Rafter Span Calculator

1. Pick the lumber size, from 2×6 up to 2×12.
2. Choose the on-center spacing: 12, 16 or 24 inches.
3. Select the species group stamped on your lumber and the roof live load your code requires.
4. Optionally type your planned span in feet to get a pass or fail check.
5. Read the maximum allowable span in feet and inches.

Here is what each result line means:

Remember that the span is measured horizontally, not along the slope of the rafter. If you need the actual rafter length, run the planned span through a rafter length tool afterward.

A rafter span is the horizontal distance a rafter can safely cross between its supports, usually the exterior wall plate at the bottom and a ridge board, ridge beam or interior bearing wall at the top. It is measured on the level, not along the sloped length of the board. This is what the span tables mean when they list a maximum span for a given size of lumber.

What is the maximum, and who sets it? Building codes publish span tables so that a builder does not need to run structural calculations for every ordinary roof. The tables account for the strength of the wood, how much the rafter is allowed to bend under load, the weight of the roofing and sheathing, and the live load from snow or maintenance traffic. Pick a size, spacing, species and load, and the table gives the longest span that is acceptable.

A rafter that is too long for its size will sag over time, crack finishes, and in a heavy snow can fail. A rafter that is far larger than needed wastes lumber and money. The calculator helps you land on the right size the first time.

Unlike a beam formula, the rafter span is not computed from a single equation on this page. The values come from published tables, which were produced by the American Wood Council using engineering formulas for bending, shear and deflection. The calculator stores the relevant rows and returns the match for your four selections.

1. Take the lumber size, spacing, species group and live load you chose.
2. Find the matching entry in the built-in reference table.
3. Show the value in feet and inches.
4. If you entered a planned span, compare it with the maximum and report pass or fail.

The reference values assume Grade No. 2 lumber, a deflection limit of span over 240 (L/240) and a 10 psf dead load on the roof. Those are the common assumptions for ceilings that are not finished with brittle materials. Different assumptions give different spans, which is why the disclaimer below matters.

Suppose you plan a shed roof using 2x8 Spruce-Pine-Fir rafters at 16 inches on center, designed for a 20 psf live load, and the horizontal span from the wall plate to the ridge support is 14 ft.

Lookup: the table gives 13 ft 9 in for this combination, which is 13.75 feet. Your planned span of 14 feet is 3 inches too long, so the check returns a fail.

Fixes: move the rafters to 12 inches on center, where the same 2×8 SPF reaches 15 ft 1 in, or step up to 2×10 lumber, which spans 17 ft 6 in at 16 inches on center. Either option passes with room to spare.

Species Group

Southern Pine and Douglas Fir-Larch are stiffer and stronger than Spruce-Pine-Fir and Hem-Fir, so they reach farther at the same size. The difference can be a foot or more on a 2×8.

Lumber Grade

The reference values here assume Grade No. 2. Select Structural, No. 1 or No. 3 lumber and the allowable span changes. Check the grade stamp before you rely on a number.

On-center Spacing

Closer rafters share the load among more members, so each carries less. Dropping from 24 inches to 16 inches on center can add several feet to the span.

Roof Load

A 30 psf live load, common where snow is heavier, cuts the allowable span by roughly 10 to 12 percent compared with 20 psf. Dead load from heavy roofing such as tile also reduces the span.

Deflection Limit

The tables limit sag to span divided by 240. A stricter limit, such as L/360 for a plastered ceiling, shortens the allowable span further.

Ceiling Attachment

Rafter ties or ceiling joists at the wall plate resist outward thrust. Without them, a different table or an engineered design is needed.

The same species and spacing can reach very different distances depending on the depth of the board. These values are for Spruce-Pine-Fir No. 2 at 16 inches on center and a 20 psf live load.

Moving from a 2×8 to a 2×10 adds about 3 ft 9 in of span, and from a 2×10 to a 2×12 adds about 4 ft. Deeper lumber costs more per board, but it can let you drop an interior bearing wall or a beam, so compare the whole framing system rather than only the price per piece.

Planning a New Roof

Use a rafter span calculator early to choose a size before you order lumber.

Checking an Existing Plan

Confirm that rafters drawn on a set of plans fit within the tabulated span.

Comparing Options

Compare 16 and 24 inch spacing, or a different species, and see how much span each option buys.

Shed, Garage and Porch Projects

This is a typical use case for small structures where a quick reference is enough to plan a permit drawing.

Remodels and Re-roofing

Check whether old rafters can carry a heavier roof or a higher snow load before you start.

1. Measuring Along the Slope

Tables use the horizontal span. Measuring along the sloped length of the board overstates the span and sends you to a bigger size than needed.

2. Ignoring the Grade

The reference values are for No. 2 lumber. Using them for a lower grade can overstate what the wood can do.

3. Forgetting the Snow Load

Choosing 20 psf where your location needs 30 psf or more is one of the most common and most serious errors.

4. Mixing Species

Do not assume Southern Pine values for a pile of Hem-Fir. Read the grade stamp on the actual boards.

5. Skipping Rafter Ties

A table span assumes the walls are tied together. Without ties or a ridge beam, the roof can push the walls outward.

6. Treating the Result as Final

Local amendments, wind and a changed code edition can alter the answer. Always confirm with your building official.

The values are reference figures derived from the AWC span tables. They are rounded to the nearest inch and are meant for planning, not for final design or permit submission.

It depends on species, spacing and load. For Spruce-Pine-Fir No. 2 at 16 inches on center and 20 psf, a 2×8 reaches about 13 ft 9 in. Southern Pine reaches about 15 ft 8 in.

A 2×6 spans roughly 9 to 13 feet depending on species and spacing. At 16 inches on center and 20 psf, Spruce-Pine-Fir reaches about 10 ft 4 in.

A 2×10 at 16 inches on center and 20 psf spans about 17 ft 6 in in Spruce-Pine-Fir and about 20 ft in Southern Pine, for No. 2 grade.

A 2×12 at 16 inches on center and 20 psf spans about 21 ft 5 in in Spruce-Pine-Fir, and up to about 24 ft 4 in in Southern Pine, for No. 2 grade.

No. The span is the horizontal distance between supports. The sloped length of the board is longer, so do not use it in the span tables.

Southern Pine and Douglas Fir-Larch generally allow the longest spans. Spruce-Pine-Fir allows the shortest of the four groups on this page.

It means the centers of neighboring rafters are 16 inches apart. Closer spacing, such as 12 inches, lets each rafter carry less load and span farther.

Often yes. Table spans assume the exterior walls are held together by ceiling joists or rafter ties. Without them, a different design is needed. Ask your building official.

Yes. Moving from 20 to 30 psf shortens the allowable span by roughly 10 to 12 percent in the reference values used here.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• AWC Span Tables for Joists and Rafters 2021 (American Wood Council, 2021 edition).
• IRC 2021 Chapter 8 Roof-Ceiling Construction (International Code Council).
• Southern Pine Council Maximum Span Comparisons (Southern Forest Products Association).