In order to build a slanted roof, one of your shed's walls must be higher than the other. If the shed walls are level with each other, you will need to build up one wall either by adding top plates or by constructing a small frame wall and adding it to your existing wall prior to constructing the slanted roof.
Measure the total span of the shed. The length of your common rafters will be based on the unit rise and total run, which is the total span minus the thickness of one wall. To measure this, hook your tape to the outside of one exterior wall and measure to the inside of the opposite exterior wall.
Measure the total rise of the shed slant roof. The total rise is how many inches your roof rises per 1 foot. A good way to get the unit rise is by measuring how much higher your one wall is from the other and then add the thickness of the common rafter to this number minus the seat cut. Take this number in inches and multiply it into your total run in feet. When you have this number, divide it by 12. Twelve is how many inches are in a foot. This will tell you how long the common rafter will need to be to span the shed from outside of one wall to the inside of the other wall. When you purchase the common roof rafters you will add extra length to the common rafters for the overhangs on each side and the thickness of one wall. The overhang length will be up to you.
Measure and cut your common rafter seats. Seat cuts are notches that let common rafters rest on the wall plates. First, measure how long you want your overhang to be and mark a line; this is where your seat cut line will be located. Using a carpenter's square, make a mark on the outer edge of your square at the 12-inch mark for your unit run, and then put a mark on the other outer edge at your unit rise. Next, place the square on your common rafter with the marks you made flush with the edge of your rafter on your line. The unit rise mark will be on your line, which will be called a heel plumb line when marked, and the unit span mark will be facing toward the other end of your common rafter. The heel plumb line will run parallel to your exterior wall. Take a pencil and mark the outer edge of the square. Next, slide the square to the left until it equals the width of your shed top plate and mark a line; this is the seat cut line.
Cut the other seat cut from your common rafter. Measure the total run of the common rafter from the heel plumb line. Next, place the carpenter's square on your mark and repeat step three. When finished, both of the seat cuts will be facing the same direction.
Cut out the remaining rafters, then lay them out and nail them together; you can use block stiffeners and fillers. For the overhangs, you will make a chalk line where you would like them to end. Using a sliding T-bevel, mark your cut-off point. Next, install your fascia board and fascia rafters on each end. With the fascia rafters, ladder-framed lookouts will work best. After framing the slant roof, do the layout and nail plywood.