Guidelines for Building a Large Rooftop Deck

Weight

• Before you consider building a rooftop deck, you must be sure that your roof can support the deck's weight, and there are several types of weight load that the deck will impose upon the roof. The weight of the deck itself is a permanent load, called a dead load, that will exert a constant force on the roof's structure. Temporary weight loads, called live loads, will come from wind, rain, snow and the traffic of everyday use. A general guideline is that a deck-supporting roof must be able to handle a load of 55 pounds per square foot, but the final judgment on whether a roof is up to the task or not should always come from a qualified structural engineer.

Drainage

• A deck will alter the profile of a building's roof and, consequently, the roof's ability to adequately shed water. The deck must be installed with an appropriate system of roofing membranes and moisture barriers, and the deck surface itself must be sloped so that water runs off the deck and the roof rather than remains on the surface of the deck. The deck is likely to bow into a concave profile when weight is placed on it, so the potential deflection of the decking material must be taken into account; in some cases the deck can be designed with a convex camber to counter any potential downward deflection.

Wind

• The horizontal surface of a deck is susceptible to the upward force of wind, and deck materials must be adequately fastened to resist that force. The deck attachments must also be able to withstand lateral forces exerted on the deck by wind or seismic activity, as well as forces caused by thermal expansion and contraction or settling of the roof structure.

Landscaping

• Although a typical rooftop deck is positioned well above most landscaping, the position of large nearby trees should be considered when you are designing a deck. Trees can provide seclusion for an otherwise exposed deck, or they can block potentially valuable views. Landscaping on the deck itself should take views into consideration, and it also must fall within the deck's weight load limits.