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.
Rafter design code#
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.
Rafter design how to#
Our goal is to provide a complete guide for sizing and spacing rafters for your projects. We’ll discuss 16” and 24” spacing, how to calculate the number of rafters required, 19.2” spacing, and the spacing when using metal roofing and for lean-to and porch roofs. In this guide, we’ll explain rafter spacing, code requirements, roof rafter span tables, and rafter spacing calculators. The wider the space between rafters often results in rafters of greater depth too. Depending on all contributing factors though, 16” and 24” are commonly used for most buildings. Standard industry roof rafter spacing is 12”, 16”, 19.2”, and 24” on-center, however other spacing may be used by a Structural Engineer. Plus, you have to select the correct rafter spacing to span the distance based on all the factors that affect your design. You have to consider live, dead, wind, and snow loads, select the correct species, grade, and dimension of lumber for the rafter span, and decide if there will be a ceiling or loft supported by the rafters. Designing and building a roof for a shed, porch, cabin, garage, or home can be a daunting task.
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