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.

• 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 we see, 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 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.

• 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 is measured properly

The current method for assessing moisture in a concrete slab before installing floor coverings is in-situ relative humidity probe testing to ASTM F2170. 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 that gives you is the internal relative humidity of the slab, which is the thing that actually governs whether a floor covering or its adhesive will fail. AS 1884:2021 makes the in-situ probe the normative test.

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 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 needs slab age, evidence of what was installed underneath, repeat testing across seasons, and depth profiling.

Why the meter on the surface tells you almost nothing

A two-pin moisture meter measures electrical conductivity between two pins. 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. Soluble salts are electrolytes, so a dry but salty surface conducts as well as, or better than, a genuinely wet clean one. On salt-loaded masonry it is common to get readings above 100% moisture content, which is a physically impossible figure because it leaves no room for the material itself. A capacitance meter is better behaved but reads a shallow and poorly defined depth, typically 10 to 30 mm, and is affected by density, surface finish, reinforcement and salts.

Both instruments are screening tools for finding where to look. Neither reads what is happening at 40% of slab depth, which is where the number that decides your flooring lives. A quote written on the strength of a surface reading is not a diagnosis.

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.

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 we do. The other is chemical damp-proof course injection into the mortar bed of external brickwork.

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. Test firstIn-situ RH probes to ASTM F2170, read after equilibration, together with slab age, whatever evidence exists of a membrane, and a walk around the outside looking at levels, falls and stormwater discharge. If the number and the site do not warrant a barrier, we say so.
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 are recorded on the job, not estimated afterwards.
5. Then the new floor goes downOnce the barrier has cured, the flooring installer lays the new covering over it. We do not lay flooring, and we do not select it. Give your installer the test results 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.
• 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 that are not us. We 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 the reason we test and look outside before we quote anything.

• 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 we 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.

Can you inject a concrete slab the way you inject brickwork?

No, and neither can anyone else. 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. If it was a surface meter, get an in-situ relative humidity test to ASTM F2170 before anything else, drilled to about 40% of slab thickness and left to equilibrate. That 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.

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 testing and the look outside come 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, and it needs drilled samples, oven-dry moisture content and hygroscopic moisture content before anyone quotes on it.

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. Test the slab, then read the limit the adhesive manufacturer actually publishes.