Building A Carport

by Darwin Hall

carport design

A carport is a covered parking space, usually attached to the house, that is not completely enclosed by walls.

There are two improvement projects that involve carports.  One is to convert an existing carport into storage space or living space by enclosing it.

The other is to build a new carport.  Sometimes a new carport is needed to replace an old one.

It can also be a roundabout way of adding new living space to some homes without building a room addition.  If you are wondering how to add an extra room in a home that has an attached garage, you might consider building a carport and converting the garage into an extra room.

Planning.  Consult your local building department about necessary permits and code restrictions.  Local codes may specify minimum dimensions for parking spaces, minimum driveway area, or maximum driveway slope.

In addition to setback limits and specifications concerning the maximum percentage of the lot that can be covered by site improvements, these restrictions will effect the size and location of your carport.

Planning will be much easier if you have a driveway beside your house with a setback that allows room for a new structure.  If your driveway is on the property line or within the setback, you will probably have to build a fire wall.

Check with your building department before you make any plans; you may not be allowed to build at all.

Building A Carport

The actual plan for building a carport is quite simple.  It should be at least 12 feet wide and 20 feet long for one car, or 20 by 20 feet for two cars.  The roof should have a minimum slope of 1 in 12, but a slope of 3 in 12 or steeper is better in snow areas.

carportThe roof usually slopes away from the house, so the house must be tall enough for the low side of the carport roof to be at least 8 feet high.

Otherwise, the roof will have to cantilever over the top of the house to attain the proper slope.

Floor and footings.  Although a gravel floor may be adequate in some cases, the best design incorporates a concrete slab at least 4 inches thick.

Most codes require that a new slab extend into the ground even farther, usually to a minimum depth of 12 inches or to the frost line.

To build a new slab, excavate to the depth of the concrete plus 4 inches to accommodate a sand base.  Excavate around the perimeter for a footing 12 inches wide.  Build forms around the outside of the footing, so their tops are at the desired final elevation.  Place a 3-inch layer of sand on the bottom of the slab area — not in the footing trench.

Cover the sand with a layer of 6-mil polyethylene sheeting to provide a vapor barrier, and cover the sheeting with the rest of the sand.  Put steel reinforcing bars in the footing and steel mesh in the slab.  When you pour the concrete, place a metal anchor at each post location while the concrete is still fresh.  The slab should slope away from the house.

See also:  Maintaining A Garage

If the existing driveway does not have a sufficient footing to support the posts or a wall, build individual footings for all the posts.  Break holes in the driveway at each post location, and excavate to the depth of the footings.  Fill the holes with concrete and place post anchors in them.

Posts.  Locate posts 6 to 8 feet on center, depending on the size of your beam.  A 4 by 4 post is adequate for most carports, but 4 to 6 stock and steel columns are also common.  When the concrete is cured, install the posts.

Attach each one to the post base with 1/2-inch-diameter carriage bolts.  Brace each post in two directions with diagonal braces after plumbing it with a level.  Do not cut them to length until all are erected.

When all the posts are up, measure and mark both corner posts at the height at which the beam will rest.  Stretch a chalk line between these marks and snap it against the posts.  Repeat the procedure on the other posts.  Mark cut lines, saw off the tops of the posts, then attach the beam on top with a metal beam connector.

An alternative method is to measure longer posts and bolt a 2-by stringer to each side.  The inside stringer should be slightly higher than the outside stringer, so that the sloped rafters will rest on both.

Rafters.  Rafters, supported by a beam and a ledger bolted to the house wall, will span the width of the carport.  To install the ledger, mark a level line along the wall of the house at a height that yields the required slope.  Two-by rafter stock makes a fine ledger.

Cut to length and mark the rafter spacing.  Nail joist hangers where the rafters will attach.  Determine the location of the studs and secure the ledger to them with lag bolts.

If the span is long, consider using 4-by lumber.  Once you have selected stock, you next task is to cut a pattern rafter.

Using a carpenter’s square, mark the plumb cut at the house end.  After making the plumb cut, calculate the dimension of the bird’s mouth by using rafter framing techniques.  Finish cutting the pattern rafter, then test it to see if it fits.

Make any necessary adjustments, and cut out the rest of the rafters.  You may have to notch the bottom of each plumb cut slightly so that it will fit into the joist hanger.  Nail the rafters into the joist hangers and toenail them into the beam.  Install blocking over the beam and at the midpoint.  If the rafters overhang the beam, attach a fascia board.

The Roof.  Roofing plywood should be at least 1/2-inch thick.

Watch the VideoNail plywood over the rafters, laying sheets perpendicular to the rafters and staggering end joints at least 4 feet.  Install flashing around the edges and along the house wall.  Install roofing trim to match the house, and install a gutter and a downspout on the outside edge of the roof.

Dotted Line Post Separator

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