Perfect Roof Rafters Focus your efforts on the pattern rafter, and the rest of the roof falls into place easily. Birdsmouth, which consists of two cuts a horizontal seat cut. Roofing Resources Take the mystery out of getting roofing done right. Cutting Common Rafters Plumb cut, typically called the ridge cut, at the top end of the rafter. Scribing a Common Rafter The length of the rafter tail will determine how far. Or 22 in the in field, and the fraction ½, the 1 in the n field and 2 in the d fieldĮnter the ridge board the same way as the overhang using the appropriate fields.Īnd the 8 for the roof pitch has only one field. Laying Out a Common Rafter For simple gable or shed roofs, you need to learn this basic building block of roof framing. ¾ inch: the 3 here is the n value, the numerator, and the 4 is the d value, the denominatorįor the overhang 22 ½' can be added to the in field as 22.5 Otherwise, enter the 44 feet in the ft field Rafter Shortening Allowance As in the case with a common rafter, the line length of a hip rafter does not take into account the thickness of the ridge piece. If you know what 44' 6 ¾' is in decimal, you can add this in the building width ft field The roof pitch is 8:12 (it rises 8 inches in a one-foot run). The overhang for the rafter is 22 and ½ inches, and you are using a 1 ½ inch ridge board. The building width is 44 feet 6 and ¾ inches If you enter a value for the ridge board, then the calculator will subtract half of this amount from the rafter length. The closer the rafters are to each other, the stronger. Typical rafter-spacing codes will specify a minimum spacing of 16 inches, 19.2 inches or 24 inches. If your local building authority has a rafter-spacing code, you may not space rafters wider than required by code.

If you add an eaves overhang dimension, then the calculator will add the amount the rafter sticks passed the wall to the rafter length. Rafters must be strong enough to hold the roof sheathing and the shingles. The calculation includes results for hip/valley factor, slope factor and the roof slope in degrees. It’s cleverly hidden in the 24-inch scale along the edge of the square. The altitude is the number above the table. When you have these two pieces of information, you can then determine the rafter length and lay out the cuts.Calculate the length of a rafter from the roof slope ratio of inches per foot and a building width measurement. On the first line of the rafter table (length of common rafters per foot run), the base of the triangle for all the entries is 12. Timber frame construction detail showing a timber frame common rafter to plate connection, fastened with GRK screws. Even when you’re working from a set of building plans, which list this dimension, always verify it by measuring between the top wall plates, from the outside face of the sheathing on one side to the outside face of the sheathing on the other. Common rafter the main support rafter of the slope between eaves, wallplate and ridge Cripple rafter the rafter connecting a hip and valley Crippled jack. The second starting point is the building width. For example, an 8-in-12 (or 8-pitch) roof angle would rise 8 in. The run, or horizontal part, is always 12 (except in the case of a hip rafter), but the rise, or vertical part, varies. The two starting points for any rafter are the roof pitch - that is, the rise and run - and the building width. After it’s been laid out and cut and its fit has been checked, it serves as a pattern for the rest. Laying out rafters involves some math, but only for the first one. Common spacing for trusses on this type of building is often between 2 and 4 feet, but trusses built with advanced designs may allow spacing as great as 8 or 12 feet. The same concept holds true for roof trusses. They’re also common in remodeling work and in small jobs such as entry-porch roofs. So the space between rafters can be greater without reducing the maximum span. Although trusses dominate new construction in most parts of the country, traditional cut rafters are still the go-to approach to roof framing in some areas.