House rafter

House rafter
Video House rafter

Designing and building a roof for a shed, porch, cabin, garage, or home can be a daunting task. You have to consider live, dead, wind, and snow loads, select the correct species, grade, and dimension of lumber for the rafter span, and decide if there will be a ceiling or loft supported by the rafters. Plus, you have to select the correct rafter spacing to span the distance based on all the factors that affect your design.

Standard industry roof rafter spacing is 12”, 16”, 19.2”, and 24” on-center, however other spacing may be used by a Structural Engineer. Depending on all contributing factors though, 16” and 24” are commonly used for most buildings. The wider the space between rafters often results in rafters of greater depth too.

In this guide, we’ll explain rafter spacing, code requirements, roof rafter span tables, and rafter spacing calculators. We’ll discuss 16” and 24” spacing, how to calculate the number of rafters required, 19.2” spacing, and the spacing when using metal roofing and for lean-to and porch roofs. Our goal is to provide a complete guide for sizing and spacing rafters for your projects.

Roof Rafter Spacing and Sizing

What Is Rafter Spacing

Rafter spacing is the distance between parallel rafters or roof joists. It is measured from the center of one rafter to the center of the next, also known as ‘on-center’, which makes it easier to determine the number of rafters based on the building’s length. The typical on-center spacing, according to the International Residential Building Code (IRC), is 12”, 16”, 19.2”, and 24”, which means the actual space between the rafters is 10.5”, 14.5”, 17.7”, and 22.5”.

Rafter joist spacing is only one aspect of roof design and building that affects the span, along with load variables, wood species, and grade. The greater the space between rafters, the shorter the distance they can span. Thicker dimensional lumber, such as 2x10s and 2x12s can span further at 24” centers than 2x4s or 2x6s at 12” spacing for comparable wood species, grade, and load variables. So, always check the prescriptive code tables, or with a Structural Engineer before building.

Roof Rafter Spacing Code Requirements

The International Residential Building Code (IRC), which is the basis of most building codes in North America, identifies the standard spacing between rafters as 12”, 16”, 19.2”, and 24” on center. The most current IRC, published in 2021, along with previous editions, provides rafter span and spacing tables for different variables in Chapter 8.

Roof rafter spacing requirements include rafter span, live and dead loads on the roof, deflection, wind and snow loads, wood species, grade, rafter size and thickness, and whether a ceiling will be attached or not. Additional factors include slope, eave and rake overhangs, roof tie-downs, rafter support bracing, collar ties, ridge connection, roof openings, ventilation, and roof finish. Live and snow loads vary from 10 to 70psf with dead loads of 10 and 20psf and deflections of L/∆=180 and L/∆=240.

Another important factor is location. Code requirements vary based on different climate conditions in different geographic regions. So, it is always important to check with your local building department or other building experts for rafter or roof joist spacing requirements.

Roof Rafter Span Table

Span is typically measured horizontally along the rafter projection from the ridge to the outer edge of the supporting outside wall plate. It is also commonly expressed as a ratio of rise to run, or a fraction of the rise over the run. The following table compares the span for common #2 grade dimensional lumber of different species and the three typical spacings between rafters. The information collected is from Table R802.4.1(1) of IRC-2021 and the American Wood Council.

Rafter

Spacing

Species (Lumber Grade #2)

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Live load=20 psf, Dead load=10 psf,

L/∆=180

Southern Pine Douglas Fir Redwood 2×4 12” 10’-4” 10’-10” 9’-4” 16” 9’-0” 9’-2” 8”-11” 24” 7’-4” 8’-2” 7”-10” 2×6 12” 15’-7” 16’-10” 15”-6” 16” 13’-6” 14’-7” 14’-1” 24” 11’-0” 11’-11” 12’-1” 2×10 12” 23’-5” 26’-0” 26’-0” 16” 20’-3” 23’-9” 22’-10” 24” 16’-6” 18’-5” 18’-8” 2×12 12” 30’-2” 30’-2” 30’-7” 16” 23’-10” 26’-1” 26’-6” 24” 19’-6” 21’-4” 21’-7”

Rafter Spacing Calculator

There are numerous rafter spacing calculators available, but our preference is the American Wood Council’s online Span Calculator. You can select ‘Max Span’ or ‘Span Options’, wood species, dimensions, grade, member type, deflection, spacing, live load, dead load, service conditions, and whether it’s incised lumber or not. Once all the choices are selected, you click on ‘Calculate’ and it provides the maximum horizontal length and other helpful information.

When choosing a Rafter Span Calculator, look for one that provides the greatest selection of variables for the most accurate answer. Remember, a calculator doesn’t replace the Building Code or the need for approval from the local Building Department. For complex roof designs, check with a Structural Engineer.

16 or 24 Rafter Spacing?

16 or 24 Rafter Spacing

Rafter spacing depends on a variety of variables and engineering calculations for structural integrity. Typically, though, the closer the rafters are together, the more support provided to the sheathing and roofing finish, the further the rafters can span, and the stronger and stiffer the roof.

Roof joists should be nominally 2” wide and 4” to 12” deep, although larger dimensions can be used. Common industry spacing for rafters is 12”, 16”, 19.2”, and 24” on-center with other spacing used where design, climate conditions, and Engineers identify them.

Spacing rafters at 16” centers is common for many roofs sheathed in OSB or plywood. The sheathing has more support and less noticeable sag over time under asphalt roofing materials. At 16” centers, there are three support members every four feet versus two when spaced at 24” centers. Remember, the outer two support rafters are shared, so only count as one. Using rafters spaced at 16” centers also means smaller dimensional lumber or a lower grade of lumber could be used compared to wider spaced rafters.

Spacing rafters at 24” centers means each rafter supports 50% more than rafters spaced at 16” centers. It also means rafter members need to be deeper or of a higher grade and species to span the same distance. However, switching to 24” centers also decreases the number of rafters required by up to 50%. Roofs sheathed in planks or strapped for metal or other roofing finishes commonly are set at 24” centers, and the planks or strapping span the gap between the rafters and provide additional stiffness and support.

At 16” centers, the space between rafters is 14-1/2” while it is 22-1/2” between those spaced at 24” centers. Wood species, grade, deflection, and loads all affect the span. A #2 S-P-F 2×4 at 16’ centers can span 9’-5” but only 8’0 at 24” centers with a live load or 20psf, dead load of 10psf, and deflection L/∆=180, so span and spacing are linked to other building factors. If unsure which spacing to use, discuss the design with your local building department or a Structural Engineer.

How to Calculate Roof Rafters: How Many Rafters Do I Need?

How to Calculate Roof Rafters

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Calculating the number of rafters needed for a project can be done with some rafter calculators, but it isn’t impossible to do with paper and pencil or a standard calculator. Use the structures supporting outside wall length in inches and divide it by the on-center spacing between parallel rafters, and then add 1 for the ‘0’ rafter. For a gable roof multiply the number of rafters by 2 for the two sides or slopes of the roof.

For example:

A building that is 30’ long with a gable roof will require 24 rafters at 16” centers and 32 at 24” centers.

30ft x 12” = 360” (length in inches) ÷ 16” on-center = 22.5 rafters -> round up and add 1 = 24 rafters per side, so, multiply by 2 and you’ll need 48 rafters for the roof.

30ft x 12” = 360” ÷ 24” on-center = 15 rafters + 1 = 16 rafters per side x 2 = 32 rafters

It should be noted that some roofs have an overhang at the gable ends known as the rake. It typically bridges the gable wall and attaches to the end rafter at each gable. The outer board or fascia isn’t a rafter and may be of 1” thick lumber, or of the same 2-by material as the rafters. However, they are sometimes included in the rafter count, which would increase the rafter total by 4, two for each gable end.

What Is 19.2 Rafter Spacing?

Rafters or roof joists, or even studs and floor joists at 19.2” on-center use 5 structural members over 8-feet instead of 6 pieces at 16” centers or 4 at 24” centers. One less rafter every 8’ on a large build does add up to savings, especially when running wires, driving nails, or insulating for less thermal transmission. Additionally, the 19.2” on-center spacing is useful when using engineered I-joist/truss rafters with the wider flange.

I-truss rafters typically have a gap of 15.7” between parallel flange edges when spaced at 19.2” on-center. A gap that isn’t much different from the 14.5” between trusses at 16” centers. As a result, 19.2” rafter spacing is commonly used by contractors when using engineered structural members. Having said that, 19.2 spacing can be used for dimensional lumber too but may require a change in lumber depth, grade, or species.

Rafter Spacing for Metal Roof

Rafter Spacing for Metal Roof

Metal roofing, typically steel or aluminum, comes in a variety of profiles and compositions and may be referred to as tin, steel, or corrugated metal. The width of the panel or sheet is commonly set by the manufacturer and the length by the ridge to eave measurement. However, the length can also be precut by the manufacturer and designed to overlap or interlock to keep the weather out.

The rafter or roof joist spacing for metal roofing may need to be set to support rigid metal panels designed and engineered to fasten directly to the rafters. The rafter spacing will be set by the steel panel manufacturer to provide a structurally sound roof. Rigid metal panels are pre ordered and manufactured for the build, so lead time usually allows for rafter placing based on the panels.

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In most roofs though, the metal roofing is flimsier and is supported by and fastened to a plywood or OSB deck, or to strapping fastened to the rafters. The rafters are commonly spaced at 16” centers when sheathed to provide the greatest support. Rafters that are strapped are either at 16” or 24” centers with strapping placed every 8” to 12” on-center to provide support. The spacing of the straps usually depends on the size of the metal roofing being attached.

A sheathed roof deck provides ample support for the metal roofing and is an additional barrier to insects and weather. Strapped rafters create an open web for the metal to be fastened to, making the underside visible from below, but also more insect-friendly. Rafter spacing is usually based on span, the dimensional lumber used, and load factors, so it’s best to check with the experts before driving in nails or screws.

Lean-to Rafter Spacing

Lean to Rafter Spacing

A lean-to roof typically attaches to the side of another building and projects down and outward to be carried by a wall or on a beam supported by posts. A lean-to rafter can also be used to make a shed-style roof on a variety of buildings too, providing a single roofline with a high wall and a lower wall supporting the rafters. The width of the building or distance from the wall to the support beam determines the rafter span, spacing, and slope.

Lean-to rafter spacing commonly is 16” and 24” on-center depending on the type of roofing finish selected. Rafters at 24” centers with strapping to support metal roofing are less expensive than those at 16” centers covered with OSB or plywood and metal roofing. The span dictates the rafter depth and spacing based on the wood species and grade used, load variables, and deflection.

Depending on design factors, budget, and all the other factors, #2 SPF 2x4s at 12” centers work to span 8’. However, increasing load and deflection factors may require planks of greater depth or a stronger timber species and grade. Some builders prefer to use 2x6s at 16” or 24” centers as there are fewer rafters and less work, so unless local codes or the building department has set requirements, it is often a matter of preference and budget.

Rafter Spacing for Porch Roof

Rafter spacing for a porch roof depends on the distance the board must span, the loads it must carry, and the deflection factors, as well as wood species and grade. The design of the porch roof is a factor too. It could be a lean-to style, flat pergola style, gabled, or some other style or a mixture of styles. The roof may be closed to protect from the elements or finished with slats to protect from the sun but not rain, so purpose affects spacing too.

Roofs in warm climates may only need to support light loads so may be further apart, but winds may be a factor that affects structural requirements. In regions that experience heavy snow loads, span and spacing may have other restrictions and need to be closer together. The roof finish may also affect the spacing between roof joists. Typically, porch roof rafter spacing is either 16” or 24” on-center.

Conclusion

The industry standard for roof joist or rafter spacing is 12”, 16”, 19.2”, and 24” on-center, typically though, rafters are spaced 16” or 24” on-center. The gap between members can be different provided it meets code requirements.

The wood species, grade, load factors, deflection, and span affect the gap between rafters and rafter dimensions. Greater space and span often require rafters of greater depth than shorter spans or narrower spacing.

Hopefully, you have a better understanding of roof rafter spacing and sizing, and are ready for your next project.

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