Martin writes: How do you properly build a concrete deck (with a roof) over a living area and prevent water leakage from rain and snow? What are the code requirements? The location is in Eastern PA.
Steve Bliss, of BuildingAdvisor.com, responds: Thanks for your inquiry. Unfortunately, the building code does not offer a lot of guidance on the best building details for this type of construction. The building code is primarily concerned with minimum standards for life-safety, structural integrity, and to some extent energy usage and basic waterproofing.
However, you need to go well beyond code to build a reliable leak-proof deck over a living space. You can build the structural deck out of concrete, as it sounds like you are doing, or wood-frame construction. Either will work. The drainage system, flashing details, and waterproofing membrane that go over the structural deck are the key to success.
The most common approach is to use a heavy-gauge EPDM membrane (typically 60 mil) as the roofing material, usually placed over rigid insulation. Heat-welded TPO membrane is another good choice. Some key details for a leak-proof rooftop deck are:
- The roof deck surface should sloped away from the house a minimum 1/8 inch per foot so the water will drain and not “pond.”
- The finished walking surface of the roof deck surface should be below the level of the interior floor by at least 1-1/2 inches.
- The roof deck should be properly flashed with a highly durable material that is compatible with the deck material (copper, lead, EPDM, etc.) where it adjoins the house.
- Proper drainage at the edge of the deck must be provided, usually with gutters.
- Special attention must be paid to railing posts and any other penetrations of the roofing surface. If possible, avoid any penetrations by attaching posts to the eaves structure at the edge of the deck.
The EPDM should be installed by a professional roofer with extensive experience in single-ply membrane roofing. If you are using a wood or composite walking surface, fasten the decking to sleepers (unattached strips of pressure-treated wood), which should be set on strips of EPDM to protect the roofing membrane.
It’s best to build the wooden deck in removable sections for cleaning and maintenance of the rubber roof, and repair of the membrane if ever needed. A properly constructed EPDM roof should last 40 or more years. Still leaf debris and organic matter that gets through the slats in the decking will build up on the rubber roof and hold moisture. If not periodically flushed out, this will shorten the life of wood or composite decking.
Careful detailing of this type of construction is critical. The devil is the details, so you want a builder who has built at least of few of these things successfully and has worked out reliable details. Or you might consider hiring an architect or engineer (again, with experience in rooftop decks) to oversee this part of the project.
There are other options that you allow you to walk directly on the roofing surface. One is Duradek, a slip-resistant PVC-based material originally designed for boat decks. The system can be applied directly to concrete and has other components such as an aluminum railing system and must be installed by certified installers.
Whatever the system, make sure you get a long-term warranty against defects in materials and workmanship — and keep your fingers crossed. I built a rooftop deck on a summer home in 19 years ago with EPDM, sleepers, and pressure treated decking and it hasn’t leaked yet. The railing posts are fastened to the roof overhang, so there are no penetrations through the membrane.