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Building Over a Patio Slab

One way to add a room — IF your patio can take it

Ask This Old House Crew
Photo by Matt Kalinowski

I am planning to enclose my concrete, exposed aggregate patio and turn it into a finished part of my house. I would like to use some tongue-and-groove oak I salvaged as the flooring. How should I go about it?
— Mike, Jupiter, Fl.


Tom Silva replies: I see several possible problems that might prevent you from taking on this project at all. First, the patio slab may not be safe to build on. Just because it's concrete, you can't assume that it will provide a suitable foundation for framed walls. Building codes require a reinforced floor slab at least 3½ inches thick. Plus, it has to have poured-concrete footings beneath the perimeter to hold the exterior walls that will be built. These footings would have to extend at least 16 inches below grade even in a mild climate like yours. The codes may also specify how far the wall framing must be from grade —that's partly to provide termite protection, a real concern in your part of the country. I think you should first invite your local building inspector out for a visit.
If the inspector okays the slab, here's what you'll have to do. First, seal it against moisture migrating from below. Use a pump sprayer or brush to coat the slab with a waterproofing masonry sealer, following the instructions on the best way to clean and prepare the surface. Then cover the slab with a layer of 30-lb. roofing felt followed by a layer of 6-mil plastic sheeting. Lap the seams about 6 inches or so. Next, lay pressure-treated sleepers face-down on the plastic, spacing them no more than 16 inches on center (12 inches on center is better). If the patio slopes away from the house slightly, as it should, you'll have to shim up the sleepers so the finish floor will be level.
Anchor the sleepers with countersunk masonry screws or spring spikes. You'll have to drill pilot holes for either type of fastener. Because exposed aggregate can be tough, use a hammer drill and have some extra masonry bits on hand.
Once the sleepers are set, install a ¾-inch tongue-and-groove plywood subfloor. Now you can lay your oak flooring.


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