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Wet It Be! A complete guide to fixing a damp basement


"We definitely have a mildewy smell in our basement, but I’m not sure where the moisture is coming from."

- Cliff, via e-mail

Here’s a trick for figuring out whether your dampness is caused by condensation or moisture seeping in through the foundation. 

Start by cutting a one-foot-square piece of foil or heavy plastic (like from a garbage bag).  Run duct tape along all four sides of the square and secure the patch to the basement wall, pressing the tape firmly to seal all edges.  Leave the patch on the wall for at least 24 hours.  When time’s up, inspect the patch to see which side is damp. 

If the side that was open to the basement air is damp, you have a condensation problem. 

If the side that was next to the wall is moist, you’re getting water seepage from the outside.  Proceed to next question.

"Help!  My basement walls are actually wet!  Is there something I can do to correct this?"  

- Mary, via e-mail

There is always something you can do for an overabundance of moisture.  The real test is how much you enjoy it. The following problems cause wet foundations:

Problem #1: You have aluminium siding on your home and some of it is loose, broken or missing entirely, so wind-driven rain gets into the walls where gravity sucks it into the basement. 

Solution:  Cut around the damaged area with a utility knife.  Clean the surrounding surface with rubbing alcohol.  Cut a piece of new or matching siding allowing one inch of overhang on all sides.  Run a bead of silicone sealant around the entire perimeter on the wrong side of the patch.  Press the patch into position and tape it there for an hour while the sealant sets up.

Problem #2: Your basement windows are leaky. 

Solution: Re-do exterior caulking around windows and replace the glazing putty if it's ossified. Coat surrounding wood surfaces with a fresh coat of exterior paint to seal possible entry points for water.  You can also buy clear plastic covers for basement window wells to limit their exposure to rainwater. 

Problem #3: Your gutters and/or downspouts are leaking or clogged to the point of overflowing, so gallons of rainwater spill out of the gutters and run down your foundation, seeping through the cinderblock walls into your basement. 

Solution: Clean those gutters and repair gaps, holes, or leaky joints with gutter sealant.  If downspouts are plugged up, give them a high colonic by blasting water into them from below with a hose set on 'stun'.  (Hint: Backsplash happens.  Wear something that isn’t transparent when wet.)

Problem #4: Your downspouts are draining too close to your home’s foundation. 

Solution: Add plastic or metal drainage extensions to the ends of downspouts to deposit water as far from your foundation as possible (a minimum of six feet). 

Problem #5:  The grading around your home is incorrect, so rainwater is flowing toward your foundation instead of away from it.  Soil around your home should slope downward and away from the exterior walls and window wells at a pitch of one inch per foot, so water always drains away from the foundation.  (In many cases patios sink and act like collection ditches holding water against the house; hence the wet walls in your basement.)

Solution:  This can be a huge job that necessitates the rental of a skid-steer loader to re-grade your entire yard.  Charge neighbour kids five bucks a ride and you’ll cover the cost of renting the beast.  

Problem #6:  If none of the above solutions work, your house may have drainage issues below ground level, i.e. an encroaching water table or underground spring.  This requires excavation, waterproofing and luck, the kind of luck you may not have.  So hire a professional.  

Solution:  If you have an acute situation with water pouring through your basement walls during a dinner party, emergency tactics are necessary. Apply hydraulic cement to leaks or cracks on the interior side of foundation walls.  Applying hydraulic cement is dead easy and stops an active leak in its tracks.  Widen your cracks first with a cold chisel, undercutting the edges (the cement adheres better to an undercut opening than it does to a v-shaped trench).  Mix a small batch of hydraulic cement to the consistency of icing, then trowel it into the crevice, remembering that you have a short amount of working time before the hydraulic cement becomes as hard as a hockey puck. 

Remember, a job well done is a job that’s never over.  And that makes home repair about as close to immortality as some of us will ever get, so enjoy it. 

Safety reminder

  • Hydraulic cement is extremely alkaline and will corrode your skin and mucous membranes.  Wear gloves, goggles and a dust mask.  If you get any on you, (or up your nose; it happens), rinse with vinegar to counteract the alkalinity.


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