A crack in a basement wall or a car park ceiling starts to weep after rain. Someone looks at it and says it just needs injecting. That may well be part of the answer. But injection seals a crack. It does not tell you where the water comes from, and it does not deal with why the crack is there.
This guide explains polyurethane grout injection in a basement. It covers the kind of leak the method suits, what it leaves untouched, and the questions worth having answered before work starts. HydroIQ investigates and reports. We do not inject cracks, and we do not do any repair work. Injection is work for a licensed contractor, such as our sister company S&S Remedial.
What is polyurethane grout injection?
It is a repair method, and a licensed contractor carries it out. A liquid resin is pumped under controlled pressure into concrete. It can go into cracks, into cold joints (the line where one pour of concrete meets a later pour) and into hollow spots inside the concrete. The resin goes in through small fittings called injection ports.
The resin expands when it meets water. It then sets into a flexible seal. Because the seal can bend a little, it can cope with small ongoing movement in the concrete.
The work is usually done from inside the basement. That is the dry side of the wall or slab. It can be done while the concrete is still damp.
Common places for it include:
- basement ceilings and walls
- cold joints in below-ground structures (a cold joint leak in a basement is one of the leaks the method is used on)
- gaps around pipes that pass through concrete
- retaining walls
- podium slabs (a raised outdoor slab, often with a car park below)
What kind of crack is it meant for?
Polyurethane injection suits a crack that is actively leaking and wet, with water pushing on it from the ground. That push is called hydrostatic pressure, which means the pressure of water held in the soil. If there is water coming through a basement wall crack below ground level, this points towards the kind of leak the method is meant for. You still need to know where the water is coming from.
Not every crack is like that. Two other kinds come up often:
- A dry crack that affects the strength of the structure. Polyurethane does not restore strength. A different injection, using a rigid epoxy resin (a resin that sets hard), is used for that. A wide structural crack may need a structural engineer to look at it.
- A fine hairline crack with no leak and no movement. This is usually treated as cosmetic and does not call for injection. If you are not sure which kind you have, have it looked at.
Polyurethane vs epoxy crack injection: what is the difference?
Polyurethane is the flexible, water-reactive option. It is aimed at stopping water. Epoxy is the rigid option. It is aimed at strength. Each one does a poor job of the other's work.
What does injection not deal with?
Injection seals the crack itself. It does not treat what caused the crack. Building movement and poor drainage are two common causes, and both are left as they were.
Work on the inside face of a basement wall or slab deals with moisture coming through it. It does not fix the drainage or the water pressure on the outside. Water pressing through a below-ground wall needs to be stopped on the outside, or the wall needs to be waterproofed from the inside as a designed system. Where water is coming through cracks under pressure, the cracks are usually injected before any inside membrane system (a waterproof layer) is applied. So injection is often one part of a wider repair, not the whole job.
Injection also does not address:
- water tracking down from a balcony, podium or planter box above
- a leaking water pipe, which is work for a licensed plumber
- persistent damp rising through a slab from the ground with no active leak, which is handled by a different repair
- condensation (water from the air forming on cool surfaces), which poor ventilation can add on top of water coming through the walls or floor
A basement can have more than one source of damp at once. On the Gold Coast and in coastal Brisbane, rising damp is common, so it is worth knowing which cause you have. Rising damp needs the path from the ground cut off, and crack injection is not that repair. Lateral damp (water in the soil pressing sideways through a wall) is another cause, and our guide Rising damp, lateral damp or condensation? How to tell them apart sets out the patterns in full.
What signs show water is moving through basement concrete?
Look for dark staining, damp patches and white salt deposits along cracks and joints, often in the ceiling. Rust-coloured staining around cracks and pipe penetrations (places where a pipe passes through a slab or wall) can also point towards water getting through.
The white deposit is called efflorescence. It is a powdery or crusty salt left behind when water has moved through concrete or masonry (brick, block or stone). It is a useful visual clue. But water can travel a long way inside concrete first. Where the deposit sits is not always where the water gets in.
For buyers, white powder and rust stains on basement cracks point towards water having moved through the concrete. They do not say where it came from or whether it is still happening. Our guide Musty smell in a basement or downstairs room: what it points to lists the other clues worth noting.
Basement crack injected still leaking? Some possible reasons
It is a known pattern that leaks sometimes carry on after earlier injection work, and that new cracking can appear. There is no single reason. Some possible reasons:
- The crack may have been sealed while its cause was not treated, so the concrete may have kept moving or cracking.
- The water may have been coming from somewhere else, such as a balcony, podium, planter or pipe, and found another way through.
- Water may have travelled inside the concrete, so the crack that was injected may not have been the one where it entered.
- Some leaking cracks may not have been found. The full extent is often only clear once work begins or during rain, so the scope of injection can change.
- There may have been more than one source of damp, such as rising damp or condensation as well as a leak.
Each of these is a reason to find the source and the cause before more work is done.
Basement car park ceiling leak: does injecting the cracks stop water from above?
With a basement car park ceiling leak, the water can be tracking down from a balcony, podium or planter box above. It comes through cracks and construction joints (joins built into a slab where one section of concrete was poured after another) in the slab below. Injecting the ceiling cracks seals the path at the bottom. It does not deal with how the water is getting in at the top. Our guide Musty smell in a basement or downstairs room: what it points to covers ceiling stains in more detail, and Balcony leaks: why the tiles aren't the problem explains how the areas above can fail.
Should I inject a leaking basement crack?
This page cannot answer that for your building. What it can do is give you the questions that need answers first. A contractor needs most of them before choosing a crack repair.
- Where does the water actually get in?
- Is it coming from the ground, from an area above, or from a pipe?
- Is the crack still moving?
- Is water there now, and is it under pressure?
- How wide, deep and long is the crack?
- Which side of the wall or slab can be reached?
- Is there rust staining or exposed steel?
- Is the aim waterproofing, strength or looks?
- Is injection one part of a wider, designed waterproofing system?
- Is there rising damp, lateral damp or condensation as well?
Some of these, such as whether a crack is still moving or whether steel is affected, are for the contractor or a structural engineer to judge. The first questions, about where the water comes from, are what an investigation is for.
How is the source of the water found?
HydroIQ starts with the history of the leak and a visual inspection. It then uses thermal imaging (a camera that shows surface temperature), moisture mapping (readings on a grid compared with a known dry area), pressure testing (checking whether a pipe holds its pressure, including plumbing under a slab), dye testing (adding coloured dye to water so its path can be traced) and controlled water testing (flooding an area above one section at a time while the underside is watched). No one method names the cause on its own, so they are used together. Our methods are non-destructive. If a small inspection opening is ever the only way to confirm a finding, we discuss it with the owner first and keep it to the minimum. Our guide How a waterproofing investigation actually works goes through each step.
What should the report give the contractor?
A useful report sets out what was seen, what was measured and what was concluded, with photos and readings. It says where the water gets in and how far the damp has spread. It describes what needs fixing in terms a contractor can price and plan from. With that in hand, a contractor can say whether injection fits, and whether it is the whole job or one part of it.
Who injects the crack, and who finds the source?
- HydroIQ investigates and reports. An investigation is a paid service. We do not carry out repairs, which keeps our findings independent.
- Crack injection, membranes and any other repair work are for a licensed contractor, such as our sister company S&S Remedial.
- A leaking water pipe is work for a licensed plumber.
- A wide structural crack may need a structural engineer.
The National Construction Code (the national set of building rules) requires wet areas and external areas to be built so that water does not damage the building or make it unhealthy. The code changes, so check the current edition.
If the building is newer, timing matters. Under Queensland Building and Construction Commission (QBCC) rules, much waterproofing is treated as a non-structural defect, and the time to complain is measured in months. Check the current limit straight away, because the rules change. Our guide The builder won't fix a leak: your options in Queensland explains the process.
How do I arrange a basement leak investigation?
If you have a leaking basement crack, or water on a car park ceiling, and someone has suggested injection, get in touch with HydroIQ to arrange an investigation (a paid service). We will look for where the water comes from and write it up in a report you can hand to the contractor of your choice.
Common questions
General information for South East Queensland home owners, not advice about your building or a legal matter. Standards and time limits change; check the current edition or rule before relying on a figure here.