DIY: Deck Above Porch

Coat the porch roof with elastomeric paint and allow it to dry to create a rubbery, waterproof surface under the deck. Lay roof walkway mats under the planned deck area to protect the roof as you work on it; leave them in place to support the duckboards.

Climb a ladder, measuring tape in hand, to check the distance between the centers of the roof joists in the porch ceiling. Design the duckboards so that their sleepers, or crosspieces, line up with the porch ceiling joists. For example, if your ceiling joists are either 16 or 24 inches on center, you could make duckboards roughly 4 foot square, with respectively either four or three sleepers to align with the joists below.

Sketch a deck plan on graph paper that incorporates uniformly sized, square duckboards of up to 6 feet square, or smaller if you cannot handle moving such a large size. Note in your sketch the relationship of the duckboards to an existing door to the porch roof. Plan to run at least one duckboard to right under the door threshold to keep visitors from stepping right on the roof surface. Note with dotted lines the porch ceiling joists on the plan and align the duckboard sleepers with them.

Cut your 2-by-4 sleepers with a chop saw or circular saw and lay them out on a work surface. In the example of a duckboard roughly 4-feet square, over a porch with joists 16 inches on center, evenly space four 4-foot-long 2-by-4s so that the center of one is 16 inches from the center of the next. Where two duckboards will abut each other over a joist, design a shorter abutting side so it falls on the midpoint on the joist.

Lay your decking, for example, 1-by-6 pressure-treated pine lumber, perpendicular to the sleepers. Cut the decking with a chop saw to length, so it fits from one sleeper end to another. Fasten the decking to the sleepers with galvanized decking screws, two at each junction of the deck board with a sleeper. Lay the completed duckboards on the porch roof according to your plans.

Bolt 4-by-4 posts cut to 48 inches through the band joist atop the porch wall with two ½-inch galvanized bolts per post base. Space the posts to code, typically no more than 6 feet apart. Construct top and bottom rails of 2-by-4s with 2-by-2 balusters spaced every 3 ½ inches between the rails and screw these to the posts with long wood screws.