3/16/2024 0 Comments Roof rafter span tableThe imposed load should be calculated in accordance with BS 6399:Part 3:1988 Code of practice for imposed roof loads, as a rule of thumb for altitudes not exceeding 100m you can use a uniformly distributed load of 0.75 kN/m² and for most other areas exceeding 100m but not exceeding 200m you can use 1 kN/m², for parts of Scotland, Pennines, North East England and heights over 200m you will need to refer to BS 6399 Part 3. 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. Listed spans are for dry-service conditions unless the table is labeled as Wet-Service. The load duration factor, CD, is 1.0 unless shown as 1.15 for snow or 1.25 for construction loads. Deflection is limited to the span in inches divided by 360, 240 or 180 and is based on live load only. The calculations used for these span tables are based on rafters having two equal spans. Applied loads are given in pounds per square foot (psf). 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 local building code inspector is the final authority, and your local building codes and inspectors may have different requirements. 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. Table 2308.10.4.1, Rafter Tie Connections, relates rafter slope, rafter tie spacing, and roof load ratings or snow load ratings and also roof span in feet. TABLE R802.5.Dead load per square metre (in kN/m²) supported by purlin The braces shall be spaced not more than 4 feet (1219 mm) on center and the unbraced length of braces shall not exceed 8 feet (2438 mm). Purlins shall be continuous and shall be supported by 2-inch by 4-inch (51 mm by 102 mm) braces installed to bearing walls at a slope not less than 45 degrees (0.785 rad) from the horizontal. Purlins shall be sized no less than the required size of the rafters that they support. Installation of purlins to reduce the span of rafters is permitted as shown in Figure R802.5.1. CEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics without storage, live load = 10 psf, L/Δ = 240) CEILING JOIST SPACING (inches)ĬEILING JOIST SPANS FOR COMMON LUMBER SPECIES (Uninhabitable attics with limited storage, live load = 20 psf, L/Δ = 240) CEILING JOIST SPACING (inches)
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