Damp coming up through a concrete slab

Why a slab is not a wall

Capillary damp needs three things at the same time: liquid water in contact with the base, a continuous absorbent path upward, and a surface where the water can evaporate. Remove any one of the three and the movement stops. In a masonry wall, the damp-proof course removes the middle one. A silane or siloxane is delivered into a continuous mortar joint, roughly every 120 mm, and diffuses to form a water-repellent zone through the wall thickness that capillary moisture cannot climb past.

A slab has none of that geometry. The absorbent path is the entire plan area of the concrete, and the evaporating surface is the whole floor. There is no horizontal joint at 150 mm above ground to work into, and drilling a grid of holes through a 100 mm slab achieves nothing except a grid of holes. The barrier for a slab exists in exactly one place, and it is installed once, before the concrete is poured.

That leaves two ends of the problem you can genuinely work on: the water arriving underneath and at the slab edge, and what happens at the top surface before a floor covering goes over it. Everything honest on this page lives at one of those two ends.

Where slab moisture actually comes from

There are five causes worth knowing, and they are not equally likely. Two of them are construction defects you cannot undo, one of them fixes itself with time, and two of them were created after the house was finished, usually by a weekend and a trailer load of soil.

Where you are matters as much as which of the five you have. On the Gold Coast and through coastal South East Queensland the ground works against a slab harder than it does inland: shallow water tables through the low-lying canal and reclaimed estates, saline groundwater with marine aerosol adding chloride on top of it, and subtropical rain that arrives faster than 1970s site drainage was ever built to shed. None of that invents a sixth cause. It just means the five below bite more often here, and it is why so many local floors end up with salt on them.

• No under-slab membrane, or a damaged one. Membranes get punctured by reo chairs, star pickets and boots during construction, or stop short at the edge beams. The Housing Provisions require the membrane to underlie the entire bottom surface of the slab, extend under internal and edge beams to finish at ground level, lap not less than 200 mm at all joints, and be fully sealed where punctured. Miss any one of those and there is a path.
• Permeable or under-cured concrete. A slab that was poured wet, finished badly or cured poorly is more porous than it should be, and moves more water for the same conditions underneath. You cannot tell this by looking at it.
• Ground water sitting against the slab edge. The site does not shed water. AS 2870 and the NCC require a minimum fall of 50 mm over the first metre away from the building, with the low point of the grading 1.5 to 2.0 m out. Where that does not happen, every storm parks water against the exposed edge of the concrete.
• Post-construction landscaping. New garden beds, raised lawn, paving and concrete paths built up against the house bury the slab edge, hold water against it and block the drainage path. This is the most common cause a licensed waterproofer sees on site, and it is also the cheapest to correct.
• Construction moisture in a young slab. A slab less than one to two years old is still giving up its mixing water. Readings decline steadily over successive tests. The treatment is time and ventilation, not a product.

Slab edge dampness, and the signs that travel with it

Slab edge dampness is moisture soaking into the exposed edge of the slab or footing and migrating inward. It is one of the ordinary findings on this coast, it has a recognisable signature set, and the single most useful thing about it is where the evidence sits relative to the damp course.

Damp that is worse below the damp course than above it is not rising damp climbing a wall. It is the slab edge. That one observation redirects a great many quotes, and it costs nothing to make.

• Persistent dampness on the exposed slab or footing edge, wet long after the rest of the wall has dried
• White efflorescence below the damp course
• Drummy or delaminating render below the damp course
• External paint blistering below the damp course
• Damp carpet edges against external walls, and pungent odours in still rooms
• Watermarks on skirting boards
• Tile bond failure concentrated near external walls
• Rust staining on metal fixings near the slab edge

How a slab gets assessed, and how it gets measured

There are two different questions here, and different people answer them. The first is what is putting water into the slab, which is a site question: ground levels, falls, garden beds, stormwater discharge, the age of the slab, whatever is known about what went in underneath, and where the damp shows up relative to the damp course. That is what a licensed waterproofer look at on the site visit, with a moisture meter used where it helps, and for most houses it is the question that decides where the money should actually go.

The second question is a number, and it only matters when a floor covering is about to go down. The method for that is in-situ relative humidity probe testing to ASTM F2170, which AS 1884:2021 makes the normative test. Sleeved holes are drilled to a specified depth, typically 40% of slab thickness where the slab dries from one side only, capped, and left to equilibrate before reading. What it gives you is the internal relative humidity of the slab, which is the thing that actually governs whether a covering or its adhesive will fail. It is commissioned through the flooring trade or a concrete moisture testing specialist as part of laying the floor. It is a flooring compliance test, not a damp diagnosis, and the two get confused constantly.

Acceptance is commonly stated as not exceeding 75% RH where no manufacturer's limit applies. Reporting on the exact figure in the 2021 edition is inconsistent, with some sources citing 75% and others a revised in-slab value of 80%, so confirm it against the standard itself and against the flooring manufacturer's own requirement, which may be stricter and which is the one that governs your product warranty.

The honest limit of the probe test: it tells you the slab is wet, not why. It cannot separate residual construction water in a young slab from ongoing ground moisture through a failed or missing membrane. That distinction comes from slab age, evidence of what was installed underneath, repeat testing across seasons, depth profiling, and somebody walking around the outside in the rain. A number without the site tells you half the story, and the site without a number will not satisfy a flooring warranty. They answer different halves of the same problem.

What a moisture meter can and cannot tell you on concrete

A moisture meter is a useful instrument in the hands of somebody who knows what it is doing, and it gets used on site where it helps. It is worth understanding what the reading is, because the number on the screen is not a moisture content.

A two-pin meter measures electrical conductivity between its pins, and it is calibrated for timber, where the relationship between conductivity and water content is well characterised. Concrete is not timber, and the meter has no idea it has been moved. It also reads salt as readily as it reads water, because soluble salts are electrolytes, so a dry but salty surface can conduct as well as a genuinely wet clean one. On salt-loaded masonry it is common to see readings above 100% moisture content, which is a physically impossible figure, because it leaves no room for the material itself. A capacitance or search-mode meter behaves better on concrete, but reads a shallow and poorly defined depth, typically 10 to 30 mm, and is affected by density, surface finish, reinforcement and salts.

So the meter is used for what it is genuinely good at: comparing one part of a floor or wall against another, following a wet area out to its edge, and deciding where to look harder. Read alongside ground levels, drainage, the height and shape of the damp and where the salt has banded, it is a real input into a real diagnosis. What it is not is the figure your flooring installer needs, because that figure lives at 40% of slab depth and no surface instrument reaches it.

What is supposed to be under there, and why it cannot be put back

The barrier for a slab-on-ground is the under-slab membrane, and it goes in once, before the pour. AS 2870 is the governing standard for residential slabs and footings, and it is the source of the branding you will see printed on the roll: AS 2870 Concrete underlay, 0.2 mm Medium impact resistance, or High impact resistance where a damp-proofing membrane rather than a vapour barrier is required. AS 2870 also recommends damp-proofing membranes under slabs in areas prone to rising damp and salt attack, which is a fair description of a good deal of this coastline.

There is a state variation worth knowing if you have moved here or are reading a specification written interstate. South Australia and New South Wales vary the NCC to require high impact resistance rather than the medium impact resistance accepted elsewhere, a direct response to salt damp risk in saline soils. Queensland accepts medium.

There is also a currency point. NCC 2025 was published in 2026, but adoption is jurisdiction-specific and Queensland deferred to 1 May 2027, so as at 2026 the operative edition here is still NCC 2022. Confirm the adopted edition for your project date before relying on any clause number.

None of which helps you if the slab is already down. You cannot retrofit a membrane under an existing slab. There is no product and no process that does it, and anyone claiming otherwise deserves a great deal of scrutiny. What you can do is reduce the water reaching the slab from outside, and manage the top surface before a new floor goes over it.

Hydropoxy: an epoxy moisture barrier on the top of the slab

This is one of exactly two things a licensed waterproofer does. If your slab turns out to need it, that is work for a licensed waterproofer — they are the ones who actually do the work. The other thing they do is chemical damp-proof course injection into the mortar bed of external brickwork. That is the whole list.

The barrier goes on the inside face of the slab, which is the side away from the water. In the trade that is called negative-side treatment, because you are treating the face the moisture reaches last rather than the face it reaches first. It does not dry the slab and it does not fix the cause. It manages moisture at the interface between the slab and the floor covering, so a covering can be laid over a slab whose internal relative humidity is higher than that covering would otherwise tolerate. That is the whole job, stated plainly, and it is worth understanding before you buy it.

1. Look at the site first — that inspection normally costs nothingA deep visual inspection inside and out, the ground levels, falls, garden beds and stormwater discharge, the condition of the slab edge, the age of the slab and whatever is known about what went in underneath, and a moisture meter where it helps. Where a flooring installer has already run in-situ RH probes, bring those numbers to the visit. If the site does not warrant a barrier, a competent contractor will say so rather than sell you one.
2. Grind back to sound concreteDiamond grinding removes old adhesive, curing compound, laitance, coatings and whatever else the surface has accumulated. The epoxy has to bond to concrete, not to the residue of the last four floors. This is dusty, silica-generating work and is done with on-tool extraction.
3. Prepare and make goodVacuum, patch, treat cracks and joints, and profile the surface so the membrane keys into it. Perimeter and penetration detailing is set out at this stage, because the edges and the pipe penetrations are where these systems are let down.
4. Apply the membraneA two-part epoxy moisture barrier is applied to the top of the slab at the coverage rate the product specifies, in coats, with the specified cure time between them. Coverage rate and coat count come off the product data sheet, not off how much is left in the tin.
5. Then the new floor goes downOnce the barrier has cured, the flooring installer lays the new covering over it. A licensed waterproofer does not lay flooring, and they do not select it. Give your installer any test results you have and the product data sheet.

When it is the right answer, and when it is not

A negative-side epoxy barrier is appropriate in a fairly narrow set of circumstances. It is worth being clear about both halves of that, because the situations where it is wrong are more common than the situations where it is right.

• Right: new flooring is going down over a slab that has tested elevated to ASTM F2170 and will not come down to the covering manufacturer's stated limit within a timeframe you can live with.
• Right: residual construction moisture in a slab you cannot afford to wait out, where the readings are declining but not fast enough.
• Right: a membrane that was never installed or never worked, where the external causes have already been addressed as far as they practically can be, and the remaining moisture is a flooring problem rather than a building problem.
• Wrong: where there is standing water or a head of water under or against the slab. A surface-applied barrier is not tanking, and hydrostatic pressure is a drainage and structural waterproofing question that has to be engineered as one.
• Wrong: where the cause is a live leak that has not been found. A cracked stormwater line, a slow supply leak, a shower membrane or an air-conditioning condensate line discharging under the house will keep feeding the slab through anything you put on top of it.
• Wrong: where the cause is a ground level or drainage problem that can be corrected for a fraction of the price. Correct it, wait, retest, then decide.
• Wrong: where the damp you are worried about is in the wall rather than the floor. A barrier on the slab does nothing about slab edge dampness showing up in the brickwork above it, and nothing about genuine rising damp above the damp course.
• Wrong: where nothing is going on top. This system lives under a floor covering. It is not a wearing surface and it is not a treatment for an exposed slab.
• Wrong: where the slab is settling, moving or structurally cracked. That is an engineer's question before it is a coatings question.

The work outside that usually matters more

In most slab cases the highest-value money is spent outdoors, by trades other than the waterproofer. They do not do landscaping, excavation, drainage, plumbing or concrete cutting, and you will get a better job for less from the people who do it every week. Here is what usually needs doing, roughly in order of value.

Correcting ground levels and paving typically runs $500 to $6,000, and site drainage works $1,500 to $12,000. Gutter, downpipe and stormwater repair runs $400 to $6,000. Against a five-figure flooring failure, that is cheap arithmetic, and it is why the site visit starts outside the house before anybody talks about a product.

• Grade the first metre to fall away from the building, minimum 50 mm over that metre under AS 2870 and the NCC, with the low point 1.5 to 2.0 m out.
• Get stormwater discharging well clear of the slab, and check the gutters and downpipes in heavy rain rather than in fine weather.
• Pull garden beds back and leave a clear strip at least 300 mm wide, preferably more than 500 mm, surfaced in coarse gravel. Gravel limits rain splash while still letting soil moisture evaporate to the sky instead of into the concrete.
• Replace sprinklers near the house with drippers kept at least 500 mm off the wall.
• Cut back paving, paths and landscaping that have buried the slab edge, so the edge can be seen and can dry.
• Then wait and retest. Slabs respond slowly, and a change judged at three months proves nothing either way.

What it costs

Questions that actually get asked

There is white powder on my concrete floor. What is it?

Efflorescence. Salt has been carried through the concrete by water and left behind on the surface when the water evaporated, because water evaporates and salt does not. In itself it is cosmetic. What matters is whether it comes back: a bloom that returns a few weeks after you remove it means water is still arriving and still evaporating, so there is a live source to find. Dry-brush or vacuum it off. Do not acid-clean it, because acid cleaning introduces chloride salts, and chloride is the one that never leaves. It goes wet again at about 75% relative humidity, which in South East Queensland is most of the year.

Is damp in a slab actually common here, or am I being sold something?

It is common here, and for reasons you can go outside and look at. Large parts of the Gold Coast and the coastal strip are low-lying canal and reclaimed estates with a water table close to the surface, the groundwater is saline, the sea air keeps adding chloride, the rain arrives in subtropical volumes, and a great deal of the older housing stock has had lawn, paving and garden beds built up against it over forty years. Slab edge dampness and genuine rising damp in the brickwork above are both ordinary findings in that environment rather than exotic ones. The thing worth being careful about is which of them you have, because the cure is different, and more often than you would expect the answer is a downpipe rather than a damp course.

Can a concrete slab be injected the way brickwork is?

No, and nobody can do it. Chemical damp-proof course injection works by delivering a silane or siloxane into a continuous mortar joint at roughly 120 mm spacings, so it can diffuse through the pore structure and form an unbroken water-repellent zone across the wall thickness. A slab has no mortar joint and no wall thickness to cross. The genuine options for a slab are reducing the water reaching it from outside, and treating the top surface before new flooring goes down.

My flooring installer says the slab tested too wet. What are my options?

First, ask which test they ran. A surface meter is a screening tool and does not read the depth that governs a flooring warranty; the in-situ relative humidity test to ASTM F2170 does, drilled to about 40% of slab thickness and left to equilibrate, and it is the method AS 1884:2021 points at. Then there are three routes: wait and retest, which is free and is the right answer for a young slab; find and fix whatever external cause is feeding it; or install a moisture barrier over the slab. Also check the stated limit for your specific covering, because it may be stricter than 75% RH and it is the one that governs your product warranty.

The house is under two years old and the slab reads high. Is that a defect?

Often it is just construction moisture. A slab less than one to two years old is still releasing its mixing water, and the readings should decline steadily across successive tests. Time and ventilation, not a product. If the readings are not declining, or the site drains toward the house, or you can see water standing against the slab edge after rain, that is a conversation to have with your builder while the statutory warranty period is still live. Start there, not with a coating. If it turns into a genuine dispute, that is the point where paying for an independent forensic investigation earns its keep — an investigation-only consultancy do that sort of investigation and do not do rectification work, so they have nothing riding on the answer.

Does an epoxy moisture barrier dry the slab out?

No, and it is not meant to. It manages moisture at the interface between the slab and the floor covering, so a covering can be laid over a slab reading higher than that covering tolerates. The slab stays as wet as it is. That is exactly why the look outside comes first. If moisture is arriving because of something fixable, the fixable thing should be fixed, and in a fair number of cases the barrier then stops being necessary at all.

Will treating the slab stop the damp in my walls?

No. Different element, different mechanism, different fix. Slab edge dampness shows up in the wall as efflorescence and drummy render below the damp course, and a barrier applied to the floor inside does nothing about it. Rising damp in masonry above the damp course is a separate diagnosis again: it is read from ground levels, drainage, the height and shape of the damp on the wall, and where the salt has banded, and an experienced inspector will tell you which of those is driving it. Drilled samples and laboratory analysis belong to the cases where the answer is going to be argued about — a building dispute, an insurer, or a wall where competent people disagree.

Do I need this if I am laying tiles rather than vinyl?

Possibly not. Ceramic tiles fixed with a cement-based adhesive tolerate slab moisture far better than vinyl, engineered timber or a glued carpet, which is why tiled floors survive on slabs that would destroy a vinyl. Tolerant is not the same as immune, though, and moisture arriving from below tends to concentrate at perimeters and cause bond failure near external walls. Get the slab tested, then read the limit the adhesive manufacturer actually publishes.