Purlins are to have minimum end bearing of 80mm. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. Holes and notches cannot be drilled or cut into the purlins unless proved adequate by specialist calculations. The calculations used for these span tables are based on rafters having two equal spans. Since continuous rafters load the purlin more severely, this is the case considered. The rafters may be continuous or may consist of shorter lengths joined at the purlin. These span tables are based on purlins for a single span, if the purlins are two span continuous or more over intermediate supports you can conservatively consider the longest span between supports. It is also assumed ceiling joists will be used to transmit the horizontal component of thrust occurring at eaves level to complementary rafters. The major axis of the purlin is perpendicular to the rafter slope. These span tables do not apply to trussed rafter roofs. The dead loads shown at the top of span table above do not include the self weight of the purlin, however, the purlin self weight is included (in addition to the dead loads) in the calculations used to determine permissble clear spans. The allowable clear spans have been calculated in accordance with BS 5268-2:2002 Structural use of timber - Part 2: Code of practice for permissible stress design, materials and workmanship and BS 5268-7.6:1990 Structural use of timber - Section 7.6 Purlins supporting rafters. It is common practice to install rafters with the ends running long, then cut them later.Dead load per square metre (in kN/m²) supported by purlin ![]() If you plan to have an overhang on your rafter tails, the rafter length will be longer. It is the length of a line beginning at the inside of the Birdsmouth drawn parallel to the edge of the rafter. Note that this is not the actual end-to-end length of the rafter. Rafter Length - The rafter length result from this calculator represents the Line Length of the Rafter(L). The angle information is valuable when performing roof framing tasks using tools that are not roof-pitch calibrated, such as the Bevel on a circular saw, which is degrees calibrated. ![]() The roof pitch calibrations on a Speed Square are also used for quickly marking the pitch cut angles on the head and tail ends of rafters.Īngle - Angle(A) is the roof's Pitch(X) represented in degrees. If you know a roof's Pitch(X), you can use the information to lay out rafters with a Framing Square. Pitch/Slope - Pitch and slope, often used interchangeably on the job site, refer to a roof's Pitch(X) represented as X in 12, where X is the number of inches of vertical rise for every 12 inches of horizontal run. Roof Pitch Calculator Results (Explained) The roof pitch chart is a visual representation of the actual slope of a roof, given the roof's Pitch(X)/Angle(A). Select this option if you know the rafter's Rise(M) and the roof's Pitch(X) and need to determine the Run(R) of the rafter. Run(R) - This option places the Rise(M) and Pitch(X) fields into the Input section and moves the Run(R) to the Results section. ![]() Select this option if you know the rafter's Run(R) and the roof's Pitch(X) and need to determine the Rise(M) of the rafter. Rise(M) - This option places the Run(R) and Pitch(X) fields into the Input section of the calculator and moves the Rise(M) field to the Results section. The Pitch(X/12) represents the number of inches of Rise(M) for every 12 inches of Run(R). Pitch(X) - This option (the default) places the Rise and Run fields into the Input section. ![]() The Roof Pitch Calculator Input Options (Explained) To determine the ( Y) height, measure from the inside of the Birdsmouth up the calculated Pitch (X in 12) to the top of the rafter. To determine the finished height of the Ridge Beam, add the Rise ( M) and the ( Y) height.
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