Keep water moving
Follow the slab’s existing slope and leave its drainage path open beneath the new frame.
Backyard build · Philadelphia rowhome
How to build a platform deck over a concrete slab
A practical sleeper-frame plan for turning a hard, uneven patio into a warm 8-by-10-foot outdoor room—without tearing out the slab first.
The short version
Build above the slab, not on it.
Build a low platform deck over a concrete slab with a sleeper frame made from ground-contact pressure-treated lumber. Anchor the frame with concrete screws, level low spots with plastic shims, keep the slab’s drainage path open, and protect joist tops before fastening the decking.
8 × 10 ftFinished platform
3 phasesFrame, protect, deck
16 in.Joists on center
Ground contactLumber rating
The transformation
Same footprint. Entirely different backyard.
The concrete was functional, but it never felt like a place to stay. The platform created one level, defined surface for cooking, seating, and everyday family use.
The concrete slab dilemma
Treat the slab as a base layer—not a finished floor.
When you have an existing concrete slab patio, it can feel like a solid but uninspiring space. In our Philadelphia backyard, we had a basic leveled concrete pad with a fire pit area. It was functional, but lacked warmth and definition.
The solution was an 8-by-10-foot floating platform deck built right over the concrete. This approach saves you from having to dig deep frost footings while providing a sturdy, permanent-feeling structure.
Four rules before the first cut
The details that keep a low deck from becoming a moisture trap.
Separate wood and slab
Use a sleeper system or adjustable supports so the finished surface is not laid directly on concrete.
Level with plastic
Correct small dips with plastic shims. Wood shims can wick moisture and deteriorate quickly.
Protect the frame
Ground-contact lumber and joist tape help the low frame withstand its moisture-prone location.
The bench list
What this build used
Always buy ground-contact-rated pressure-treated lumber when framing this close to concrete and soil.
Materials
- Ground-contact pressure-treated 2×6 joists
- 5/4-inch decking boards
- Concrete screws, such as Tapcons
- Plastic horseshoe shims
- Polymer-coated exterior deck screws
- Joist tape
Tools
- Hammer drill with masonry bit
- Circular saw or miter saw
- Impact driver
- 4-foot level
- Tape measure and speed square
The build path
Three phases from bare slab to finished deck.
The photo sequence captures the preparation and perimeter framing; the written path carries the project through joist protection and decking.
Site preparation
Phase 01
Layout and fasten the sleepers
The biggest mistake you can make is laying decking directly onto concrete. Moisture will become trapped, and your boards will rot within a season. Instead, use a sleeper system—laying pressure-treated joists flat, or on edge depending on the desired height, across the concrete to act as the subframe.
Critical check
Orient the sleepers so they do not dam the slab’s natural drainage path.
We used a hammer drill to bore through the sleepers and into the concrete, then secured the frame with concrete screws. Check for level as you work; if the slab dips, use plastic shims under the joist. Never use wood shims, which can wick moisture directly from the concrete and deteriorate rapidly.
Perimeter framing
Phase 02
Frame and protect the core
With the outer perimeter and main anchors set, fill in the rest of the joists. Standard spacing is 16 inches on center, ensuring the deck boards have plenty of support and will not flex when walked on.
Protection layer
Apply joist tape along the top edge of every piece of framing before decking goes down.
Joist tape seals around the deck screws and stops water from pooling on top of the framing, significantly extending the life of the deck.
Phase 03
Lay the decking from the edge you will see
Finally, the rewarding part: seeing the surface come together. Start from the most visible edge and work your way back. If you are using wet, pressure-treated lumber straight from the hardware store, butt the boards tightly together; they will shrink as they dry, naturally creating a drainage gap.
- 01
Square the first board. Every course that follows takes its line from this edge.
- 02
Fasten into each joist. Keep the screw pattern consistent across the surface.
- 03
Trim the final edge cleanly. Finish with one deliberate line instead of chasing every board individually.
The finished transformation
The platform became an outdoor room.
By adding a grill station, string lights, planters, and artificial turf around the platform, the once-drab concrete pad became an entirely new outdoor room.
Building a platform deck over concrete is less intimidating than digging post holes and mixing concrete, making it an incredibly satisfying weekend project for a DIYer. It set the stage for many backyard gatherings and, ultimately, helped me perfect the design for our future, much larger catio project.
Project questions
Platform deck FAQs
The practical checks to make before framing over concrete.
Do I need a permit to build a floating platform deck?
In many jurisdictions, a ground-level platform deck that is not attached to the house, is under 200 square feet, and sits less than 30 inches off the ground does not require a permit. However, always check with your local building code office as regulations vary by municipality.
Can I lay decking framing directly on a concrete slab?
No, you should never place decking boards directly on concrete. Doing so prevents proper water drainage and airflow, leading to rapid rot. Always use a sleeper system or adjustable deck supports to elevate the joists off the concrete. If using sleepers, be sure to use ground-contact pressure-treated lumber and consider joist tape.
How do I secure the sleeper frame to the concrete?
The best practice is to pre-drill holes through your sleepers and into the concrete slab using a hammer drill and a masonry bit. Then, secure the frame using concrete screws (like Tapcons) that penetrate at least 1 inch into the slab.
What is the best way to handle an uneven concrete slab?
If your existing concrete has slight dips or slopes, you can level your sleeper joists using plastic shims. Avoid wood shims as they will rot quickly when in contact with moisture. For more severe unevenness, consider using adjustable deck pedestals instead of a sleeper system.
Does the concrete slab need to drain?
Yes. Ensuring water can flow off the concrete and away from your house is critical. When laying out your sleepers, orient them so they do not block the natural slope and drainage path of the water. Avoid boxing in water beneath the deck.