How Do I Build a Porch Floor and Roof?

Building a concrete porch and basic wooden roof

• 1

  Measure and mark the area for your concrete porch with wooden stakes and builder's twine. Then excavate the area about 10 inches deep -- 6 inches for gravel base, 4 inches for concrete. Dig holes at outside corners for 4-by-4 posts; make holes about one-third the final height of your post, i.e. 3 feet for an 9-foot post. If your span is more than 10-12 feet, dig holes in the center of each side for more support. If there is a long span across the front of the house, set posts about three feet apart on either side of the center.

• 2

  Set your posts in concrete. It is best to mix concrete and tamp it in firmly around the post, but you can also fill the hole with dry concrete, tamp it firmly, then wet it down to set it. Let posts set at least overnight. Check that posts are plumb and form square corners. Make sure posts are the same height, and the correct height. Your slant roof needs a slope of at least 1 inch for every foot. For example, if your outside posts are 9 feet high and your porch is 12 feet long, you will need the posts at the house end to be 10 feet high.

• 3

  Compact your dirt with a hand or mechanical tamper. Then build a 2-by-4 board form around the edges, to the outside of the posts. Lightly coat the wood with vegetable oil to prevent concrete from sticking. Make sure corners are square and stake the forms securely in place. Then add gravel in layers, compacting each layer, until the base is about 6 inches thick; it should come up to the bottom of your 2-by-4 forms. Lay welded wire or similar reinforcing on top of the gravel.

• 4

  Pour your concrete. Place a rubberized or asphalt-type expansion form against the house side if the slab will butt up to a foundation. Fill the concrete to the tops of the forms, then screed it with a 2-by-4 or similar tool to compact it and level it. When the concrete starts to set, smooth it with a trowel; you can lay a board across the top of the forms to give you a working surface as you trowel. Let the concrete set and remove the forms.

• 5

  Nail or screw on a 2-by-6 inch ledger board (a supporting board that fastens to beams or joists in the house wall to support the roof) to the house wall. Nail or screw another 2-by-6 across the outside corner posts. Make sure both boards are level and the slope between them is correct. Nail metal joist hangers on the two 2-by-6s 24 inches apart, then nail in 2-by-6 joists from the wall to the outer edge of the roof. Joist hangers can be bought at any building supply store -- they fit over the top of one 2-by-6 and the ends of the abutting board to secure it.

• 6

  Sheath the roof in oriented strand board (OSB) nailed to the roof joists. Then lay roofing paper and nail down shingles. Put a metal drip edge under the shingles on all sides. You also could install corrugated metal or other roofing, but make sure it conforms to residential codes and matches the style of the house.