Concrete Crack Repair: When to Fill, Inject or Call an Engineer

Fine shrinkage cracks can often be left alone, but a crack that widens, lengthens, offsets or keeps recurring needs a structural engineer before any coating or overlay goes on. Epoxy injection suits non-structural cracks that have stopped moving. Most guidance here comes from the United States, so confirm with a local professional.

Hairline or structural?

The width rule of thumb

A structural engineer quoted by a US publication says a web of hairline cracks, or widely spaced cracks narrower than about 1/8 inch (roughly 3 mm), usually means the concrete shrank as it cured. Unless they grow wider or leak, those can usually be left alone. Cracks 1/8 inch or wider deserve a closer look, and the same publication says to consider a professional once a crack is wider than 1/4 inch (about 6 mm).

Warning signs from the sources

• One side of the crack sits higher than the other (an offset).
• Closely spaced cracks, or repeated cracking in the same places.
• Cracks in beams, columns, retaining walls or other load-bearing parts.
• A crack that widens, lengthens or shows movement over time.
• Water seeping through the crack.
• Doors or windows that no longer close properly, which can signal a structural shift.

What an Australian inspector says

An Australian inspection company lists likely causes of slab cracking as ground movement, foundation settlement, thermal expansion and contraction, or drying shrinkage. It notes shrinkage cracking is common in newly poured slabs and is often considered cosmetic or something to monitor. It advises a structural engineer if the crack widens, lengthens or shows signs of significant movement, and a qualified concreter for further advice.

Common crack types

Hairline and crazing

Hairline cracks are thin and superficial, usually under 1/8 inch. Crazing appears as a spiderweb pattern and is caused by concrete drying before it has fully cured. Neither is usually structural, though the US publication says to fix them early to stop water getting in.

Shrinkage and control joints

Concrete shrinks as it cures. Control joints, cut about an inch deep, are meant to channel those cracks into straight lines where they do no harm. A crack that follows a control joint is behaving as designed.

Settlement and structural cracks

Settlement cracks come from ground shifting or settling unevenly beneath the slab. Structural cracks are wider and deeper and may point to tree roots or an unstable foundation. These are the ones to refer to an engineer, because filling them treats the symptom.

Repair methods

Concrete caulk

For mild hairline cracks on horizontal surfaces that do not bear significant weight. It is applied with a caulk gun and smoothed. It keeps water and debris out but does not restore strength.

Epoxy filler and two-part epoxy

Epoxy filler suits small to mid-size cracks. A US contractor quoted in the publication recommends two-part epoxy as extremely durable, with compressive strength greater than the concrete itself.

Injection ports

For large or deep cracks the publication describes injecting multi-part epoxy through ports fixed along the crack: clean the crack, set the ports over nails about a foot apart, seal the surface with epoxy about 1/8 inch thick, let the sealer cure for 6 to 10 hours, inject from the lowest port until epoxy appears at the next, then let it cure for 5 days before cutting off the port necks. Doing this properly is professional work for anything that matters.

When epoxy injection works

The conditions

Epoxy injection is generally suited to non-structural or moderate cracks. Before using it, the cause of the crack should be identified and no more movement expected. Done properly, two-part injected epoxy is reported to restore the concrete's strength, while a flexible sealant only keeps water and debris out.

When it should not be used

It should not be used where the concrete is still moving, or on cracks caused by rusting steel, because the steel keeps corroding and new cracks form. Water matters too: the publication says to check that the crack is dry first, and that if dampness returns after drying, water is still seeping in and the cause needs fixing before the repair.

Before you coat or resurface

A moving crack comes back

A crack that moves will reflect through a coating or overlay. Resurfacing and coatings are surface fixes: see concrete driveway resurfacing vs replacing and epoxy moisture barrier. If a crack leaks, the slab may also be wet: see concrete slab moisture testing.

Fill and stabilise first

A coatings manufacturer says cracks and defects should be filled and stabilised before epoxy goes on. A coating job that skips this tends to show the crack within a season.

Prevention

The US publication lists sealing concrete every 2 to 3 years against moisture, fixing drainage and grading so water runs away from the slab, installing expansion and control joints, using reinforced concrete in high-stress areas, and keeping roots away. These are general US recommendations rather than NSW standards.

When to call someone

An engineer

Call a structural engineer for a crack that is wider than 1/4 inch, keeps growing, is offset, sits in a load-bearing element or leaks. We found no Australian standard or engineering-body guidance in our research, so do not rely on this page for a structural decision.

A concreter

A qualified concreter can advise on filling, injecting and resurfacing once the cause is known. Ask which licence they hold: see which flooring work needs a licence.

Find contractors

NSW Flooring lists businesses by service and region, from Google Maps, retrieved 22 October 2026. Examples for concrete repair and resurfacing: concrete resurfacing western sydney, concrete resurfacing north shore & ryde and concrete resurfacing penrith & blue mountains, or browse concrete resurfacing contractors in NSW. Read the directory's methodology and polished concrete vs grind and seal.

Quick answers

Can I just fill a crack and coat over it?

Only if it is a stable, non-structural crack. A moving crack will come back through the coating.

Who decides if it is structural?

A structural engineer, in the sources we read.

Is a hairline crack a defect?

An Australian inspection company classes hairline slab cracking as a low-severity defect that is often cosmetic or needs monitoring. It is still worth sealing in wet areas.

How wide is too wide?

The US sources use about 1/8 inch as the point to look closer and 1/4 inch as the point to consider a professional. These are rules of thumb, not Australian standards.