Q: My daughter's house has an unusual layout. The single-car garage is on the same level as the basement and is attached to the basement, but is not under the house itself. It is off to the side, so it has its own roof. The roof is a flat concrete slab that doubles as a patio area and also serves as a walkway to a side entry door.
The garage roof leaks whenever it rains or when snow melts in the winter. Plus, because it is flat, water pools on top. Building up one side of the roof to encourage it to drain seems unlikely because the door threshold is right on top of the slab. What can we do to stop the leakage and water pooling?
A: Water penetrating and seeping through the concrete means the slab won't last long. In the winter, water trapped inside will freeze, expand and start to crack the concrete. Additionally, the water can lead to mildew and mold problems in the space below.
Sealing, or waterproofing, the slab would stop the leakage. Adding a drain to the low spot would help get rid of water that is pooling on top.
While a film-forming sealing product, like paint, would help stop the leakage, most don’t last more than a few years. In this situation, with the slab being used as a patio and a walkway, the service life might be even shorter. And it wouldn’t be a good idea to degrade traction on the concrete by coating it with a slick film.
An ideal sealer would seep into the pores of the concrete and do its work below the surface, without affecting the concrete's traction and durability. Silane-based sealers are the state of the art in such products. Thoro's Concrete and Masonry Sealer is one example. A water-based material, it penetrates and reacts chemically with minerals in the concrete to form a barrier to both water and chloride (salt).
Silane sealers can be sprayed, brushed or rolled onto concrete. Because they don’t lie on the surface, they’re protected from abrasion, wear and weathering. Their exclusionary effects can last for decades.
A slab garage roof like on your daughter’s house is not the only place to use a silane sealer. Concrete driveways and sidewalks also can benefit from these products, gaining protection from water and salt.
As for the standing water on the roof, either remove the water every time, as needed, with a push broom or squeegee, or install a drain in the slab. The latter would involve drilling a hole through the concrete and then sealing and mechanically mounting a PVC pipe flange to the underside of the slab.
Run a PVC pipe along the garage’s ceiling, conveying the water outside the structure. A drain grate on the top would keep large debris out of the system.
To locate the proper spot for a drain, use chalk to draw an outline around the last remaining pool of water as the slab is drying out after a rain. The center of that “lake” will be the lowest point on the slab, and that’s where the drain should go.
If several pools are on the roof, additional drains could be added and their pipes joined below the slab to drain simultaneously.
Long term, putting a three-season porch with a roof over the slab would eliminate all these problems, and might increase the enjoyment of, and add value to, the home.
Zolton Cohen is a Kalamazoo-based newspaper columnist and former American Society of Home Inspectors-certified home inspector. Write to Zolton B. Cohen, Around the House, P.O. Box 2007, Kalamazoo, MI 49003.
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