![]() It also provides the distance between the short points of the ridge cut and the birdsmouth cut of the rafter, 156.343, which converts to 156 5/16 inches. This triangle provides the height to the bottom of the ridge, 69.875, which converts to 69 7/8 inches. This comes to 11.65, which is the common multiplier needed to expand the 6-12-13.42 triangle into the larger, 69.87-139.75-156.34 triangle. To find how many 12-inch increments there are in 139 3/4, convert 3/4 to. Opting to have the hypotenuse of the big triangle run along the underside of the rafter means the base of the large triangle equals 139 3/4 inches. In the drawing of the 24-foot-wide building (“Ridge Height and Common Rafter Layout”) shown below, the short point (sometimes called the heel) of the birdsmouth cut of the rafter lands 2 inches in from the inside of the building’s 2圆 wall. A 6-12 right triangle has a diagonal-or hypotenuse-of 13.42.įirst, establish the size of the base of the large triangle. To find the diagonal for a triangle with a rise of 6 and run of 12 (for a roof with a 6-in-12 pitch), look up the first entry beneath the number 6 on the framing square’s top scale. The task now is to expand this little, 6-12-13.42 triangle into a much larger triangle that preserves the angles of the little triangle and fits the roof you are building. The hypotenuse of the triangle is provided in the decimalized number just under the number indicating the altitude.įor a 6-in-12 pitch roof, then, assume a base of 12 and look under the number 6 to find the hypotenuse, or 13.42 (see annotated framing square, below). It’s cleverly hidden in the 24-inch scale along the edge of the square. ![]() The altitude is the number above the table. 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. #Rafter length table how toThese triangles are well-camouflaged on the square, but they are there, and learning how to decipher them in the 1970s proved to be a turning point in my own quest to learn how to frame a roof. Unlike the brace table, which uses the same isosceles triangle for the first 13 entries and the familiar 3-4-5 triangle for the final entry, the rafter table provides the base, altitude, and hypotenuse for 34 different right triangles, each with angles that are different from all the rest. #Rafter length table codeWith the purlin/bracing installed, the long rafter span would be broken up into two shorter spans, thus allowing the rafter to carry additional load.īefore you make your final decision on lumber sizing and span, consult your local building code officials to determine design loads for your area.The front of a standard framing square is etched with rafter tables on the wide blade and the octagon scale on the narrower tongue. You can break up a long span of rafters by installing a purlin and bracing on the underside. When calculating the span of joists or rafters, use the horizontal distance between to vertical supports. The #1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller #1 grade joists or rafters in your design, youĭepending on the availability and cost of various size framing lumber, it might be more economical to use a larger size joist or rafter in your design, because you can typically use fewer of them. For example, you can span a #1 grade (no knots) southern yellow pine joist a greater distance than a #2 grade of the same species (with knots). The lengths and sizes of joists vary depending on the species and quality of the lumber used. ![]() To calculate maximum spans additional species of lumber, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.Ĭonsiderations when using the Rafter and Joist Span Tables Contact your local building code officials to determine the building code for your area. Some other regions have special wind and seizmic requirements that should be considered when designing wood structures. For example, some regions get large amounts of snow, whereas others do not get any. Before you can determine the maximum spans for joists and rafters, you must know the load requirements for your given area. ![]()
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