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Home / Code Guides / Rafter Spans

Rafter Span Tables — How to Size Rafters Under the IRC

Sizing rafters from the IRC is a lookup, not a calculation — but you have to enter the tables with the right inputs and understand what is holding the walls together at the plate line. The tables live in R802.4.1, and whether you need a structural ridge depends on your ceiling joists.

RequirementModel Code StandardSection
Span tablesRafters are sized from Tables R802.4.1(1) through R802.4.1(8), selected by roof live load or ground snow load (20 psf live, or 30/50/70 psf snow) and by dead load and ceiling attachmentIRC R802.4.1
Table inputsWithin each table, allowable span depends on lumber species, grade, member size, and on-center spacing — the span is measured horizontally, not along the slopeIRC Tables R802.4.1(1)–(8)
Ridge boardA non-structural ridge board must be at least 1-inch nominal thickness and at least as deep as the cut end of the rafter — it aligns rafters but carries no loadIRC R802.3
Rafter ties requiredOpposing rafters push the walls outward, so the assembly needs continuous ties — usually ceiling joists lapped and nailed at the plate per the heel-joint connection tableIRC R802.5.2, Table R802.5.2
Ridge beamWhere ceiling joists or rafter ties cannot provide continuous ties — cathedral ceilings, joists running the wrong way — the ridge must be a beam designed by accepted engineering practice and supported at each endIRC R802.3
Raised tiesRaising rafter ties above the plate sharply increases forces at the connection — the heel-joint table applies adjustment factors, and ties raised more than one-third of the rise generally push you to a ridge beamIRC Table R802.5.2 (adjustments)

Worth knowing

  • Enter the table with your jurisdiction’s ground snow load from Table R301.2 — using the 20 psf live load table in snow country is the classic mistake.
  • Span is horizontal projection from plate to ridge, not the rafter length you cut. A 12:12 roof has rafters much longer than the tabulated span.
  • Collar ties in the upper third of the roof resist wind uplift at the ridge — they are not a substitute for rafter ties at the plate line, which is where wall spread is resisted.
  • Notching and boring rules for rafters mirror joists: no notches in the middle third, depth limits at the ends, and bird’s-mouth seats cut so the remaining depth over the plate stays adequate.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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