A carport is structure used to provide shelter for vehicles, just as a garage does. The difference between the two is that while a garage is a fully-enclosed structure, a carport is open-sided, using only a roof to shelter the vehicles contained inside it. Building a carport can provide some protection to your vehicles at a lower cost than constructing a fully-enclosed garage.
Drive stakes into the ground around the perimeter of the carport's location, forming a square that measures 20 feet by 20 feet.
Dig six post holes for the carport, three on each side. Place one post hole in each corner, and one in the center on the sides of the carport. Dig the post holes 6 feet deep with an augur. Place concrete tubes into the holes for the post footings.
Mix a batch of concrete, following the instructions on the packaging.
Pour about a foot of concrete into the first hole and set a 20-foot long, 4-by-4 post into the hole. Check the post for plumb (vertically straight) with a level, and continue filling the hole with concrete. Check the post for plumb again.
Install the other five posts into the ground, using the same technique, and allow the concrete to cure for four to five days.
Excavate the ground for the carport's concrete base in between the posts with a 1 1/2-ton excavator, using a shovel around the post bases. Dig deep enough to go beneath the frost line in your area. Consult your local zoning board to learn how deep this is.
Drive wooden stakes into the ground every 3 feet and nail 2-by-4 boards to them, laying the boards on edge against the ground. This is the form for the slab when you pour the concrete.
Place a vapor barrier over the bare soil in the hole. This is a plastic sheet that protects the foundation from moisture in the ground.
Cover the vapor barrier with sand or gravel, following local building codes regarding which material to use and how deep the layer needs to be.
Lay wire mesh or rebar on top of the base, again following local building codes.
Fill the hole with concrete. For a project this size, you may be better off hiring a concrete contractor to do the pour and finish the slab. Allow the concrete to cure for at least four to five days.
Attach a string to the top-front corner of the front post on each side of the carport, and run the strings back to the last post on each side. Attach the strings on the back posts 8 inches down from the top of the post. Trace the line created by the string onto the posts, then remove the strings.
Cut off the tops of the posts, following the lines from the string. This creates a slope for the roof that allows water to drain off.
Cut two 2-by-8 beams to a length of 20 feet. Lift each beam against the posts on the outside edges -- you'll need help with this -- until the top of the beams are flush with the top of the posts. Clamp the beams into position.
Drill two holes through the 2-by-8s and the posts where they meet. Connect the beam to the posts with M12 galvanized bolts. Connect galvanized angle brackets to the insides of the joints with four M12 galvanized bolts, then remove the clamps.
Measure and cut 2-by-6 beams for the rafters. The rafters should match the distance between the two inner edges of the beams on the sides of the carport.
Mark the location for the rafters on the side beams and nail galvanized metal joist hangers to the side beams. Keep the rafters no more than 3 feet apart.
Place the rafters into the joist hangers and nail them to the joist hangers with galvanized nails.
Measure the distance between the rafters and cut 2-by-8 boards to fit. These boards are the blocking, which helps to strengthen the roof. Install the blocking using the same process you used to install the rafters.
Nail the roofing material to the rafters, blocking and outer beams with galvanized nails. You can use sheets made of PVC or steel.