Flat roof decks are very popular among homeowners, because you can use your otherwise unused roof space, to create a private retreat for yourself, and to host parties / barbecues and other gatherings outside. Have a roof deck also increases the value of your home, as most people want to have a roof deck / patio when buying a home. Even many condos / apartment buildings have roof-top decks, patios and balconies, for these same reasons.
However, a roof deck poses certain design difficulties and the waterproofing membrane under the roof decks often leak, which requires taking the deck apart and fixing the roof, which can be a very costly project. It is best to use proper roofing material from the beginning, and to design the roof deck with proper water drainage, so ponding water under the deck does not create leak problems.
Most popular types of flat-roof deck assembly:
Most common roof decks today are floating wood or composition deck installed over waterproofing / flat roof materials – typically a rubber roof (EPDM), but other roofing membranes and materials are also used, such as PVC roofing, Modified Bitumen & Tar / Asphalt built-up roofs.
On top of the roofing material, a floating roof deck is constructed, and laid down on slippers (deck rafters laying on top of the roofing membrane). Railing posts are typically installed around deck perimeter, by ether attaching them to the roof framing/rafters, to the outside of the roof, or built into deck. Second and third methods eliminate roof penetrations (which often cause leaks), but are not as sturdy, as posts mounted to roof rafters.
Issues with flat roof decks and roofing materials under them:
Most of the roofing materials used under roof decks will leak if there is ponding water on the roof. Ponding water can happen due to inadequate roof drainage or roofing material failures. Most flat roofing materials (except for PVC roofing) rely on adhesives to seal the seams and flashing around penetrations, and these adhesives break down with time. Ponding water further increases the rate at which seam adhesive fails, and causes the roof to leak.
It is best to foresee all these issues before constructing a roof deck and to design proper drainage of the roof, such as creating a slope on the roof (typically 1/4″ per 1 foot roof pitch) or installing a center roof drain where design and local conditions do not allow for roof to drain off the edge. Roof drains can be problematic as the can clog, and water will back up and stand on the roof, which will likely cause a leak.
Another issue that you should consider is the clearance between the door sill and the roof. Often, this clearance is just an inch or two and often this is where the roof may/will leak. Architects and home builders should make this clearance at least 4 inches, even if it creates a step-down from the door leading onto roof deck.
Choosing flat roofing material for your flat roof deck:
Considering that most roof decks will leak due to seam / flashing failure, which most often will happen due to seam adhesive break-down, we recommend using IB PVC flat roofing membrane as the waterproofing material under roof deck. PVC roofing has hot-air welded seams, and once the seam is welded properly, the overlapping sheets of membrane become essentially one piece, and these seams do not come apart.
Regardless of the roofing material you choose for your roof deck, proper roof slope should be added either during construction or using tapered (sloped) insulation system. It is also recommended to use the thickest membrane you can get, and to go above and beyond on sealing the seams, and probing them, before installing the floating deck. It is best to use a single sheet of membrane if possible, and to have no seams at all.
Optionally, many PVC roofing manufacturers have developed a special Roof Deck membrane such as IB Deck Shield flat roof decking membrane, which has a special slip-resistant surface that is safe to walk on even when roof is wet, and does not require a floating deck on top of it, which reduces the overall roof deck costs, and is much easier to repair if there is a leak, as roof is exposed. Note, that such roof deck materials are very durable, and extremely puncture resistant when installed over proper substrate, such as DensDeck or plywood.
Roof deck repairs:
If you are building a new roof deck, or using a floating deck – it is best to build it in sections. In case the roof leaks, you can easily remove smaller deck sections to repair the leak, rather than taking the whole deck apart, which will be very costly. It is also “required to have membrane slip-sheets installed under deck rafters to separate the deck from the roofing material.
When roof deck leaks, you should remove one or more sections of the deck, to find the leak, and repair it. Always check the seams to find if there are any voids. Also check roof to wall flashing, and how the membrane is attached under the door sill. If you are hiring a contractor to repair your roof deck, hire only roofing contractors that have extensive experience with flat roofing or specialize in flat roofing. Do not hire roofers who install mainly asphalt shingles roofs or handymen. These contractors will 99% of the time will not be able to properly find and repair your roof leaks.