How do you seal a crack in a foundation?
Updated August 2026
1–2 hours plus 24 hours of cure · Moderate DIY
For a quiet, non-structural crack in a poured wall the fix is a $20–60 polyurethane or epoxy injection kit, about 1–2 hours of work, and 24 hours of cure before you test it. You clean the crack, glue injection ports along it every 6 to 8 inches, cap the surface with the paste in the kit, then inject from the lowest port upward until resin appears at the port above. Read the crack before you buy anything, though. Wider than a quarter inch, running horizontally, stepping through block, or growing month over month means the wall is moving, and resin does not stop movement.
What you need
- A polyurethane or epoxy crack-injection kit with ports and surface paste
- The dual-cartridge caulk gun most kits require
- A wire brush and a shop vacuum
- A cold chisel and hammer
- Safety glasses and nitrile gloves
- A pencil and painter tape for monitoring
Before you start: These resins bond to skin as readily as to concrete, so wear nitrile gloves and sealed eye protection, and put a fan in a basement window before you open a cartridge. Read the crack before you seal it. A horizontal crack, a stair-step crack through block, or anything wider than a quarter inch can mean soil pressure is pushing the wall in, and injecting it hides the evidence without stopping the movement. A structural engineer's inspection runs $400–800 and is the right first call in those cases.
Steps
- Measure and date the crack before you buy anything. Draw a pencil line across each end, note the width against a coin (a quarter is about a quarter inch), and tape a paper telltale over the middle. A crack that has not moved in 30 days is a candidate for sealing.
- Rule out the disqualifiers: horizontal cracks, stair-step cracks through block, anything wider than a quarter inch, one side pushed out of plane, or sticking doors and a sloping floor upstairs. Those get an engineer first.
- Wire-brush the crack open and vacuum out the dust. Injection resin bonds to clean concrete and to nothing else, so chisel out any old patching compound, hydraulic cement, or paint sitting in the groove.
- Dry the surface. If water is actively weeping, wait for a dry spell, or choose a polyurethane kit, which tolerates damp and expands on contact with moisture, rather than an epoxy kit, which does not.
- Glue the injection ports along the crack every 6 to 8 inches, starting at the lowest point, each one centered over the crack line.
- Spread the surface paste over the crack and around each port base, leaving the openings clear, and let it cure per the label, usually 20 to 60 minutes. That cap holds the resin inside the wall under pressure.
- Inject from the bottom port with slow, steady pressure until resin shows at the next port up. Cap the filled port, move up one, and repeat to the top.
- Cure 24 hours, snap the ports off flush, then test by running a garden hose against the outside of that wall for 10 minutes.
- Fix the water source while you are at it. Extend downspouts 4 to 6 feet out and regrade so the soil drops about 6 inches over the first 10 feet from the wall.
Or hire it out
A foundation contractor charges $350–800 to inject a single crack with a warranty, and $1,500–3,500 when the repair needs an exterior dig and a waterproofing membrane. Interior drain tile with a sump pump is $5,000–12,000. That spread is the whole argument for injecting a quiet hairline crack this spring rather than after the finished basement floods.
Done. Now the part everyone skips: doing it again on schedule. Log it in OnOtto and the next reminder for foundation crack sealing sets itself.
Log this task, get the next reminderFrequently asked questions
Which cracks are normal and which are trouble?
Thin vertical or diagonal cracks under about 1/8 inch are usually shrinkage from curing, and most poured walls develop one or two in the first year or two. The other kinds matter more: a horizontal crack partway up the wall points at soil or frost pressure, stair-step cracking in block follows settlement, and a crack wider at one end than the other means the wall is rotating. Width plus direction is the tell, not length.
Can I just patch it with hydraulic cement or caulk?
A surface patch seals the face and leaves the other 7 or 8 inches of wall thickness open, so water keeps tracking through the crack and finds an edge to escape at. Hydraulic cement is the right product for a round hole or a tie-rod leak, not a moving crack. Injection is different because the resin fills the crack from face to face and stays slightly flexible, which is why kits cost $20–60 instead of $8.
Do I seal from inside or dig outside?
Inject from inside for a single leaking crack: a fraction of the cost, workable in any weather, and no damage to the landscaping. Digging outside earns its $1,500–3,500 when the wall leaks in several places, when the crack sits under a patio you cannot reach, or when the footing drain has failed and needs replacing anyway. Try the inside fix first unless a pro shows you otherwise.
How often should I be checking the foundation?
Twice a year, and the easiest hook is your gutter cleaning: you are already outside walking the perimeter, so look at the exposed foundation and the grade while you are there. Photograph any crack with a tape measure next to it so next year has something to compare against. See how often to inspect a foundation, and pair it with cleaning the gutters, since roof runoff at the footing causes most of these leaks.