How to Fix a Leaky Porch
A thrifty, DIY way to seal the floor and keep water out of the basement
Water is leaking through our front-porch floor into the finished basement underneath. Right now, the floor is covered with indoor/outdoor carpeting over plywood. Is there a way to seal the floor to stop the leaks? —Jackie Dripps, Morris, MN
Long ago, people solved this problem by stretching heavyweight cotton canvas over a tar-saturated membrane called Irish felt and then painting the canvas with multiple coats of oil paint.
At the East Boston TV project in 2006, we used an updated approach on a second-floor roof deck: an elastomeric roofing system made by Acrymax. It’s basically a layer of polyester fabric bedded in multiple coats of a thick acrylic paint that can withstand foot traffic and ultraviolet rays. The finished product actually looks a bit like painted canvas. Even better, the installation process is simple enough for a careful DIYer to handle.
In your case, the process starts by taking off the old carpeting and plywood. Then you screw down fresh sheets of ¾-inch exterior-grade tongue-and-groove plywood and cover the seams and screwheads with a coat of elastomer and strips of the fabric. Next, roll another coat of elastomer over the entire porch deck, lay sheets of the fabric onto this wet base coat, and apply more elastomer over them. When the surface is dry, apply at least two top coats of a different elastomer, one formulated to protect against dirt, sun, and abrasion. These top coats should be a different color from the base coats so that when the base color shows through, you’ll know it’s time to recoat.
When finished, the system’s total thickness, including the plywood, comes to about 7/8 inch. And the square-foot cost for materials runs between $2.50 and $3.