Seal a weeping basement wall from inside, in the right order

Quick answer: Tape foil to the wall to learn whether it's seepage or condensation. If it's seepage, fix the gutters and grading outside first. Then chisel out cracks, plug them with hydraulic cement, and coat the wall with masonry waterproofer. If the wall is bowing or cracking sideways, stop and call a pro.

Safety before anything else

Water and electricity in a basement is a bad pairing. If the wet wall has outlets, a panel, or a sump pump cord running along it, shut off those circuits at the breaker before you touch anything. Never stand in standing water near a live outlet or an extension cord. If the water is deep enough to reach a plugged-in appliance, cut the power at the main and call an electrician.

If water is reaching a furnace or water heater, shut off the gas to that appliance and get a licensed pro. Don't relight anything that's been wet.

Wet walls grow mold fast. Wear an N95 respirator, safety glasses, and rubber gloves when you scrub and chip. A respirator runs about fifteen dollars. Lungs don't come cheaper.

Diagnose first: is it actually coming through the wall?

Half the "leaky" basement walls I've seen weren't leaking. Warm, humid air hit cold concrete and dripped. That's condensation, and a sealer won't touch it.

Run the foil test. Tape a two-foot square of aluminum foil flat against the wall and seal all four edges with duct tape. Leave it for a day or two.

• Moisture on the outside of the foil: condensation. Your problem is humidity. Run a dehumidifier (roughly $200 to $250 for a decent 50-pint unit) and vent the dryer outdoors if it isn't already.
• Moisture under the foil, or the wall behind it is dark and wet: water is coming through the concrete or block. Keep reading.

Then look at when it happens. Does it weep the morning after a hard rain? Then the water is coming from the yard. Does it weep in dry weather too? That points to a high water table or a plumbing leak, and the second one is a plumber's job, not a sealer's.

Rule out the floor drain while you're down there. A backed-up one can make a wall and floor look like they're seeping from outside. If it gurgles or sits full, unclog it with a plunger and a snake before you buy a single bucket of sealer.

Stop the water outside first

This is the step most people skip because it's outdoors and unglamorous. It's also the one that works.

Walk the house during a rainstorm. Yes, in the rain. You're looking for:

• Gutters overflowing or clogged. Clean them. Fix sagging sections so water runs to the downspout.
• Downspouts dumping at the foundation. Extend them at least four to six feet away with a flexible extension, about $10 to $20 each.
• Grading that slopes toward the house. The dirt should fall away, roughly six inches over the first ten feet. A few wheelbarrow loads of fill and a rake fix a surprising number of wet walls.
• Window wells full of leaves or water. Clear them. A clear plastic cover runs about $30 to $60.
• Flower beds piled against the foundation, holding water like a bathtub.

Give it a couple of rainstorms before judging the result. Plenty of wet walls dry out right here and never need the rest.

Prep the wall

Sealer sticks to clean, bare masonry and nothing else. Skipping prep is how a $100 bucket peels off in sheets.

1. Move everything away from the wall. Pull up wet carpet and pad. Throw out anything you can't dry within a day or two.
2. Scrape off old paint and flaking material with a stiff wire brush ($8 or so) and a scraper.
3. Scrub off the white, chalky crust. That's efflorescence, mineral salts left behind as water evaporates. A wire brush gets most of it, and a diluted white vinegar solution handles stubborn patches. Rinse and let it dry.
4. If you see black or green growth, scrub it with a detergent solution and a stiff brush. Don't bleach and walk away. Remove the growth, then dry the surface.
5. Vacuum the dust.

Plug the cracks and active leaks

Any crack that's actually weeping needs a plug before the coating goes on.

Chisel the crack out into a V, wider at the back than at the face, so the patch locks in. A cold chisel and hammer, about $10 to $15, does it. Wear the glasses.

Hydraulic cement is the material. Quikrete's water-stop version is easy to find, and a bucket is usually $15 to $30. It sets in a few minutes, so mix only a handful at a time, to the thickness of stiff putty. Press it into the crack with a gloved hand or a putty knife and hold it for about three minutes. If water's still pushing through, mix another handful and hold again. Let it cure the full time on the label.

You'll feel the first time it works. The weep just stops. Don't celebrate yet, because the wall behind it is still wet.

Coat the wall

Masonry waterproofing paint, Drylok being the name most hardware stores stock, goes on after the patches cure. A five-gallon pail runs roughly $100 to $150 and covers a good stretch of wall. Check the label for whether the surface should be damp or dry when you start. Different products say different things.

Apply two coats with a stiff masonry brush or a thick-nap roller, working it into the pores. Two coats beat one thick coat. Give each the full dry time, and don't skimp on the second.

Keep the room ventilated, and run a fan.

What this fix can and can't do

An interior coating holds back light seepage and damp. It doesn't stop water pressure. If the soil outside is saturated and the water has nowhere to go, it can still push behind the coating and blister it off the wall. That's the honest limit of this job. Sealing from inside manages water that's already in the wall. It doesn't remove the water.

If the wall was finished, wet drywall behind it needs to come out. You can patch the drywall once the concrete has stayed dry for a few weeks.

When to call a pro

Stop and get a licensed foundation or waterproofing contractor if you see:

• Horizontal cracks, or a wall that's bowing inward
• Stair-step cracks in block, or any crack wider than about a quarter inch
• Water running out of the wall rather than seeping
• The problem coming back after you've fixed the gutters, grading, and coated the wall
• Water in the basement after every rain

Those can mean the wall is under serious pressure. A real fix may involve exterior excavation or an interior drain tile and sump system, which often runs into the thousands. Codes vary by area on drainage, sump discharge, and electrical work, and some places require permits. Ask your local building department before any of it.

Leave it better than you found it

When the wall's dry, take the extra step. Haul the old pad and junk to the curb, rinse the buckets, and put a hygrometer on the shelf ($10 to $15) so you'll know when the humidity creeps up. Set your stored boxes on pallets or shelving, off the floor. If you ever sell, a dry basement with a paper trail of fixes is worth real money.

Keep your gutters clean in the fall. The cheapest waterproofing job you'll ever do is a ladder, a bucket, and an hour.