How to Cut Garage Roof Rafters
You can make your own roof rafters, although it's nowhere near as easy as buying them pre-cut and assembled, and requires very careful measuring and cutting. You should only attempt this if you’re an experienced carpenter. Nonetheless, by doing it yourself you do have more control, and with some size buildings, for which you can’t find pre-cit rafters, you might need to do it yourself.
Step 1 - Rafter Length
Before you do any cutting for the roof rafters you’ll need to make a number of measurements and calculations. The first thing to understand is how the pitch of the roof affects the length.
Begin by measuring the width of the building. After noting this figure, decide on the pitch of the roof. A 6/12 is standard, meaning that the roof rises 6 inches in height for each 12 inches in length. Where the width of the building is 10 feet, half of that (where the ridge stands) will be 5 feet, or 60 inches. With a 6/12 pitch this means that the length of the rafter must be 67 1/16 inches (the formula is run multiplied by secant).
Step 2 - Measuring the Rafter Angle
Start by having the board for the rafter (usually 2 feet, 6 inch by 12 feet pine) on flat ground. Mark on the top right hand corner of the wood. Lay a carpenter square so the board sits inside the square’s notch. If you do it this way the length of the board will be on the long part of the square and the width will be on the short part of the square.
Check the width of the wood. If sold as 6 inches, it will actually be 5 ½ inches. Make a mark on the length of the board, on the opposite side to the reference mark you’ve made, that is half that measurement, or 2 ¾ inches. Join the two marks you’ve made on the roof rafter. With your saw, cut along this line. Repeat for each of the rafter’s you’re putting up.
Step 3 - Rafter Length
The next thing to calculate is the length of the rafter, which will largely be determined by the overhang you’re planning. The simplest way is to hold the rafter against the ridge and see how far you want the roof rafter to hang over the side of the building. With that done, lay the rafter down again.
Step 4 - Cutting
Rather than go through more complex calculations, draw a line on the driveway as if the top of the roof rafters were touching the ridge. This line will go straight down and be as long from the peak of the ridge to the joists in the building.
From here, draw another line at 90 degrees extending out to the board. You will also need to know how wide the framing on the building is. In this case, say 2 inches. Where the line meets the board, extend it by 2 inches. Draw another line from this going down 90 degrees. Cut out this notch, and then cut the bottom of the rafter along a line parallel with the line you’ve drawn.