14 Aug Decking over Concrete
Is your old concrete patio an eyesore? You can avoid the cost and mess of tearing it out by just screwing down wood sleepers and install decking over concrete.
A concrete slab in this condition has to be either broken up and carted away or decked over. A slab with this much damage can’t be fixed. But it can be covered up—and this article will show you how. The results look just like a deck, but getting them is much easier and less expensive than building a deck from scratch. In most cases, this project is also less expensive than a new patio installed by a contractor. Local contractors estimated costs of $170 per cubic metre for concrete plus labour costs to remove this patio and pour a new slab. You could probably replace your patio yourself for less than the cost of this project, but DIY demolition and concrete pours are big, backbreaking jobs.
Will It Work On Your Patio?
Even if your patio is in terrible shape, you can install decking over concrete. Cracks, craters and seasonal movement along cracks are no problem. But beware of these three situations:
- If an area is badly cracked and sinks noticeably year after year, any decking over concrete will also sink and develop a low spot. In most cases, settling concrete stops sinking eventually, so delay this project until it does.
- This project raises the level of your patio by 2-1/2 to 3-1/2 in. (depending on the thickness of your decking and whether or not you put spacers under the sleepers). So any door thresholds adjoining the patio must be at least that far above the concrete. If not, this project won’t work for you.
- Stairs connected to the patio can complicate this project. To keep step heights equal, you’ll have to raise the treads by the same distance you raise the patio (2-1/2 to 3-1/2 in.). On concrete steps, that’s a straightforward job: You can treat them just like the patio, screwing sleepers to the treads and risers and decking over them.
The cost of this project depends mostly on the decking you choose. Decking ranges from about $2.50 per lineal metre. for treated wood to more than $10 per lineal metre. for a top-grade manufactured product. For looks and durability, we used two different colors of decking made from PVC, but there are less expensive alternatives.
Aside from standard carpentry tools, you’ll need a hammer drill for this project. You can get a hammer drill for less than $150 that will do the job. But consider spending $150 or more. Even a very small patio will require more than 50 holes, and a more powerful drill will make that chore a lot easier. Also consider buying an impact driver. Impact drivers pack a lot more torque than standard drills or drivers and will drive concrete screws much better. Most models are cordless, but you can still find inexpensive corded models online.
Decking Over Concrete
Sleepers screwed to the concrete allow you to screw on decking over concrete. Spacers and flashing tape protect sleepers from moisture and rot. Fascia boards hide exposed edges for a neater look.
Mark the sleeper locations on the patio. Don’t forget about extra sleepers to support stairs or railings. The layout of your sleepers will depend on the layout of your decking over concrete. If you want a standard decking over concrete design—all the deck boards running one direction—all you need are rows of parallel sleepers. If you want a more complicated decking pattern, like the one shown here, you’ll need doubled sleepers to support any boards that run perpendicular to the others (see Figure A). We also installed sleepers to support the steps we later added to the concrete stoop
Solve Water Problems First
This corner of the patio had settled by more than 2 in. over the years. That meant a big reservoir after rain—and water in the basement. So we filled the reservoir with exterior-grade self-leveling compound. After the first batch hardened, we poured on a thin coat and gave it a slight slope so water would run away from the house.
Self-leveling compound hardens fast, so you can get on with the project. But it’s also expensive. If you’re not in a rush, you can get similar results for less than one-third the cost with concrete topping mix.
We also took a couple of other water-fighting steps. To prevent water from seeping down along the foundation, we caulked the gap between the patio and the house. At the other end of the patio, a corner of the slab had sunk slightly below the level of the soil and rainwater pooled there. To correct that, we shaved off the sod with a spade, dug out a couple of inches of soil and replaced the sod.
Drill through sleepers, spacers and into the concrete with a hammer drill, then drive in concrete screws. Overhang the sleepers along one edge of the patio and trim them to exact length later.
The sleepers don’t have to be level‐they can follow the slope of your patio. But they do need to form a flat plane. If your patio is in good shape, you’ll get a flat plane automatically. If your patio has ridges and sunken areas, you’ll spend lots of time fussing with shims.
To preview the situation, lay a straight board across the patio in a few spots. Look for the highest hump in the patio and fasten your first sleeper there. Then work outward from the high spot, adding sleepers and checking for flatness along each sleeper and across them. Add shims to raise low spots.
Screwing down sleepers with concrete screws is simple, but there are some things to keep in mind:
- Screws should penetrate the concrete by at least 1 in., so 3-1/4-in. screws are perfect. In low spots, where we had to stack up shims, we switched to 3-3/4- in. screws.
- As you drill, dust compresses around the drill bit. That slows you down, strains your drill and overheats the bit. To clear the dust, pull the bit completely out of the hole once or twice while drilling each hole.
- Drill the holes 1/4 to 1/2 in. deeper than the screw will reach. Extra depth provides a space for dust and grit, so screws are easier to drive.
- Have extra drill bits on hand. As a bit wears, it doesn’t just drill more slowly; it also bores a slightly smaller hole and screws become harder to drive. We replaced each bit after about 40 holes. When all the sleepers are screwed down, take a few minutes to double-check for flatness. Set a 4-ft. straightedge on each sleeper, both across it and along it. If you find spots that are 1/16 in. or more out-of-plane, back out the screw and add or remove shims
Why Use Spacers?
You could lay your sleepers directly on the concrete, but we bought a 1/2-in.- thick PVC trim board and cut spacer blocks from it. Here’s why:
- Spacers let you run sleepers parallel to the house so decking can run perpendicular to the house (if that’s what you prefer). A patio typically slopes away from the house so that water runs off. If you run sleepers parallel to the house and set them directly on the concrete, each sleeper will block runoff. But with spacers, water can run under the sleepers.
- Spacers allow for longer decking screws. We wanted to use Cortex screws, which come with cover plugs made from the same material as the decking. They’re easy to use and almost invisible. But they’re 2-3/4 in. long; that’s too long to sink into our decking if we use sleepers only.
- Spacers let the sleepers dry out. If kept damp, common grades of treated wood will eventually rot. Spacers keep the sleepers off the damp concrete so they can dry.
Lay the decking over concrete
Install the deck boards just as you would on a standard deck. When you reach the end of the patio, position the last board, mark the overhanging sleepers and trim off the excess.
Before decking, we covered the sleepers with flashing tape. Without it, water soaks the tops of the sleepers and the decking prevents the wood from drying. Common grades of treated timber will rot if kept permanently damp, and flashing tape is the best insurance against that.
Installing deck boards over sleepers is just like installing them over standard deck framing. We began with the darker “accent” boards, screwing them into place temporarily to act as guides for the “field” boards. When we reached the end of the deck, we removed the center divider board and cut it to final length. Then we removed and mitered the border boards and trimmed the sleepers to final length.
To cover the ends of the sleepers, we used a deck “fascia” board made from 1/2-in.-thick PVC. We cut the fascia into strips and screwed them to the sleepers.
Aside from the decking, here’s an estimate of what you’ll need to cover 100 sq. ft. of patio. Exact quantities depend on the shape of your patio and the layout of the decking.
- 90 linear ft. of treated 2×4
- 90 linear ft. of flashing tape
- Sixty 3/16” x 3-1/4” concrete screws
- 1/2”-thick PVC trim or deck fascia (for spacers), plastic shims, 3/16-in. masonry drill bits (minimum drilling depth of 3-1/2 in.)
The simpler you can make your deck plan, the faster and cheaper it will be to build.
If you’re a beginning DIYer, don’t let all the details on our deck scare you away. Your project can be a whole lot easier if you choose a simple decking design. With all the deck boards running in the same direction, all you need are sleepers centered 16 in. apart. You won’t need to do the careful measuring required by our design, or make angled miters and other fussy cuts. A simple deck design makes this project almost impossible to get wrong. If you have a badly uneven patio like ours, flattening the sleepers with shims may take hours. It’s tedious but not complicated.
You can slash the cost by using treated wood deck boards instead of manufactured decking. And if you live in a drier climate where the sleepers won’t stay damp, you can skip the spacers and flashing tape.
The crack factor Cracks in your patio drastically limit your options. That’s because cracks tend to move. Some grow wider over time, while others shrink and widen as the soil’s moisture levels change. Some become uneven as one side of the crack sinks. Any movement happens so slowly that it’s hard to detect. But if you know—or even think—that you have moving cracks, you have to choose a patio upgrade that can “float” over moving cracks without becoming damaged.