Roof Rafter Calculator
Calculate roof rafter size, spacing, and span requirements. Professional rafter sizing calculator for stick-built roofing with load analysis and lumber specifications.
Rafter Sizing & Span Analysis
Enter span, loads, and lumber grade to determine the correct rafter size and spacing for your project
Enter Your Roof Measurements
Input your roof measurements to calculate results instantly
Roof Rafter Sizing and Layout
Rafters are the sloped structural members that form the roof plane, running from the ridge to the eaves. Unlike trusses, rafters are cut and assembled on-site (stick framing), giving builders flexibility for complex roof shapes, additions, and exposed ceiling designs. Proper rafter sizing ensures the roof can handle dead loads, live loads, and environmental forces without excessive deflection.
Spacing Options
Rafters are typically spaced 12", 16", or 24" on center. Closer spacing allows smaller lumber sizes but uses more material. At 16" OC, a 2×8 SPF #2 rafter handles most single-story residential spans up to 15 feet. At 24" OC, the same lumber is limited to about 12 feet. Heavy roofing materials like concrete tile usually require 12" or 16" spacing regardless of span.
Deflection Limits
Building codes limit rafter deflection to L/180 for live loads and L/240 for total loads, where L is the rafter span. For a 12-foot span, maximum deflection is 0.8 inches under live load. Exceeding deflection limits causes visible sag, cracked finishes, and pooling water on low-slope sections. If deflection calculations are marginal, go up one lumber size — it's cheap insurance.
Collar Ties and Ceiling Joists
Rafters create outward thrust at the wall plates. Ceiling joists or rafter ties in the lower third of the rafter span resist this thrust. Without ties, walls spread outward and the ridge drops. Collar ties placed in the upper third of the rafter span prevent ridge separation but don't resist wall thrust — they serve a different structural purpose and are not interchangeable with rafter ties.
When to Choose Rafters Over Trusses
Stick-framed rafters make sense for cathedral ceilings, complex roof shapes, small additions, tight site access where a crane can't deliver trusses, and when you need usable attic space. Trusses are faster and more economical for straightforward rectangular plans. Many roofs combine both — trusses for the main span with stick-framed rafters for dormers and intersecting gables.