Rafter Calculator - Lumber Size, Count & Board Feet

Rafter sizing at a glance

2x8

handles a 12 ft span at 16 in o.c. with 30 psf snow and asphalt shingles

2x10

needed when you push past 14 ft span or switch to heavy tile roofing

16 in

standard on-center spacing for residential rafters across the US

1.5 ft

typical overhang (rafter tail) added to the slope length

How to use this calculator - step by step

Rafter span - horizontal distance from the bearing wall to the ridge. For a 28 ft wide house with a center ridge, the span is 14 ft.
Spacing - center-to-center distance between rafters. Standard is 16 in. Use 12 in for tile roofs or 24 in for light metal roofs.
Roof pitch - angle in degrees. Common: 4/12 pitch = 18 deg, 6/12 = 27 deg, 8/12 = 34 deg.
Roofing material - weight per square foot. Metal is lightest (1.5 psf), clay tile heaviest (12 psf).
Ground snow load - from ASCE 7 maps or local building code. The calculator adjusts for roof slope automatically.
Building length - dimension along the ridge, for counting total rafters.
Lumber price - optional, per board foot for cost estimate.
Read results: minimum lumber size, rafter count, total board feet and cost.

Rafter span table - SPF No.2, 16 in o.c., 30 psf snow, asphalt shingles

Lumber size	Actual (in)	Section modulus (in3)	Max span (ft)
2x6	1.5 x 5.5	7.56	~9
2x8	1.5 x 7.25	13.14	~12
2x10	1.5 x 9.25	21.39	~15
2x12	1.5 x 11.25	31.64	~19

Max span values are approximate for SPF No.2 at the stated conditions. Actual spans depend on load duration, deflection limits and local code. Always verify with IRC span tables or an engineer.

Things most people get wrong

Span is not the building width. A 28 ft wide house has a rafter span of 14 ft (wall to ridge). The full building width is split by the ridge board.

Metal roofing does not mean smaller rafters. The roofing weight difference between metal (1.5 psf) and shingles (2.5 psf) is only 1 psf. Snow load at 30 psf dwarfs that difference. Spacing matters far more.

Steeper pitch does not always mean bigger rafters. Steeper roofs shed snow (lower live load) but the rafter is longer along the slope. These effects roughly cancel out for typical residential pitches (4/12 to 12/12).

Board feet is not linear feet. One board foot = 1 in x 12 in x 12 in (144 cubic inches). A 2x8 that is 14 ft long is 1.5 x 7.25 / 12 x 14 = 12.7 board feet, not 14.

Practical examples

Example 1: Ranch house, 12 ft span, 16 in o.c., 6/12 pitch, shingles, 30 psf snow, 40 ft long
Result: 2x8, 62 rafters, rafter length 15 ft, total 843 bf

Example 2: Cape Cod, 14 ft span, 16 in o.c., 8/12 pitch, shingles, 40 psf snow, 36 ft long
Result: 2x8, 56 rafters, rafter length 18.5 ft, total 939 bf

Example 3: Shed roof, 10 ft span, 24 in o.c., 4/12 pitch, metal, 20 psf snow, 24 ft long
Result: 2x6, 26 rafters, rafter length 12.5 ft, total 223 bf

Example 4: Mountain cabin, 12 ft span, 12 in o.c., 10/12 pitch, metal, 50 psf snow, 30 ft long
Result: 2x6, 62 rafters, rafter length 17.5 ft, total 746 bf

Example 5: Tile roof, 10 ft span, 12 in o.c., 6/12 pitch, concrete tile, 30 psf snow, 50 ft long
Result: 2x6, 102 rafters, rafter length 13 ft, total 912 bf

FAQ - Frequently asked questions

What size rafter do I need for a 12 foot span?

For a 12 ft span at 16 in o.c. with asphalt shingles and 30 psf ground snow load, a 2x8 SPF No.2 rafter is typically sufficient. With heavier loads (tile roof, 50 psf snow), step up to 2x10. Always check local IRC span tables for your specific conditions.

Can I use 2x6 rafters?

2x6 rafters work for spans up to about 8-9 ft with light loads (metal roof, low snow). They are common for porches, small additions and shed roofs. For primary roof structures on homes, 2x8 is the practical minimum in most jurisdictions.

What is the difference between rafter span and rafter length?

Span is the horizontal distance from the bearing wall to the ridge. Length is the actual board measurement along the slope, which is always longer than the span. A 12 ft span at 6/12 pitch (27 deg) produces a slope length of about 13.4 ft, plus the overhang tail adds another 1-2 ft.

How do I find my ground snow load?

Check the ASCE 7 ground snow load map or call your local building department. Most US building departments post snow loads on their website or provide them over the phone. Typical values: Southeast 5-10 psf, Mid-Atlantic 20-30 psf, Upper Midwest 40-50 psf, mountain areas 50-100+ psf.

Does collar tie or ridge beam change the rafter size?

A structural ridge beam (supported at both ends) reduces the effective rafter span to the distance from the wall to the beam. This can allow smaller rafters or wider spacing. Collar ties in the upper third of the rafter span resist spreading but do not reduce bending - they do not change the required rafter size.

How many board feet in a 2x8x14?

Board feet uses nominal dimensions: 2 x 8 / 12 x 14 = 18.67 bf. However, this calculator uses actual dimensions (1.5 x 7.25) for structural sizing, and actual dimensions for the board-feet total to give a more accurate material volume. The lumber yard price per board foot is based on nominal dimensions - factor this in when estimating cost.

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