Crack filling is the step most homeowners rush through — or skip entirely — before applying driveway sealer. It's also the step that determines whether your sealing job holds up through Ontario winters or starts failing within a season. Getting crack filling right is straightforward once you understand the types of cracks you're dealing with and the right products for each.
Why Asphalt Cracks Form in Ontario
Understanding why cracks form helps you assess their severity and choose the right repair approach. Ontario's climate is particularly harsh on asphalt for a specific set of interconnected reasons.
Thermal Expansion and Contraction
Asphalt expands in summer heat and contracts in winter cold. This expansion and contraction places the asphalt surface under repeated tension — and asphalt, like any material, has a fatigue limit. Over years of cycling, the surface develops tension cracks, usually at right angles to the direction of traffic or in patterns following the stress points in the sub-base beneath.
Freeze-Thaw Infiltration
Once a hairline crack forms, water enters. When that water freezes, it expands by approximately 9%, forcing the crack wider. When temperatures rise again, more water enters the now-wider crack. Ontario's 30–80 annual freeze-thaw cycles mean this process repeats relentlessly, turning hairline cracks into structural failures over a few seasons if left unaddressed.
Sub-Base Movement
Sometimes cracks indicate movement or failure in the compacted aggregate base beneath the asphalt rather than fatigue in the asphalt itself. These cracks tend to be wider, more irregular in shape, and sometimes accompanied by slight height differences (one side higher than the other) across the crack — a sign of differential settling.
Pro Tip: Take photos of cracks before filling them. If the same cracks reappear the following season in the same location with greater width, this may indicate ongoing sub-base movement that crack filler alone won't solve — a professional assessment may be needed.
Understanding Crack Types and What They Mean
Not all cracks are created equal, and diagnosing the type correctly prevents you from applying the wrong solution.
| Crack Type | Width | Appearance | Cause |
|---|---|---|---|
| Hairline / Shrinkage | Under 3mm | Fine lines, often in patterns | Normal aging, thermal cycling |
| Medium / Structural | 3–25mm | Visible gaps, may follow traffic patterns | Freeze-thaw progression, fatigue |
| Edge Cracks | Varies | Along driveway perimeter | Lack of lateral support, tree roots |
| Alligator / Fatigue | Network pattern | Interconnected cracks resembling scales | Base failure, overloading |
When to Fill Cracks vs. When to Replace
Crack filling is the right solution for isolated, non-structural cracks. It is not a remedy for base failure or widespread deterioration. Knowing the difference prevents expensive mistakes.
Fill These Cracks
- Hairline cracks (under 3mm) with no height differential across the crack
- Medium cracks (3–25mm) with no height differential and isolated locations
- Edge cracks at the perimeter of the driveway (once any root issues are addressed)
- Cracks that have been stable in width for one or more seasons
Consider Professional Assessment (May Need Patching or Replacement)
- Alligator cracking over any significant area — this indicates base failure that crack filler cannot resolve
- Cracks with height differential across them (one side higher than the other), indicating differential settling
- Cracks wider than 25mm or deep enough that you can see the sub-base below
- Any crack that was filled last season and has reappeared at the same location wider than before
"Homeowners sometimes call us after spending money on crack filler and sealer when what they really needed was a section replacement. Filling over alligator cracking or cracks with a height difference is money wasted — those cracks will be back within a season."
— David, D&D Home Services Co-Founder
Crack Filler Products: What to Use
Several types of crack filler are available for DIY use. Each has appropriate applications and limitations.
Cold-Pour / Liquid Crack Filler (for cracks under 12mm)
Cold-pour crack fillers come in jugs or squeeze bottles and are ready to use without heating. They're asphalt-based liquids that you pour directly into the crack. They're easy to use, inexpensive, and effective for smaller cracks. They cure by evaporation, so warm, dry weather is important. Allow 24 hours to cure before sealing over them.
Trowelable / Paste Crack Filler (for cracks 6–25mm)
For wider cracks, a trowelable filler — thicker in consistency than cold-pour — provides better structural fill. Applied with a trowel or putty knife, it can be leveled flush with the surrounding surface. It's more work than cold-pour but produces a neater finish. These products also need 24–48 hours to cure before sealing.
Hot-Pour Crack Filler (professional-grade)
Hot-pour rubberized crack filler is the professional standard for durable crack repair. Heated in a specialized melter unit until liquid, it's poured into cracks and bonds extremely well to the asphalt walls. When cured, it remains flexible through temperature changes rather than becoming brittle. This is what professional contractors use and why professional crack filling typically outlasts DIY repairs. Equipment cost makes it impractical for homeowners.
Backer Rod (for wide cracks, over 12mm)
For cracks wider than 12mm, it's not practical or cost-effective to fill the full depth with filler. Instead, a foam backer rod — a cylindrical foam rope available at hardware stores — is inserted into the crack to reduce the fill depth before applying filler on top. This saves material and actually produces a stronger repair by controlling where the filler bonds.
Step-by-Step Crack Filling Process
Follow this sequence for the best results when filling cracks before sealing.
- Clean the cracks thoroughly. Use a stiff wire brush to remove loose material, vegetation, and debris from inside the crack. A leaf blower or compressed air is excellent for clearing dust after brushing. Any material left in the crack prevents filler adhesion to the crack walls.
- Remove any vegetation. Weeds or grass in cracks must be removed — roots and all where possible. Applying filler over live vegetation is pointless; the plant will grow through it. A targeted weed killer applied a few days before, or manual removal with a crack cleaning tool, works best.
- Ensure the crack is dry. Filler does not adhere to wet crack walls. Wait until after at least 24 hours of dry weather to proceed. A heat gun can be used to dry stubborn damp cracks on cool days.
- Insert backer rod for wide cracks. For cracks over 12mm wide, push backer rod into the crack to within 6–10mm of the surface. This forms the base for your filler layer.
- Apply filler. For cold-pour, pour slowly into the crack from a low height to prevent bubbles. For trowelable filler, apply with a margin trowel and work it into the crack. Fill slightly overfull — the filler will settle as it cures.
- Tool flush to surface. Once the filler has settled slightly (typically 5–15 minutes), use a trowel or squeegee to level the excess flush with the surrounding surface. A smooth, level fill is important for the sealer to coat evenly over the repair.
- Allow 24 hours minimum cure before sealing. Most cold-pour fillers need at least 24 hours at temperatures above 10°C before sealer is applied over them. In cooler temperatures, extend to 48 hours.
Pro Tip: Overfill cracks slightly — about 10% higher than the surrounding surface. Cold-pour filler settles and shrinks as it cures. If you fill exactly level, you'll end up with a slight depression once cured. A slight overfill becomes flush after the shrinkage.
When Professional Crack Filling Makes Sense
DIY crack filling with consumer cold-pour products is effective and appropriate for minor to moderate cracking. But there are scenarios where professional crack filling is the better investment.
- Extensive cracking: If you have more than a few isolated cracks, the time and material cost of DIY crack filling begins to approach the cost of professional service — with inferior product quality.
- Wanting hot-pour durability: If you want the crack repair to last as long as the seal coat rather than needing refilling next season, hot-pour professional repairs are significantly more durable than consumer cold-pour products.
- Uncertain about severity: If you're unsure whether cracks indicate base failure or can be repaired with filler, a professional assessment before spending money on DIY products is wise.
- Combining with sealing: When D&D Home Services performs a professional driveway sealing job, crack assessment and filling is included as part of the complete service — ensuring consistent, professional-grade prep before the sealer goes down.
Key Takeaways
- ✓ Ontario freeze-thaw cycles turn hairline cracks into structural problems if not addressed annually.
- ✓ Identify crack type first: hairline and medium cracks can be filled; alligator cracking indicates base failure.
- ✓ Clean cracks thoroughly before filling — vegetation, debris, and moisture prevent filler adhesion.
- ✓ Use backer rod for cracks over 12mm wide to control fill depth and save material.
- ✓ Overfill slightly — cold-pour filler settles as it cures, so fill a little high and level after.
- ✓ Wait 24 hours after crack filling before applying sealer; 48 hours in cool temperatures.
Need professional crack filling and sealing in Kitchener, Waterloo, Cambridge, or Guelph? D&D Home Services handles the complete job — from crack assessment through sealing — with commercial-grade products. Get a free quote today.
