Fix the Water Outside Before You Patch the Crack Inside

Quick answer: Figure out whether the crack is moving and where the water comes from before you touch a tube of anything. Hairline vertical cracks are a weekend job. Horizontal cracks, bowing walls and stair-step cracks in block are an engineer's job.

Most people do this backward. They see a wet line on the wall, grab a bucket of patch, and smear it on. Three storms later it's weeping again, now with a gray scab on top.

Water is pushing on that wall from the outside. A patch on the inside is fighting the soil. Sometimes it wins. Often it doesn't, unless you deal with the water first.

Safety before you touch anything

If there's standing water in the basement, treat every outlet, cord and appliance as live. Don't wade in. If the water is near the electrical panel or outlets, stay out and call your utility or a licensed electrician. Only shut off the main breaker if the panel is dry, you can reach it from dry footing, and you can do it without stepping in water.

If water is reaching the furnace or water heater, shut off the gas to that appliance at its valve. If you smell gas, leave the house and call the gas company from outside. Don't flip switches on the way out.

For the repair itself you need safety glasses and a dust mask. Chiseling concrete throws fine dust, and you don't want that in your lungs. Gloves too. Hydraulic cement and epoxy both burn skin.

Read the crack

Do this on a dry day with a flashlight, a pencil, a tape measure and a credit card.

Direction. A vertical or slightly diagonal crack is usually shrinkage or minor settling. Common, and usually fixable by you. A horizontal crack, especially near the middle of the wall, means soil pressure is bending the wall inward. That's structural.

Width. A credit card is about 1/32 of an inch thick. If it slides in easily, you've got something wider than a hairline. Past roughly 1/4 inch, or if one side of the crack sits proud of the other, stop.

Block walls. A crack that steps along the mortar joints like a staircase, or a wall that visibly bows, goes to a structural engineer. Not a waterproofing salesman. An engineer has no repair to sell you, which is the point. Expect a few hundred dollars for the visit. It's cheap next to a wrong guess.

Is it moving? Draw pencil lines across the crack at both ends and date them. Check again in a few weeks and after a hard rain. If the marks drift apart, the wall is still moving and a rigid patch will crack again.

Make sure it's a leak

Not every wet wall is a leak. Warm humid air hitting cool concrete drips water down the surface, especially in summer.

Tape a square of aluminum foil, about a foot across, flat to the wall over the damp spot. Seal all four edges. Wait a day or two. Moisture on the outside of the foil is condensation, and your fix is a dehumidifier, not a patch. Moisture underneath means water is coming through the wall.

Fix the outside first

This is the unglamorous part and the part that actually works. Walk the house during or right after a rain.

• Gutters: Clean them. A clogged gutter dumps a waterfall right beside the foundation.
• Downspouts: Extend them at least six feet from the wall. A splash block doesn't count. Flexible extensions run about $10 to $25 apiece.
• Grade: The soil should fall away from the house. Model building codes call for about six inches of drop in the first ten feet. Codes vary by area, so check yours. Where it's flat or sloping toward the house, add compacted fill, not loose topsoil that settles.

Then wait through a rainstorm or two before you decide how bad the crack is. A surprising number quit leaking once the roof water goes somewhere else.

Plug an active leak

If water is actively seeping or running, you want hydraulic cement. It sets in minutes, even wet, and expands slightly as it cures. A pail runs about $15 to $25. You'll also want a cold chisel and a hammer, a wire brush, and a small margin trowel or putty knife. Call it $30 all in if you own nothing.

1. Chisel the crack out into a V, then undercut it so it's wider at the back than the face. About 3/4 inch deep. That shape locks the plug in like a dovetail.
2. Brush out the dust and loose bits. Wet the channel with a sponge.
3. Mix a small batch. Read the label for water ratio, but you're after something like stiff putty. You get about three minutes before it grabs. Mix more than you can use and you'll throw it away.
4. Press it in with a gloved hand or the trowel. Hold it firm for the time on the label, usually a few minutes, then smooth it.

Mix and place it in short sections, working from the top down. The annoying part is that water finds the next weak spot. If it stops one place and shows up an inch over, that's the soil pressure talking. Keep going or slow down and look at the outside again.

Seal a crack that seeps or just stays damp

For a vertical crack that isn't moving, an injection kit does the job better than surface patching. Two kinds:

• Epoxy is rigid and structural. It bonds the two sides back together. Use it on dry cracks that aren't moving.
• Polyurethane stays flexible and tolerates moisture. Use it on damp or seeping cracks that may shift a little.

DIY kits run roughly $100 to $200. A pro injecting a typical crack often charges several hundred dollars, more for a long one. Follow the kit exactly. You'll glue small plastic ports along the crack about every foot, seal the surface between them with the paste, let it cure for the stated time, then inject from the bottom port up until material shows at the next one.

Don't use waterproof masonry paint as the fix. With water pushing from behind, it blisters and peels within a season. It's fine as a finish coat after the crack is truly sealed.

Dry it out

Run a wet vac, then a dehumidifier. Get fans on it. Give the wall a day or two before you judge your work.

Anything porous that stayed soaked longer than that, like carpet, cardboard boxes and insulation, goes in the trash. If mold covers more than about ten square feet, the EPA's guidance is to bring in a professional. If you borrowed a wet vac from a neighbor, return it empty, rinsed, with a fresh filter. That's the rule.

If the wall was finished, patching drywall without making it worse covers the cleanup. And if the floor drain is backing up with the water, that's a separate problem. Start with a plunger, a snake and ten minutes.

When to hand it over

Call a licensed contractor or engineer for horizontal cracks, bowing, stair-stepping, a crack wider than 1/4 inch, or any crack that keeps growing. Same for water coming up through the floor slab and anything involving excavation or wall bracing. Structural work often needs a permit, and codes and licensing rules differ by state and town. A good contractor will tell you what the permit requires without being asked.

Keep the pencil marks on that wall. Check them after the next big storm. A crack that holds still through a wet spring has told you the truth.