Another option are 2 X 8 rafters spaced 24 inches o.c. The table states that 2 X 6 rafters spaced 16 inches on center (o.c) can span a maximum distance of 13 feet 5 inches. Looking through the center of the table, we can find a couple of options. Let's use this example and find a size of rafter that will work. So if your house is 26 feet wide and the roof ridge beam is centered on the house, you will need a rafter that can span 13 feet. The maximum spans in the table above are for the horizontal distance the rafter can span (NOT the diagonal distance or actual length of the rafter). This table is simply a sample and may not be valid for your region. Sample Maximum Spans (feet-inches) Douglas Fir or Larch, No.1 or No.2 with 31.3 psf of Snow Loading Let's look at the sample table below which shows maximum rafter spans for a snow load of 31.3 pounds per square foot (psf).įraming Table for Maximum Roof Rafter Spans If you live in a region that gets winter snowfall, contact your municipal building department to find out what maximum snow loads you need to design for.ĭepending on your region, you may need to design for a maximum snow load of anything between zero to 65 pounds per square foot (or roughly 0-3 kPa). All roof framing tables are organized by maximum snow load. In most regions, the first thing to consider before sizing the wood framing members for a roof is the maximum snow load that the roof will have to bear. Initially, we will discuss the framing of gable roofs so that we can learn the basics of how to size a roof rafter, beam or joist. Each rafter is in turn fastened to the top of the exterior structural walls.Īs we discussed in Exterior House Designs there are many different house roof types. Along the top center of the house there is a beam to which are attached the rafters. The image below shows a cross-section of our simple 12' X 13' house from the joist span tables section of this tutorial. Let's consider a basic gable wood roof framing design. Where access to the roof is provided in addition to that necessary for cleaning and repair, the imposed uniformly distributed load should be taken as a minimum of 1.50 kN/m².Continuing on from Part 3: Floor Beam Span Tables of Residential Structural Design, we will now look at roof rafter and beam design. The 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. Joists are to have minimum end bearing of 40mm. Wane as allowed in BS 4978:2007+A2:2017 is permitted in all sections included in these span tables. Holes drilled at the neutral axis (centre line) are not to exceed 0.25 of the depth of a joist and are not to be less than three diameters (centre to centre) apart and are to be located between 0.25 and 0.4 of the span from the support. Notches are not to exceed 0.125 of the depth of the joist and are to be located between 0.07 and 0.25 of the span from the support. Timber to be covered, these span tables do not apply to timber which is fully exposed to the elements. These span tables apply only to flat roof and roof with a slope of up to 10 degrees. If insulation is placed on top of the flat roof joists and as such the joists are not fastened to the boarding above, the depth to breadth ratio of the joists should not exceed a ratio of 4 to 1. The dead loads shown at the top of span table above do not include the self weight of the joists, however, the joist self weights are 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 5268-7.2:1989 Structural use of timber - Section 7.2 Joists for flat roofs. * Boarding to be fastened on top of flat roof joists with a depth to width ratio greater than 4 Dead load per square metre (in kN/m²) supported by joistsĬentre-to-centre spacing of joists (in mm)
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