Can you build a deck outside on a roof?
Short answer. Yes, a deck can be built on top of a roof, such as over a garage or a flat section, but the roof underneath must be a low slope waterproof membrane, not shingles, and the structure must be designed to carry deck loads. It needs a permit, proper guards, and a plan for drainage and future roof access.
What has to be true underneath
- A waterproof membrane. Shingles shed water on a slope; they do not waterproof a flat surface under foot traffic. Rooftop decks sit over a membrane system with flashing turned up the walls.
- Slope to drain. The membrane still needs a slight slope toward drains or scuppers so water never sits.
- Structural capacity. A deck adds people, furniture and sometimes planters. The framing must be designed for deck live loads, which are higher than a normal roof's.
How the deck sits on the roof
The walking surface usually rests on sleepers or adjustable pedestals that float above the membrane. Fasteners should never puncture the waterproofing. Sections should lift out so the membrane can be inspected and repaired.
Safety and permits
- Guards: the residential code generally requires guards at least 36 inches high where a walking surface is more than 30 inches above the grade below.
- Permit: nearly every Georgia jurisdiction will want plans for a rooftop deck. See roofing permits in Georgia.
- Door threshold: the door from the house must sit high enough above the deck surface, with flashing, to keep water out.
Common failure points
The wall to membrane flashing, the door threshold, and posts for railings that penetrate the membrane. Plan those details with a designer, a roofer and a carpenter together. For the flashing basics, read roof flashing explained.