DIY Steps for Building a Deck Over a Patio Slab: The Low Down on Low Decks
By Larry Walton Photos by Bruce Smith
How to build a deck over an existing patio slab with low clearance.
Hey, some of you have been ignoring our great deck how-tos because you think you can’t build a deck over your patio slab. Check this out: My uncle kept bugging me about building a deck in his backyard. I tried to explain to my uncle that he didn’t have enough elevation at the door to build a deck over his patio. Even if we removed the austere concrete slab, the patio door threshold was barely above grade. But because of his persistence his patio area took on a remarkable transformation, and we came up with a solution that can be a benefit to many who would love to have a deck in place of their concrete patio slab. Here’s how:
Determine the joist height for the system. To get the height of the joists, subtract the thickness of the decking from the distance between the top of the slab and the bottom of the patio door threshold. In our case this was 2-1/2 inches. You will probably need a minimum of 1-1/2 inches to do this project. Anything greater than that is a bonus.
If your tolerances are very tight, you can gain another 1/2 inch of room by going with one of the synthetic decking types that measure only 1 inch in thickness.
After determining the joist height, rip all of the needed pressure-treated joist material the same size. With some of the off-fall (extra material from the rips), rip strips of 3/4, 1/2, 1/4 and 1/8 inch and cross-cut these pieces into 4- to 8-inch pieces. These pieces are now flat shim stock to be used later.
Install the Joists
Lay the joists across the concrete perpendicular to the house wall at 16 inches on center. Level the joists using the shim stock. Make sure to support the joists with shims every 12 inches because the ripped-down material has diminished structural strength and cannot span much distance.