About Header Span Calculator (IRC R602.7)
The header span calculator reads 2021 IRC Table R602.7(1) — Girder and Header Spans for Exterior Bearing Walls — to size the built-up dimension-lumber header over a window, door, or garage-door opening in a bearing wall. This tool transcribes the one-story loading condition of the table: a header supporting a roof and ceiling only, at a 30 psf ground snow load (the same column also applies where ground snow is below 30 psf and the roof live load is 20 psf or less). Spans are based on minimum design properties for No. 2 grade lumber, and the same values apply to Douglas fir-larch, hem-fir, southern pine, and spruce-pine-fir — the 2021 table is species-independent.
Pick the number of plies (2 or 3), the member size (2x6 to 2x12 — the table has no 3-ply 2x6 row), and the building width (the 12, 24, or 36 ft table column measured perpendicular to the ridge) to get the maximum allowable span and NJ, the number of jack studs required under each end of the header. Or flip to reverse mode, enter the opening span you need, and the tool lists every 2- and 3-ply header in the table that works, sorted smallest member first with its utilization percentage. Headers carrying a floor above, higher snow loads, or spans beyond the table are outside this tool — use the full code table or an engineered design.
How It Works
- Select the ply count. A header built from two members (2-ply) is the everyday case for 2x4 walls; three members (3-ply) span roughly 25% farther and suit 2x6 walls without furring. The 2021 table publishes 3-ply rows only for 2x8 and larger.
- Select the member size and the building width. Width is the dimension of the house perpendicular to the ridge — a wider building sends more roof load to the header, so spans shrink as the width grows. For a width between columns, this tool expects the next wider column; the code footnote also permits straight-line interpolation.
- Read the allowable span and NJ. For example, a 2-ply 2x10 in a 24-ft-wide house spans 6 ft 10 in and needs 2 jack studs under each end. Where NJ = 1, the code permits an approved framing anchor to the full-height stud instead of the jack stud.
- The bar chart compares the allowable span of every published size at the chosen ply and width, so you can see what one size step buys you.
- In "find a header" mode, enter the required opening span instead: every adequate ply-and-size combination is listed with its table span, jack studs, and utilization, smallest member first — pick the most economical row. If nothing qualifies, the tool says so and points you to the full table or an engineered header.
Worked Example
A one-story house 24 ft wide (perpendicular to the ridge, roof and ceiling only, 30 psf ground snow) needs a header over a 6-ft patio-door rough opening in an exterior bearing wall. In reverse mode the tool lists every adequate row of Table R602.7(1) at the 24 ft column: the smallest is a 3-ply 2x8 spanning 7 ft 3 in (87 in) at 82.8% utilization with a single jack stud each end, followed by a 2-ply 2x10 spanning 6 ft 10 in (82 in) at 87.8% with 2 jack studs each end, then the 3-2x10, 2-2x12, and 3-2x12 rows. In a 2x4 wall the 2-ply 2x10 keeps the header flush with the framing; forward mode confirms the lookup: 2-ply 2x10 at 24 ft width = 6 ft 10 in, NJ = 2.
Formulas
- Allowable header span (table lookup)
L_H = TableR602.7(1)[roof+ceiling, 30 psf][ply][size][width]- Jack studs at each end (table lookup)
NJ = TableR602.7(1)[...].NJ- Utilization (reverse mode)
U = L_req / L_H x 100%- Table basis (not a computation)
spans assume: roof + ceiling load, 30 psf ground snow, No. 2 grade DF-L / hem-fir / SP / SPF, header top laterally braced
Standards & References
- IRC 2021 R602.7 / Table R602.7(1) -- girder and header spans for exterior bearing walls (this tool: roof-and-ceiling rows, 30 psf ground snow only)
- IRC 2021 R602.7.5 -- supports for headers (jack studs / approved framing anchors)
- IRC 2021 Table R602.7(2) -- headers in interior bearing walls (not covered here)
Frequently Asked Questions
What size header do I need for a window or door opening?
Look up the smallest row of Table R602.7(1) whose span meets your rough opening width. In a one-story 24-ft-wide house (roof and ceiling, 30 psf ground snow), a 2-ply 2x8 header spans 5 ft 9 in, a 2-ply 2x10 spans 6 ft 10 in, and a 2-ply 2x12 spans 8 ft 1 in. Going to three plies stretches a 2x10 to 8 ft 7 in and a 2x12 to 10 ft 1 in. Enter your opening in reverse mode and the calculator lists every adequate row with its utilization.
How many jack studs does a header need?
The NJ column of Table R602.7(1) gives the number of jack (trimmer) studs required under EACH end of the header — in the roof-and-ceiling / 30 psf block it is 1 or 2. Longer spans and wider buildings need 2 per end because the header reaction crushes a single trimmer. Where NJ = 1, IRC R602.7.5 permits an approved framing anchor attached to the full-height wall stud instead of the jack stud, which is handy tight against a corner.
What size header for a garage door?
A 9-ft single garage door in a narrow (12-ft-column) building is reachable with a 3-ply 2x12, which spans 13 ft 2 in there — but in a 24-ft-wide house the same header spans only 10 ft 1 in, and no 2- or 3-ply row reaches a 16-ft double door at any width. Double-door headers are almost always engineered LVL, PSL, or steel, or sized from the 4-ply rows of the full code table. Check the reverse mode: if the list comes back empty, the opening needs an engineered header.
Does the lumber species change the header span?
Not in the 2021 table. Table R602.7(1) spans are based on the minimum design properties among No. 2 grade Douglas fir-larch, hem-fir, southern pine, and spruce-pine-fir, so one set of values covers all four species groups. Use No. 2 grade or better; utility or stud-grade lumber is not covered by the table.
What does "building width" mean and what if mine is between columns?
Building width is measured perpendicular to the ridge — it sets how much roof span drains onto the header, which is why spans shrink from the 12 ft to the 36 ft column. For a width between columns (say 30 ft), the code footnote permits straight-line interpolation between the 24 and 36 ft values; the conservative shortcut this tool expects is to simply use the next wider column.
Which loading conditions does this calculator cover?
Only the one-story case: a header supporting a roof and ceiling, at a 30 psf ground snow load (also valid below 30 psf when the roof live load is 20 psf or less). The full Table R602.7(1) has additional row blocks for headers that also carry one or two floors above, and columns for 50 and 70 psf ground snow — those combinations, single-member (1-ply) and 4-ply headers, and anything beyond the table need the code book or an engineered design. The spans also assume the top of the header is laterally braced by perpendicular framing; unbraced 2x8-2x12 spans must be multiplied by 0.70.