How to Measure and Cut Rafters for Small Buildings
Rafters for small or large structures are based on the same principles. Rafters are boards that extend from a roofline ridge over the walls of the structure to support a roof. They are spaced at regular intervals and can be made with boards as small as 2x4s and as large as 2x10s. Rafters are used in lieu of trusses, the other main method for roof framing, when trusses are too large or the roof design is too complex for trusses. They are made with three cuts: the ridge cut, the bird's mouth and the heel cut. Does this Spark an idea?
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Things You'll Need
- Circular saw or table saw
- Saw horses
- Plumb line
- Chalk string
- Pencil
- Builder's square
- 2x6 inch boards length of rafters plus three feet
- 4x8 foot plywood board
Instructions
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1
Measure the distance and angle from the center of the roof ridge to the edge of the wall the rafter will cover. To do this, hold a sheet of plywood on its edge on the top of the wall and slide it against the roof ridge. Level the plywood, resting on its long edge. Secure the plywood with temporary nails at the wall and the ridge. With the plywood perfectly level, snap a line with the chalk string onto the side of the plywood from the ridge to the exterior edge of the wall. Remove the plywood.
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2
Take down the plywood. From the point where the exterior edge of the wall touched the plywood, measure 5 ½ inches up (the width of a 2x6) and mark the point. From this new point to the roof ridge intersection, snap another line, then reinforce the line with a pencil so it won't rub off. Your line is now at the same angle your rafters will be, and the plywood is your angle guide for cutting the rafters.
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3
Place a 2x6 on your guide. Ensure the 2x6 is at least 2 feet longer than you need to establish your desired overhang. Lay the 2x6 along your chalk line with one end slightly overlapping the roof ridge end of the guide. Strike a pencil line between the intersections of the board with the plywood guide. This is your plumb cut line. Mark the point at which the board intersects the exterior wall edge.
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4
Determine the pitch of the roof. This is expressed as a ratio of vertical change over lateral distance. For example, if the roof rises 5 inches for every 12 inches of lateral distance, it is a 5-12 roof, 12 being the constant. If the level-distance from the edge of the roof to the centerline of the house is 6 feet and the difference between the height of the edge and center is 2 feet then the pitch is 4-12: 4 inches of rise for every 12 inches of lateral distance.
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5
Place the level where it intersects the point you marked for the exterior wall contact. Strike a square line across the board. Place the inside corner of the square on the line you marked. Slide it up 1 ½ inches from the edge of the board. Holding the inside corner at that point, rotate the square until the end that extends down the long end of the board intersects with the edge of the board. Read the distance from the square's corner to the intersection of the square and the long edge. Then read the distance from the corner to the short-end intersection. Keeping the inside corner of the square on the point 1 ½ inches up your line, shift the square until the ratio of the short side to the long side matches the roof pitch. In other words, if the roof has a 4-12 pitch, which is a three-to-one ratio, then the "long" measurement should be three times the length of the short one. Mark the angle. This notch will be the "bird's mouth" that hooks onto the wall's top-plate.
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6
Cut out the notch for the bird's mouth. Cut the roof-ridge angle parallel to and a few inches out from the plumb cut line you made in step 3. You need the extra to size the board, and if you cut off too much you will have wasted the board as a rafter.
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Place the board where it will go between the ridge and wall. Seat the notch on the wall's top plate and lower the plumb cut end until it touches the ridge-board. Little by little, trim the plumb cut end until it fits flush against the ridge.
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8
Take the board down. At the desired distance, calculated by your desired overhang, make your "heel cuts." Heel cuts are the last two cuts at the overhang end of your rafter. The end cut should parallel exactly the plumb cut at the opposite end of the rafter, so when it is mounted, the overhang end of the rafter will be plumb, straight up and down. Make a second cut on the heel, this time at a 90 degree angle from the cut you just made, along the bottom of the rafter. When mounted, this cut will be perfectly level.
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9
Repeat the process for the bird's mouths and heel cuts on the rest of your rafters. Never cut the ridge/plumb cut until you have checked it in exactly the place where it will be attached. Variations in wood will make each of these cuts slightly different. Cut the ridge cuts long, and trim back until they match the ridge board.
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References
Resources
- Photo Credit cabin image by Jim Dubois from Fotolia.com
