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Rafter span calculator for wood roofs

Calculator presetASD, NDS 2024

Design a wood rafter in Calcs.com. Start from the Rafter preset, set your size and span, and check bending, shear, bearing and deflection to NDS 2024.

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How rafter design works in Calcs.com

The Rafter preset sets up the Wood Beam (ASD, NDS 2024) calculator in Calcs.com, a standards-aligned calculation platform for structural engineers.

[ 01 ]

Start from the Rafter preset

Choose Wood Beam (ASD), then Rafter. You start from the member you're designing, with its loads and limits in place, rather than a blank beam. Rafter puts the member on a roof slope, so each roof load acts the way it does on a real roof: along the slope, on plan or across the rafter. Each value below is where the preset starts. You can change any of them.

  • Slope

    Starts at a 6:12 pitch (26.6 degrees), so a 14 ft plan length is 15.7 ft along the slope. Set the pitch of your roof.

  • Loads

    Starts with Roof Load (Gravity - Inclined): dead 15 psf; Snow Load (Gravity - Plan): snow 30 psf, roof live 20 psf; Wind Load (Aligned): wind down 30 psf, wind up -30 psf, over a 16 in spacing. Change any value or the spacing, or add line, point and moment loads.

  • Deflection limits

    Starts at L/180 for live load and L/120 long term. Set the limits your project calls for.

  • Service condition

    Starts dry. Set it to wet for a member that stays damp, which usually means exterior work.

  • Member and bracing

    Starts as a repeating member, top braced for lateral torsional buckling. Change the plies or the bracing to match your framing.

Wood Beam (ASD) presets in Calcs.com, with Rafter picked

[ 02 ]

Change it to your rafter

Pick a size and grade from the standard sections database, enter a custom section, or let Autosize choose one. Then set the plies, span, supports and bracing.

  • Size and grade

    Standard lumber, glulam, LVL, PSL and LSL sections, or a custom section, with Autosize beside the size field.

  • Span and supports

    Plan length, support type, position and bearing length for each support.

  • Bracing and flitch plates

    Continuous bracing for lateral torsional buckling, and optional flitch plates.

The Rafter preset in Calcs.com: a 2x12 D.Fir-L No. 1 at a 14 ft plan length, with the 6:12 incline pitch and its 26.6 degree beam incline highlighted

[ 03 ]

See every check as you design

Bending, shear and bearing utilization sit at the top of the sheet. Deflection is checked against the limits the preset sets.

A Rafter sheet in Calcs.com: wind, snow and roof loads resolved on a 6:12 slope over the 15.7 ft rafter, with bending, shear and bearing utilization

Try the rafter preset: change the size and span

Pick a size and a span. The sheet shows what the Wood Beam (ASD, NDS 2024) calculator returned for that rafter.

  • Utilization at a glance

    Bending, shear and bearing as percentages at the top of the sheet, as in the calculator.

  • Deflection against the preset's limits

    Live load and long-term deflection checked against L/180 and L/120.

  • Every check, as the calculator prints it

    Moment, shear and bearing utilization and both deflections, with the value behind each percentage.

14-day free trial • No credit card required

Wood Beam (ASD, NDS 2024) - RafterSavedJ1
Bending56%Shear17%Bearing19%

Key Properties

Size and Grade
Number of Plies =
Beam Plan Length =ft
Incline Pitch =: 12
Beam Incline (in degrees) =26.6 deg
Center-to-Center Spacing (= tributary width) =in
Distributed LoadsRoof Load (Gravity - Inclined): dead 15 psfSnow Load (Gravity - Plan): snow 30 psf, roof live 20 psfWind Load (Aligned): wind down 30 psf, wind up -30 psf
Continuous Bracing for Lateral Torsional Buckling
Service ConditionDryWet

Checks

Moment Utilization =1704 lb-ft / 3032 lb-ft56%
Shear Utilization =399 lb / 2329 lb17%
Bearing Utilization =540 lb / 2812 lb19%
Governing Live / Short-Term Deflection =-0.122 in (L/1534)12%
Governing Long-Term Deflection =-0.265 in (L/708)27%

Plies, spacing, loads and bracing are as the Rafter preset sets them. Every input is editable in the calculator. This sheet replays 12 solves by the Calcs.com engine on 2026-11-12: 3 sizes by 4 spans. In the calculator, any size, span, spacing or load is solved the same way.

Rafter span table: 2x8 to 2x12

Each cell is the Rafter preset solved in the Wood Beam (ASD, NDS 2024) calculator at that size and span, with every other input as the preset sets it. It shows the governing check's ratio. Above 1.00 fails.

Rafter span table, governing check ratio by member and span
Member12 ft14 ft16 ft18 ft
2x80.90BendingPasses1.10BendingFails1.58Long-term deflectionFails2.40Long-term deflectionFails
2x100.61BendingPasses0.75BendingPasses0.93BendingPasses1.16Long-term deflectionFails
2x120.46BendingPasses0.56BendingPasses0.70BendingPasses0.84BendingPasses

13 presets on one wood beam calculator

Each member preset, from Floor Joist to Deck Beam, sets up the same calculator with its own loads and deflection limits. Rafter and Hip / Valley put the member on a slope. Deck Joist and Deck Beam set wet service.

Open the Rafter preset in the calculator

  1. [ 01 ]Start your free trial. You land on the app's home page.
  2. [ 02 ]Open the Wood Beam (ASD) calculator for the edition you design to.
  3. [ 03 ]Choose Rafter from its presets. Its values load in, ready to change.
See the full Wood Beam (ASD, NDS 2024) calculator
Edition

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Pick the edition you design to. The sheet and the steps above follow it, and each edition has a Rafter preset.

These results use the preset's loads and limits. Check every input against your project before you rely on a result. The engineer of record stays responsible for the design.

Frequently asked questions

Design your rafter in Calcs.com

Start from the Rafter preset and check your size and span to NDS 2024.

Design your rafterSee the full Wood Beam (ASD, NDS 2024) calculator
Linking a calculation to a column in Calcs.com, with the AISC Steel Construction Manual reference attached

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