Besides making your deck more comfortable in both wet and sunny weather, a roof also protects the deck from the elements. The best roof choice depends on the shape of your deck, as well as the architecture of your house. The optimum time to build the roof is when you're building the deck, because you'll need larger footings to support the deck and the roof than you would need for just the deck.
If your deck is long and narrow, you may opt for a shed roof, which is the easiest type of roof to construct. It attaches to a ledger board secured to the house with bolts and extends to a point just beyond the deck railing, supported by the same posts that support the deck -- if the footings are large enough -- or by dedicated posts. The pitch of the roof is a limiting factor on the depth of the deck it can cover; the flatter the pitch, the more deck space it can cover. You need a steeper pitch if you use shingles than you do if you cover the roof with corrugated metal or plastic.
A gable roof is slightly more complicated to build than a shed roof, but it better fits a square deck or one that is deeper than it is wide. This triangular roof consists of a central ridge from which the roof slopes down on either side. You can attach one end of this roof to the building or make it freestanding by supporting it entirely on posts. Gable roofs work with many house designs, and the sides usually have a steep slope that shed water and snow more easily than flatter roofs.
The third main type of deck roof -- the hip roof -- is similar to a gable roof in that it has sides that slope from a central ridge. It has an important difference that makes it more resistant to high winds: The front and the back of the roof also slope. The edges of the roof form a rectangle that extends just beyond the edges of the deck on all sides. This type of roof requires an extensive support system and is the most complicated type of roof to build, but it can turn an exposed freestanding deck into a multiuse pavilion.
An awning provides an quick way to provide shade and some degree of rain protection for your existing deck without the need to add posts or a structured roof. You can erect an awning that is supported by posts secured to the deck, or you can install a retractable one that has its own lightweight supports. Fixed awnings, which are typically made from canvas, can be surprisingly sturdy, and with the proper pitch and tension, they can even shed snow. Retractable awnings aren't as durable, but they can be stowed before the onset of severe weather.