Many people find that as their family grows their storage space shrinks. This cramping includes the addition of a second family car. A lean-to carport is the perfect solution to protect a vehicle from the elements. The beauty of a lean-to is that it rests against the side of the house or existing garage. There are many different designs for a lean-to, but the basic principles of building are the same.
Determine the height, width and length of the lean-to carport. For this example the measurements are 7.3 m (24 feet) long, matching the length of the garage. The width of the carport is 4.6 m (15 feet). The rafters are standard size, 5 m 5cm x 15 cm (16 foot 2 x 6 inch) boards. The posts are 10 cm x 10 cm (4 x 4 inch) pressure treated lumber. Outer exterior construction uses 2.5 cm x 15 cm (1 x 6 inch) pressure treated lumber.
Install support headers to the existing garage. A support header will bear the brunt of the weight of the structure in conjunction with the support beams. Measure the length and width of the support header; the header needs to be straight and level all the way across the beam. If the support beam is not straight the lean-to structure will not be plumb.
Measure the amount of leaning, bowing and bulging of the garage the lean-to will rest against using a plumb line. A plumb line is a suspended string with a weight attached. The weight gives the string a perfect vertical line to measure backwards to the wall. Hammer a nail into the subfloor overhead in the garage. Look at and determine which part of the foundation wall of the garage is bulged or leaning the most. Hammer the nail into the subfloor overhead about a foot in front of the bulge. Stop the weighted line from swinging and make a measurement of how much the foundation has moved. This is taking a plumb line measurement.
Measure the location of the support posts using the plumb line measurements. Measure the distance between the wall surface and the vertical plumb line with a tape measure. Repeat this measurement every 40 cm (16 inches) along the garage wall. Write the measurement on the exterior wall of the garage. This will indicate where the exterior wall bulges out the most.
Find a place on the garage wall where zero movement has occurred. Take the plumb line measurement and subtract the baseline measurement. The difference between these two measurements determines the location of the support post holes in the ground for the lean-to. The zero movement measure is located closest to the garage foundation. Measure the distance between the foundation wall and the vertical plumb line at the bottom of the support wall.
Dig the support post holes. You want the exterior lean-to wall to be plumb with the exterior garage wall. The marking of the holes in the ground will not be perfectly straight, but the goal is to have the support header bear most of the weight efficiently. Dig down about 60 cm (2 feet).
Insert the support posts into the holes. Make sure the support posts are plumb when you insert them into the holes. This requires accurate measurement from the garage wall to the width of the lean-to. Tap support braces into the holes and screw them to the support posts as they sit in the holes.
Prop two end boards straight up to calculate the height of the roof. Don't dig holes for the two end boards. These boards extend higher than the support posts since they are the height of the outer roof of the lean-to. Mark the height desired on the end posts with a carpenter pencil. A common height is 2.4 m (8 feet) at the lower point and 3 m (10 feet) at the highest point. The highest point is against the garage.
Support the lean-to rafters with two front beams. One front beam connects to one of the marked end posts and the other front beam connects to the other end post. Make sure the boards at the end are plumb using the same calculations in steps 2, 3 and 4. Connect joist hangers to the support beams to allow the rafters to stay in place. A joist hanger is a metal "V" that connects the main beam to the rafter, providing support to the lean-to structure. The beam and the rafter slide into the joist hanger and a screw holds both in place.
Cut the rafters to proper length with a power saw. Use a mitre box to make sure the angles of the garage match that of the support header for the lean-to. Make another cut with the power saw at the opposite end of the rafter. Make a cut at the same angle and attach a joist hanger to each end. Attach braces to the metal joist hangers at each end of the rafters. Screw in a header to give additional support to the inside of the lean-to structure along the rafter.
Slide the front braces into place at a 45-degree angle on all sides of the lean-to roof. Attach the braces to the support posts with screws. Make sure the front braces attach to the outer part of the header. Add trip to create uniformity between the garage and the lean-to.
Screw in the sheet metal roofing material to the exterior of the lean-to ceiling. Finishing moulding is available to enhance the aesthetics of the lean-to if desired.
Use paint or stain to protect exterior wood from damage from weather.
Maintenance cost of a wood lean-to is higher than a metal structure.