Crack chasing (also called routing) is the step that decides whether a crack repair actually lasts or reappears through the new surface within a year or two. It’s a simple idea: rather than filling a crack exactly as it is, a narrow saw or grinding tool opens it into a wider, defined groove first, giving the repair material something proper to grip and enough volume to actually do its job. Skip this step, or rush it, and even an expensive flexible filler is fighting a losing battle from day one.
This guide is a deeper look at the technique itself, why it matters more than most homeowners assume, and how it fits into the wider preparation process before any resurfacing or coating goes down.
What crack chasing actually is
A crack in concrete is rarely a clean, uniform gap. It tapers, wanders, has debris and dust packed into it, and its actual width at the surface often has little to do with how wide it is a few millimetres down. Trying to fill a crack in that state, even with a good product, means the filler bonds to a thin, contaminated, irregular surface rather than to clean, sound concrete.
Crack chasing addresses this directly. A crack chasing tool, typically a diamond blade mounted on an angle grinder, or for larger jobs a dedicated crack router, is run along the length of the crack to cut it open into a consistent V-shaped or U-shaped groove, generally somewhere in the range of 6-12 mm wide and a similar depth, depending on the crack and the product going in afterwards. This does three things at once: it removes the weak, contaminated material right at the crack edges, it creates a defined, consistent channel rather than an unpredictable hairline, and it gives whatever filler goes in next enough volume and surface area to bond properly and flex with the crack rather than just sitting on top of it.
Why this step gets skipped, and why that’s a problem
On a straightforward resurfacing job, it’s tempting to treat crack filling as a quick pass: squeeze some filler along the visible line, smooth it off, move on to the overlay. That approach can look fine on the day. The problem shows up later, usually within 6-18 months, when the same crack reappears, sometimes as a hairline through the new overlay, sometimes as the filler itself popping back out.
There are a few reasons an unchased crack fails faster:
- Poor bond area. A crack filled at the surface only, without opening it up, gives the product very little surface area to key into. It’s essentially glued to the top of a gap rather than bonded into it.
- Trapped contamination. Dust, loose material and old sealer sitting inside an unopened crack prevent the filler from making contact with sound concrete at all.
- No room to flex. Cracks that are still subject to any seasonal thermal movement need a filler with enough cross-section to stretch and compress without tearing away from the sides. A hairline-thin bead of filler simply doesn’t have that capacity, however flexible the product itself claims to be.
- Uneven crack width. Where a crack narrows and widens along its length, an unchased fill will be too thin in the narrow sections regardless of how generously it’s applied at the wider points.
Our guide on hairline cracks: seal at the surface or inject covers the broader decision between surface sealing and full-depth injection; crack chasing is the preparation step that makes either of those approaches actually work as intended, rather than the corner that gets cut to save time.
The crack chasing process, step by step
- Assess the crack first. Before chasing anything, the crack needs to be identified as dormant or active, since that assessment changes what filler goes in afterwards, not the chasing step itself. See our guide on hairline cracks for how that assessment is done.
- Chase the crack open. A diamond blade or crack router cuts a consistent groove along the full length of the crack, generally into a V or U profile, wide and deep enough for the chosen filler to have a meaningful cross-section.
- Clean thoroughly. Compressed air and, ideally, vacuuming remove all dust and loose material from the freshly cut groove. Any residual dust left in the channel compromises the bond just as badly as skipping the chasing step altogether.
- Install backer rod where needed. For wider cracks, typically over 3 mm, a foam backer rod is pressed into the base of the groove before filling. This controls the depth of the sealant, ensures it forms the correct hourglass or dogbone shape for maximum flexibility, and stops filler being wasted deep in the crack where it does nothing useful.
- Apply the filler. A flexible polyurethane filler for cracks that may still see some movement, or a rigid epoxy filler for genuinely dormant, structural bonding situations. This is tooled flush with the surrounding surface.
- Allow proper cure time before any overlay, coating or sealer goes over the top. Rushing this step traps uncured product under a new surface and causes its own set of problems.
How crack chasing fits into the wider preparation sequence
Crack chasing doesn’t happen in isolation. On a professional resurfacing or coating job, it sits within the broader preparation sequence: assessing the slab, cleaning and degreasing, mechanical grinding of the whole surface, then crack chasing and filling, then moisture testing where relevant, then priming. Our complete guide to preparing concrete before resurfacing walks through that full sequence in detail. Grinding the broader surface first is actually part of why crack chasing works properly: grinding often reveals fine cracks that weren’t visible on a dirty, unground slab, so profiling before filling, not after, catches defects that would otherwise be missed.
Moisture is also part of this picture, particularly for slabs headed toward an epoxy coating rather than an outdoor overlay. A crack that’s been properly chased and filled but sits on a slab with elevated moisture vapour emission can still see the coating around it fail for entirely separate reasons. Our guide to moisture testing before epoxy explains why that check happens alongside, not instead of, proper crack preparation.
Does every crack need chasing?
Not necessarily. Very fine hairline cracks, under roughly 0.3 mm and confirmed dormant, can sometimes be bridged adequately by a quality primer and overlay system without individual chasing and filling. Once a crack is wide enough to see clearly and run a fingernail across, or shows any sign of ongoing movement, chasing before filling is the standard, reliable approach rather than an optional upgrade. On a driveway or path with numerous cracks of mixed severity, an inspection sorts out which ones genuinely need chasing and which will be adequately handled by the overlay system itself.
Crack chasing FAQs
Is crack chasing the same as crack injection?
No. Crack chasing opens a crack at the surface into a defined groove for filling. Injection is a different, deeper technique that pumps epoxy or polyurethane through drilled ports along the crack’s full depth, generally reserved for structural cracks or where water is moving all the way through a slab. Chasing is standard prep before surface filling; injection is a separate, more involved repair for a different category of crack.
Will a chased and filled crack be invisible after resurfacing?
A properly chased, filled and levelled crack under a new overlay is generally not visible as a raised or sunken line, though very slight colour variation directly over the repair is sometimes noticeable depending on the finish and lighting. The structural benefit, a repair that actually holds, matters more than any minor cosmetic difference.
How wide does a crack need to be before it needs chasing rather than just filling?
There’s no single hard number, but as a practical guide, cracks under about 0.3 mm can often be bridged by a good primer and overlay alone. Anything wider, particularly if you can feel it with a fingernail, benefits from chasing to create a proper bonding channel before filling.
Can I chase and fill a crack myself before resurfacing?
It’s possible for a very minor, clearly dormant crack with the right tools, but assessing whether a crack is genuinely dormant, cutting a consistent profile without damaging surrounding concrete, and choosing the correct filler for the situation are all easy to get wrong without experience. On any crack of real significance, or where the wrong call risks the whole resurfacing job, having it assessed and treated as part of a professional preparation sequence is the safer route.
Does crack chasing cost extra on top of a standard resurfacing quote?
Standard hairline crack filling is typically included in a resurfacing quote as part of normal preparation. More extensive chasing across numerous or wider cracks is usually itemised separately, since the labour involved scales with how much cracking a slab actually has. This gets assessed and quoted transparently at inspection rather than buried in a headline price.
Wondering whether the cracks in your driveway or slab need proper chasing before anything else happens? Send a few photos through our get a free quote form and we’ll give you an honest read on what’s needed before any resurfacing or coating work begins.