How to Build a Flat Roof Carport

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A carport is an open-sided structure that is used to provide partial shelter for a car, truck or other vehicle. It basically consists of a roof set on posts, which makes it easier to build than a garage, but the lack of walls provides much less protection from the elements -- and much less security -- than a garage does. A carport can be built as an addition on the side of a house, or as a stand-alone structure.

Spray-paint the ground to mark the location of the carport. In this example, the carport will be 15 feet long and 10 feet wide.

Mark the ground where the posts are being installed. In this instance, we'll use three posts on each side -- one in each corner and one between the corner posts.

Dig holes for the posts. The part of the post in the ground should be at least one-third the post's total length, so if you're using 15-foot-long posts, at least 5 feet needs to be buried. Using an augur can make the job much quicker and easier than digging by hand.

Insert concrete tubes in the holes to protect the footings for the posts. Pour 6 inches of gravel into the bottom of each hole and tamp it.

Drive a wooden stake into the ground at each corner, placing the stakes in line with the post holes. Tie a string tightly to the stakes on the sides. The strings will be used as a guide for setting the posts.

Prepare a batch of concrete, following the manufacturer's instructions. Pour about 1 foot of concrete into one of the holes. Slide the 4-by-4 post into the hole and check it for plumb (vertically straight), and position it so that it's flush against the string. Fill the rest of the hole with concrete, then check it for plumb again. Allow the concrete to cure for the time specified on the packaging.

Set the other posts into the ground the same way.

Dig out the ground inside the carport area with a 1 1/2-ton excavator. Dig beneath the frost line (where frozen ground is an issue) to protect the concrete slab. Your local zoning board can tell you how deep this is. Dig by hand using a shovel near the footings for the posts.

Drive wooden stakes into the ground every 3 feet around the perimeter of the hole and nail 2-by-4 boards to them.

Lay a vapour barrier over the soil inside the hole. This is a plastic sheet that prevents moisture from seeping up through the ground.

Cover the vapour barrier with sand or gravel, using whatever your local building codes call for. Add rebar or wire mesh over the sand or gravel, again following local codes.

Pour concrete into the hole until it's slightly above the wooden form. Drag a 2-by-4 board across the top of the concrete to smooth the surface. Add more concrete to any low spots that appear, then drag the board across again. Allow the concrete to cure for at least four or five days.

Clamp a 15-foot 2-by-8 beam to the outside edges of the posts, positioning the beams so that their top edges are flush with the post tops. You'll need assistance when doing this.

Drill two 1/2-inch holes all the way through each beam and post. Slide 1/2-inch galvanised bolts through the holes and secure them with nuts.

Measure and cut the rafters for the roof to match the distance between the inside edges of the beams. The rafters should be no more than 3 feet apart, so you'll need five for a 15-foot carport.

Lift the first rafter into position near the ends of the beams -- again, with help -- and attach it to the beams by placing galvanised metal joist hangers on the inside corners of where the rafter and beams meet and nailing them together. Do this for the four remaining rafters.

Cut four 2-by-8 boards for the blocking, which will run down the centre of the roof between the rafters. Measure the distance between the inner edges of the rafters to determine the length of each piece of blocking.

Install the blocking using the same technique you used to install the rafters.

Install the roofing by connecting it to the beams, rafters and blocking. You can use either PVC or steel sheets.

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