Rafter Calculator
Common rafter length, plumb and seat cuts, and the bird mouth — reference-checked.
Common rafter
= 168" (14')
= 1 1/2"
= 18" (1' 6")
Advanced
= 7 1/4"
= 3 1/2"
Results update as you typeLive
Rafter · Result
CalculatedRAFTER LENGTH
rafter length 17' 3 1/8"
- Line length
- 15' 7 13/16"
- Plumb cut
- 26.6°
- Ridge deduction
- 13/16"
- Depth above seat
- 5 1/2"
2021 IRC reference check — within the limits checked. Local requirements may differ.
Reference profile 2021 IRCA reference check is not a compliance determination; local amendments differ.
SettingChangeRoof defaults to a gable
SUGGESTED DEFAULTTop plate defaults to 3 1/2" (2×4) and the seat to the plate width (full bearing)
SUGGESTED DEFAULTBearing material defaults to wood — minimum 1.5 in; masonry 3 in
SUGGESTED DEFAULTCode references & assumptions · 8
How this result is supported
- ASSUMPTIONRun is horizontal (R802.4.1 measures rafter spans on the horizontal projection), from the outside face of the top plate to the ridge centreline; the two endpoints are inferred, not cited — PARTIAL
- PRIMARY-VERIFIED
- ASSUMPTIONRafter length is plumb cut to plumb cut, with no allowance for the style of the tail cut
- UNVERIFIED
- SUGGESTED DEFAULTRoof defaults to a gable
- UNVERIFIED
- SUGGESTED DEFAULTTop plate defaults to 3 1/2" (2×4) and the seat to the plate width (full bearing)
- CONVENTION
- ASSUMPTIONRidge deduction is half the ridge's actual thickness taken horizontally off the run (method A) — stated by three independent framing sources (Sloggatt; carpentry-tips-and-tricks; Simpson as corroboration) and consistent with R802.4.1 and R802.3. It is a trade convention, not a code-prescribed rule
- CROSS-VERIFIED
- ASSUMPTIONOverhang applied as declared (horizontal); the other reading is reported alongside because no source states the convention
- UNVERIFIED
- SUGGESTED DEFAULTBearing material defaults to wood — minimum 1.5 in; masonry 3 in
- UNVERIFIED
- CODE REFERENCEBird mouth limit from 2021 International Residential Code R502.8.1 via R802.7.1: one-fourth of the rafter depth, with no bearing exemption in the text. The tail is checked under R802.7.1.1 regardless of profile
- PRIMARY-VERIFIED
What do these evidence labels mean?
- PRIMARY-VERIFIED
- directly supported by one authoritative primary source, quoted with section/page/figure
- CROSS-VERIFIED
- independently supported by two or more appropriate authoritative sources
- DERIVED
- mathematically derived from PRIMARY-VERIFIED values; the derivation is preserved in data and in code
- INTERPRETED
- a source exists but translating it into calculator behaviour required interpretation
- CONVENTION
- recognised trade practice, not a mandatory provision
- ASSUMPTION
- product/design assumption or useful default with no authoritative basis
- UNVERIFIED
- insufficient authoritative evidence
Code references and product data are quoted in the units their source uses.
Reference checks are informational, not compliance determinations.
How rafter is calculated
This calculates COMMON rafters — the ones running straight up the slope from plate to ridge. Hips, valleys and the jacks that die into them run diagonally across two planes and use a different factor entirely; they are not covered here, and a number from this page should not be used for them.
The run is measured horizontally from the outside face of the plate to the centreline of the ridge, and half the ridge's actual thickness is then taken off it. Actual, not nominal: a 2× ridge is an inch and a half, and using two would shorten every rafter on the roof by a quarter inch.
The overhang is asked for as a measurement AND as a direction, because a tail measured horizontally and a tail measured along the slope are different lengths on the same roof. The calculator will not guess which you took — declaring it is the difference between a tail that lands and one that does not.
The bird mouth is checked against the reference profile you select. The check reports what that document says about the remaining heel depth and the seat bearing; it tells you which provision a number is near, not whether the roof may be built.
line length = run × slope factor
ridge deduction = ridge thickness ÷ 2
Taken horizontally off the run — half the ridge's actual thickness.
heel = rafter depth − (seat width × slope factor)