter as your acceptance test. A salt-contaminated wall may never read dry no matter how long you wait, because hygroscopic salts hold atmospheric moisture indefinitely. Acceptance should be judged on the free-water content of drilled samples, which means total moisture content measured by oven drying, minus hygroscopic moisture content measured at 75 per cent relative humidity.

The order that actually holds

1. Find the source and cut itGutters, downpipes, flashings and stormwater first. Then ground and paving levels against the damp-proof course, garden beds pulled back to a gravel strip at least 300 mm wide, drippers 500 mm off the wall, and the first metre graded to fall about 25 mm away. Then plumbing, wet areas and condensate lines. Then subfloor ventilation.
2. Take the salt-loaded render and plaster offAssess how much original material was lost before it was applied, then break it into small pieces by careful hammering to limit damage to the substrate. Expect much of it to be partially detached already, with salt crystallised at the interface. Brush, vacuum or lightly chisel the surface salt off. In a pre-1990 building, asbestos and lead paint must be identified and managed before this starts.
3. Test before anyone quotes a treatmentA vertical profile of drilled samples, oven-dry moisture content, hygroscopic moisture content at 75 per cent relative humidity, and salt percentage and species. High hygroscopic content with negligible free water means the source is already dead and the remaining problem is salt.
4. Desalinate where salt exceeds about 0.5 per centDry vacuum, captive-head washing, then absorbent clay or paper-pulp poultices applied wet so salt crystallises in the poultice rather than the wall. Iterative: re-sample nearby and re-treat until salt drops. Start this before any damp course goes in, because injection can displace saline moisture upward into masonry that was previously undamaged.
5. Install a damp course only if drilled samples prove live rising dampNot because a meter read high, not because the house is old, and not because there is a mark on the wall. This is step six of seven in the Australian guide's sequence, and the industry's habit of selling it to people who needed step one is the reason this page exists.
6. Replaster in a compatible systemLime or lime-cement, never gypsum where dampness may continue. Sacrificial mixes with porous particulates where salt remains, matched in stiffness to the wall, because hard brittle cement render on a soft flexible wall fails regardless of the damp.
7. Dry, then decorate in the most breathable finish you canThree to six months, up to twelve on wet thick walls. Limewash or a mineral finish ahead of acrylic, and no oil-based paint at all on a wall that is still drying. Bubbling paint on an early repaint is close to guaranteed.

Render replacement is masonry work. We do not do it.

This matters on this page more than on any other, because most of what a blown render job needs is not what we sell.

S&S Remedial are waterproofing and concrete repair specialists. We are not builders. We perform exactly two things: a chemical damp-proof course injected into the mortar bed of external brickwork, and Hydropoxy negative treatment to internal concrete slabs, where the slab is ground back, the surface prepared, and an epoxy moisture barrier applied to the top of the slab before new flooring goes down.

We do not remove or replace render. We do not install salt-retardant or sacrificial render systems. We do not plaster, repoint, poultice, cut physical damp courses, underset, do landscaping or drainage works, do plumbing, install subfloor ventilation, or waterproof wet areas.

So for a wall with blown render and bubbling paint, the trades you will need are a renderer or plasterer for the removal and reinstatement, a heritage lime plasterer if the building is old or listed, a landscaper or drainage contractor for ground levels and falls, a roof plumber for gutters and stormwater, and a QBCC-licensed waterproofer for any wet area involved. If a chemical damp course on external brickwork turns out to be part of the answer, that part is ours. If it is not, we will tell you, and we do that often.

What it costs

Questions we actually get asked

My render sounds hollow at the bottom of the wall. Does it all have to come off?

If it is drummy, it has already failed as a bond and it is also acting as a salt reservoir and a moisture trap, so in most salt-affected cases it comes off. Removal runs roughly $40 to $120 per square metre, more if asbestos or lead paint is present, which changes the cost category entirely in a pre-1990 building. Two things are worth knowing before you book it. Map the drummy area by tapping first, because it is usually larger than the stain. And on heritage fabric, assess how much original material remains before removal, because breaking it up carelessly damages the substrate underneath.

Can I just scrape the bubbled paint back and repaint it?

You can, and it will fail again, usually faster than the first time. The paint is not the problem. Salt crystallising behind the film is levering it off, and scraping removes the evidence without touching the cause. If you repaint before the wall has dried, bubbling is close to guaranteed. If the wall is salt-loaded and still cycling with the humidity, the bloom returns after every repaint regardless of how long you wait, which is a sign the remaining problem is salt rather than water.

Does rendering a wall cause damp?

Rendering does not create damp, but rendering a wall that already has damp reliably makes it worse and moves it higher. Render reduces the evaporation rate, and the steady height of rise varies inversely with the square root of that rate, so the front climbs until it finds enough wetted surface area to shed the same inflow. On the modelled 215 mm wall, rendering to 1.25 metres and cutting evaporation tenfold pushes the front above the render, taking roughly 425 days to reappear. Hard cement render also traps salt at the interface, which is what eventually blows it off.

The render is blowing but only below the damp-proof course line. What does that mean?

On slab-on-ground construction that pattern points at slab edge dampness rather than damp rising through brickwork. The signature set is persistent dampness on the exposed slab or footing edge, efflorescence below the damp-proof course, drummy or delaminating render below the course, external paint blistering below the course, damp carpet edges, tile bond failure near external walls and rust staining on nearby metal. It very often follows landscaping added after construction that buried the slab edge. The fix is drainage, ground levels and slab-edge detailing, and an under-slab membrane cannot be retrofitted once the slab is poured.

Is there a paint that will hold on a damp wall?

Not one that seals it. On a wall that is damp, drying or carrying residual salt, the governing property is breathability and you want the most permeable finish available, not the least. Limewash is the most permeable, then cement-based paints, then acrylics. Oil-based alkyd paints should not go on a wall that remains damp or is still drying. Anything marketed on the basis that it seals moisture out will seal moisture in, push the damp higher and eventually be forced off by salt crystallising behind it.

How long after the source is fixed before I can repaint?

Plan for three to six months, and up to twelve months on wet thick walls. The old trade rule of roughly a month per 25 mm of wall thickness comes from BRE Digest 163 and was framed around construction moisture, so treat it as an order-of-magnitude guide rather than a promise. Leave several weeks after replastering before decorating, with good natural ventilation, and do not wallpaper for at least twelve months. Judge readiness on the free-water content of drilled samples, not on a meter, because a salt-loaded wall may never read dry.

Should I run a dehumidifier and heaters to speed it up?

Ventilate well and be patient rather than aggressive. Drying is limited by how fast water can move through the wall to the surface, so beyond a point extra dehumidification achieves little. More importantly, increasing the drying rate increases the rate of salt crystallisation inside the masonry, and therefore the rate of decay. The Australian heritage guide notes that adding heating or air-conditioning increases the drying rate and so increases the associated decay. Steady moderate ventilation is the better trade-off, and outdoor air-conditioning units should be kept away from vulnerable old walls.