Building a deck on a flat roof is neither difficult nor time-consuming. Two people can install a brand new deck in a weekend, with little or no previous construction experience. Before beginning, however, it is important to do two things: inspect the roof to make sure it is in good condition, and acquire any permits that may be required by the local zoning board.
Step 1 - Simplify the Project
The joists of the deck, as with most flooring, will be set out on 16 inch centers. To simplify a construction project of this type, adjust the size of the project to be divisible by 16. For this project, the deck will be relatively small, but will be sized to use factory cut lumber, saving a tremendous amount of construction time. This deck will measure 16 feet by 12 feet. To build it, you will need four 16 foot 2x6, and 44 12 foot 2x6. Order one box of 16 diameter nails.
Step 2 - Order Materials
If you order the lumber from a major hardware store and let them know that you are building a deck on a flat roof, the material can be delivered on a truck with a lift. Make sure that someone will be home when the material is delivered, and you have it set directly on the roof in a convenient work area. It is a very good idea to lay down a canvas or heavy drop cloth. If you have two 4x4 posts or landscape timers available, lay them crosswise to the direction of the lumber, laying across the rafters, not parallel to them. This will lift your material up from the roof, and the drop cloth allows trimming the materials in place rather than transporting it.
Step 3 - Prepare the Materials
Set two of the 16 foot boards on each side of the spot where you want to build the deck. These are your end caps. Cut 12 of the shorter 2x6 to measure exactly 11 feet 9 inches. These are the joists for the deck. Place one flush with each end of the deck, and nail the end caps to them. Start at either end, and mark the end caps every 16 inches all the way to the other end. Cut the two remaining 16 foot boards into pieces that are 14½ inches long.
Step 4 - Assemble the Deck
Place one joist every 16 inches, lining the center of the board's edge up with the marks on the end caps. If the fit seems too tight, check the length to make sure that it is 11 feet 9 inches. If it is, use a hammer to tap the joist into the correct position and nail in place. Place the deck planks every 6 inches, which will leave a small gap between each plank. Nail the planks in place. Install the 14½ inch pieces as braces throughout the joists, with a staggered row extending across the center of the deck.
Step 5 - Add Handrails
A roof deck is almost certainly going to be required by law to have handrails. The specific requirements will vary from one location to another. When you know the requirements, use 2x2 lumber for the rail posts, and connect a 2x6 to their tops edgewise for the handrail.