The short answer
Assess the Crack First
Before sealing, look closely at the crack. Hairline cracks, under about 1/8 inch, often come from concrete shrinkage and are not urgent. Wider cracks, or those that shift over time, may point to settling or structural issues. Active water seepage changes the repair method entirely.
Check if the crack is dry or damp. For a dry, non-moving crack, a simple sealant can work. If water is pushing through, you need a hydraulic product that expands as it cures. In a wet climate, many basement cracks stay damp, so this distinction matters.
Gather the Right Materials
For small, dry cracks, a polyurethane sealant or a quality concrete crack filler is enough. These come in cartridges that fit a standard caulk gun. For damp or actively leaking cracks, choose a hydraulic cement that sets fast and expands slightly. Epoxy injection kits are for larger, structural cracks but are more involved.
You also need a wire brush, a vacuum, and a putty knife. Clean the crack thoroughly, removing loose concrete, dust, and any old sealant. A clean surface is essential, or the new sealant will not bond. Wear gloves and eye protection when handling these materials.
- Wire brush or stiff nylon brush for cleaning
- Vacuum with a narrow nozzle to pull out dust
- Polyurethane sealant for dry, hairline cracks
- Hydraulic cement for damp or leaking cracks
- Epoxy injection kit for wide or structural cracks
- Putty knife to smooth the surface after application
Prepare the Crack Surface
Use the wire brush to scrub inside the crack, removing loose particles. Then vacuum the crack thoroughly, reaching as deep as you can. If the crack is wider than 1/4 inch, you may need to chisel out any loose edges to create a sound base. Dampen the crack slightly before applying hydraulic cement, but keep it dry for polyurethane.
For very narrow cracks, widening them slightly with a chisel or angle grinder can help the sealant penetrate better. This is not necessary for hairline cracks, but for anything larger, it improves the bond. Let the surface dry completely if you are using a non-hydraulic sealant.
Apply the Sealant Properly
Load the sealant into a caulk gun and fill the crack from the bottom up, pressing firmly to force it deep. For hydraulic cement, mix small batches and push it into the crack with a trowel, then smooth it flush. Work quickly because hydraulic cement sets fast. For polyurethane, tool the bead smooth right away.
If the crack is wide or deep, apply in layers, letting each layer cure before adding the next. This prevents sagging and ensures a full fill. For epoxy injection, follow the kit instructions precisely, often using a two-part mix that needs a static mixer. Allow full cure time before painting or covering.
After the sealant cures, inspect the area. A good seal should sit flush with the wall, with no gaps left open. If you notice new cracking nearby, the issue may be structural, and you should consider a professional evaluation.
Know When to Call a Pro
If the crack is wider than 1/4 inch, growing, or accompanied by bowing or displacement, the cause may be foundation movement. Sealing the surface will not fix the underlying problem. A structural engineer or a foundation specialist can assess the situation. Water damage pros can also help if the crack has led to moisture issues.
For active leaks that return after sealing, the crack may connect to groundwater or drainage problems. In that case, exterior waterproofing or drainage improvements might be needed. Local water damage pros can evaluate the extent of moisture and recommend next steps. Policies vary, and your insurer decides what is covered.
In areas with wet winters and dry summers, basement cracks often appear during heavy rain. If you notice water stains or mold near a crack, address it quickly. A professional can help determine if the crack is letting in moisture.
- Crack wider than 1/4 inch
- Crack is actively growing or shifting
- Water seeps through after sealing
- Crack is accompanied by mold or musty odor
- You see bowing or leaning in the wall
- You are unsure about the structural integrity
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Call Now (425) 407-5186Frequently asked questions
Can I use regular caulk to seal a basement crack?
Regular caulk is not meant for concrete. It may flex but will not bond well and can peel off. Use a polyurethane concrete sealant or hydraulic cement designed for masonry. These products expand and adhere to concrete, providing a durable seal against moisture.
What if water keeps coming through after I seal it?
If water returns, the crack may be under hydrostatic pressure, meaning groundwater is pushing against the wall. Sealing from the inside may not stop it. You may need exterior drainage work or a professional water damage pro to assess the situation. Check your policy for coverage details.
Is it safe to seal a crack myself?
For small, dry cracks, DIY sealing is safe with basic precautions. Wear gloves and eye protection. For large, structural, or actively leaking cracks, the risk of improper repair is high. In those cases, consult a professional to avoid making the problem worse.