You can select ‘Max Span’ or ‘Span Options’, wood species, dimensions, grade, member type, deflection, spacing, live load, dead load, service conditions, and whether it’s incised lumber or not. There are numerous rafter spacing calculators available, but our preference is the American Wood Council’s online Span Calculator. The information collected is from Table R802.4.1(1) of IRC-2021 and the American Wood Council. The following table compares the span for common #2 grade dimensional lumber of different species and the three typical spacings between rafters. It is also commonly expressed as a ratio of rise to run, or a fraction of the rise over the run. Span is typically measured horizontally along the rafter projection from the ridge to the outer edge of the supporting outside wall plate. So, it is always important to check with your local building department or other building experts for rafter or roof joist spacing requirements. Code requirements vary based on different climate conditions in different geographic regions. Live and snow loads vary from 10 to 70psf with dead loads of 10 and 20psf and deflections of L/∆=180 and L/∆=240.Īnother important factor is location. Additional factors include slope, eave and rake overhangs, roof tie-downs, rafter support bracing, collar ties, ridge connection, roof openings, ventilation, and roof finish. Roof rafter spacing requirements include rafter span, live and dead loads on the roof, deflection, wind and snow loads, wood species, grade, rafter size and thickness, and whether a ceiling will be attached or not. The most current IRC, published in 2021, along with previous editions, provides rafter span and spacing tables for different variables in Chapter 8. The International Residential Building Code (IRC), which is the basis of most building codes in North America, identifies the standard spacing between rafters as 12”, 16”, 19.2”, and 24” on center. So, always check the prescriptive code tables, or with a Structural Engineer before building. Thicker dimensional lumber, such as 2x10s and 2x12s can span further at 24” centers than 2x4s or 2圆s at 12” spacing for comparable wood species, grade, and load variables. The greater the space between rafters, the shorter the distance they can span. Rafter joist spacing is only one aspect of roof design and building that affects the span, along with load variables, wood species, and grade. The typical on-center spacing, according to the International Residential Building Code (IRC), is 12”, 16”, 19.2”, and 24”, which means the actual space between the rafters is 10.5”, 14.5”, 17.7”, and 22.5”. It is measured from the center of one rafter to the center of the next, also known as ‘on-center’, which makes it easier to determine the number of rafters based on the building’s length. Rafter spacing is the distance between parallel rafters or roof joists.
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