I hadn’t seen my cousin in over a year, so when I had some free time I decided to make the drive to her house. I was surprised to see that she had two puffy eyes and a rag held to her nose when she opened the door. She’d been gardening, and while moving potted plants around had stumbled over a crack in her patio, stubbing her toe and smacking herself in the nose with a pot. She was embarrassed, but unharmed aside from a nosebleed.
When I checked out the scene of the crime it was easy to see how it had happened. The slab behind the house was subsiding, and deep cracks had fissured the concrete. I was able to turn over a sliver five inches long with my foot and kick it off the patio. I asked her if she’d considered replacing the slab, and her reply was that she didn’t have the time. Having a contractor over would interfere with her planting. I suggested putting a ground level deck down, but she likes gardening in bare feet and worried about splinters. I suggested putting composite decking over the concrete porch. Good quality composite decking is barefoot-friendly and looks like real wood decking. These qualities piqued her interest and we started to discuss the options.
The Challenges of Building Decks on Top of Concrete
Decks installed over concrete porches or patios aren’t all that common. One reason is that on most homes with concrete patios there isn’t much space to build a deck. Concrete patios are normally found on homes with concrete slab foundations, and the thresholds of the exterior doors are generally very close to the ground, usually three to four inches. This doesn’t leave a lot of clearance for a deck, and if you ask a contractor about it, the odds are he’ll say ‘No way,’ simply because of the clearances involved. In most cases, though, it’s possible to lay dimensional boards with the wide side down to make a frame. Your typical two-by-four, for instance, is only an inch-and-a-half wide in the shorter direction. With their widest faces towards the slab these boards will be at about the right height to build a deck over the slab.
If clearances are smaller, then it becomes more difficult to build a deck over concrete. If the height of the door threshold that leads out to the slab is fewer than three inches, then building a platform for the deck boards requires ripping—sawing boards lengthwise—to make a frame at the right height. This is a time-consuming process and it’s difficult to create even, straight boards this way, which in turn makes it harder to build a level deck.
Preparing the Surface by Leveling the Slab
Once you’ve determined your concrete slab has the required clearance, the real challenge is ensuring the frame is level so that the resulting deck is level. Even a completely intact concrete slab is unlikely to have a perfectly even surface, so you’ll likely have dips and rises that will have to be evened out before you build the deck frame. There are a couple ways to do this:
- Shims are one way to compensate for dips. These are just small pieces of wood cut to the width needed to fill in gaps under boards, and you’ll need to use at least a few of them even on an intact slab.
- Leveling compound is probably the best option for a broken slab. This a quick-drying cement that can even out rough surfaces. You’ll want to be sure to use an exterior-grade compound.
- Ready-mix cement is an alternative to leveling compound that dries much more slowly. This may make it a better choice for people who are new to concrete work as it allows the user more time to work with the cement.
To use leveling compound or ready-mix cement, identify the low-points on the slab and pour a generous amount of the compound into them. Use a straight edge to level the compound out. For finished work you would want to use professional tools, but for something that will be covered up with decking you can use a straight piece of excess wood, which will actually make it a little easier to get an even surface than if you used the tool designed for this purpose (it’s essentially a large putty knife). As you work the compound, regularly place a level on top of the concrete surface to make sure the slab is leveling out. Using slower drying ready-mix cement will give you a little more time to perform this step.
One of the things to watch out for when building a deck over a concrete slab is water drainage. Building so close to the slab means there isn’t much room for air to circulate underneath the deck, and this can trap water. The last thing you want is for that water to flow toward the home’s foundation. To prevent this, use the leveling solution to create a very slight slope to the edge of the slab away from the house. Lay a small amount of the compound down on the home side of the slab, and carefully pull it toward the opposite edge, checking with a level as you do so. Do this in two steps:
- First, place your level perpendicular to the home and check that the bubble floats past the line on the home’s side, but not too far past it.
- Then place your level parallel to the home and make sure the level remains between the lines. This ensures that the slab slopes very slightly front to back but not side to side.
Once again, only a very slight slope is required for water drainage. Once the slab is properly prepared, you can begin creating the deck’s substructure.
Installing Composite Decking Over a Concrete Porch
In most decks, two-by-fours aren’t acceptable to use as joists. They’re too thin, too flexible, and leave you with a bouncy deck surface. When they’re flat on the ground or on a slab, though, they’re called sleepers—really just joists on a surface—and can be used this way because the slab underneath is carrying most of the weight. However, they still need to be more closely spaced than usual. A 12-inch spacing center-to-center is the maximum distance you should set sleepers apart, and you may want to put them even closer to each other. You may even wish to place two two-by-fours side by side every few feet to reduce flexing. The exact way you lay the substructure will vary according to what you’re comfortable with and your local building codes. Since you’ll be drilling into concrete and mistakes will be difficult and time-consuming to fix, you’ll want to plan ahead, lay everything out, and double check all of it before starting.
- First, mark the place your sleepers will be going. Grab your measuring tape and a long piece of wood, and begin measuring and marking where your boards will go. You will have to pre-drill the holes in both the concrete and the boards. You may wish to pre-position your wood in the places it will go so all the holes match.
- Drill your holes slightly smaller than the fasteners you’ll be using. For a standard width screw of three-sixteenths of an inch, you’ll want to use a three-eighths-inch bit.
- Once everything is laid out and pre-drilled, attach the two-by-fours to the concrete with three-and-a-quarter-inch-long concrete screws.
- While you attach your sleepers, use a level to ensure that the boards are level in the direction that parallels the house.
- If you find that you need to shim an individual board then switch to slightly longer three-and-three-quarter-inch screws which will be long enough to go through the board and shim and still keep the deck anchored.
Once all the sleepers are in place you can begin laying your deck boards. There are some things to be aware of when you’re anchoring your deck boards to the frame. Since the vertical height is so small it will limit what sort of fasteners can be used to anchor your decking boards. You will have at most one-and-three-quarters inches of wood to sink a screw into between deck boards. This may not be enough for hidden fasteners that anchor boards using clips that slot into the side of the decking board. Check the dimensions before ordering, and look for a hidden fastener system that uses screws short enough to work with this small distance.
If you decide to sink your screws through the deck board, it gives you more room to work with, but going through the board also gives water a direct path into the core of the board. For this reason, if you choose to anchor your boards this way use a composite that is fully capped and dense enough to resist moisture.
One composite deck system with these features is Infinity by Fortress Deck. It’s a fully capped board for protection from moisture above and below (perfect for decks that sit close to the ground) and uses a combination of recycled plastic and bamboo flour for strength and water-resistance. The Hulk hidden fastener system has screws that are 40 mm long and will go into a two-by-four without the hitting the concrete beneath. This hidden fastener system gives your deck a sleeker, more sophisticated look while also leaving the boards caps intact and better able to resist moisture. It’s just one example of the thoughtful engineering that Fortress Building Products puts into its whole line, from decks to railings to fences. To find out more about Infinity composite decking, fill out Fortress’ contact form.