Blowing the roof off of getting your rafter size right
The International Building Code (IBC) has strict guidelines for rafter size that you must comply with for safety and legality.
The standard roof rafter is cut from a two-by-ten piece of lumber, but two-by-eights and two-by-twelves are also common, depending on the loads and lengths of the rafters.
The space between roof rafters can range from 12 to 24 inches, with 16 inches being the standard.
One of the most crucial parts of a roof to get correct is roof rafter size, including the board thickness, spacing, and span. Not only is proper sizing required to ensure your structure is up to building code, but it also ensures your structure is sound and can stand up to the loads expected from snow, wind, and other environmental factors. In this guide, we’ll discuss standard roof rafter sizes and spacing and the factors you’ll need to consider during construction.
Most roof rafters in residential construction are two-by-ten boards spaced 16 inches apart on center. Depending on a few factors, you have options for board size ranging from two-by-four boards up to two-by-twelve boards. Rafter spacing can sit at 12, 16, 19.2, or 24 inches apart, depending on board size, wood species, expected loads, and rafter length.
A key part of understanding proper roof rafter size is the load they’ll need to hold. Roof rafters support the ceiling inside and the roofing material outside and transfer the loads onto the wall studs and down to the foundation of the structure. The weight of this construction material that the rafters support is called the dead load. Dead loads remain static.
Roof rafters also support live loads, which are any other loads that the rafters might need to support. Live loads include wind, snow on the roof, and other environmental factors. All roof rafter board widths and heights can support reasonably expected dry and live loads, but higher loads demand less rafter spacing and shorter maximum rafter lengths.
You can use the rafter sizing tables provided by the 2018 International Residential Code (IRC) to find appropriate rafter spans, spacings, and board dimensions based on load.
For example, the following table includes rafter dimensions for standard two-by-ten roof rafters of the Douglas fir species spaced at 16 inches on center under different live loads, measured in pounds per square foot (psf). These figures assume a dead load of 10 psf in addition to the live loads.
| Live Load (psf) | Max. Rafter Length |
|---|---|
| 20 | 25ft |
| 30 | 24ft 1in |
| 50 | 19ft 9in |
| 70 | 17ft 1in |
The appropriate roof rafter size varies based on the density and strength of the wood, which the IRC measures based on the species. The denser and stronger the wood species is, the further apart you can space the rafters, and the longer the rafters can be for a given board width and height and given live and dead loads.
Douglas fir is the standard wood species for construction framing, as it offers a good balance between rigidity, strength, availability, and affordable roof framing cost. Following Douglas fir in terms of strength are Southern pine, Hemlock fir, and Spruce fir.
The table below includes a quick look at how these wood species compare when it comes to suitable rafter sizes. These numbers assume a dead load of 10 psf, a live load of 20 psf, and standard two-by-ten boards.
| Wood Species | Rafter Spacing (in) | Max. Rafter Length |
|---|---|---|
| Spruce fir | 12 | >26ft* |
| Hemlock fir | 12 | >26ft* |
| Southern pine | 12 | >26ft* |
| Douglas fir | 12 | >26ft* |
| Spruce fir | 16 | 25ft 5in |
| Hemlock fir | 16 | >26ft* |
| Southern pine | 16 | >26ft* |
| Douglas fir | 16 | >26ft* |
| Spruce fir | 19.2 | 23ft 11in |
| Hemlock fir | 19.2 | 24ft 6in |
| Southern pine | 19.2 | 25ft 5in |
| Douglas fir | 19.2 | 25ft 11in |
| Spruce fir | 24 | 21ft 8in |
| Hemlock fir | 24 | 22ft 9in |
| Southern pine | 24 | 23ft 8in |
| Douglas fir | 24 | 23ft 9 |
*Note: the IRC only accounts for rafters up to 26ft in length. Any rafter span above that will require guidance from a structural engineer and special approval from code enforcement to determine the maximum span.
How far apart your roof rafters are placed—referred to as roof rafter spacing—depends on the wood species, the length of the rafters, and the dead and live loads. Under a given load, rafter spacing can often increase up to a maximum of 24 inches apart on center as the length of the rafters decreases.
The table below includes an example of how roof rafter spacing is related to the maximum rafter length allowed by building code.
| Rafter Spacing (in.) | Max. Rafter Length |
|---|---|
| 12” | >26ft* |
| 16” | >26ft* |
| 19.2” | 25ft 11in |
| 24” | 23ft 9in |
| Rafter Spacing (in.) | Max. Rafter Length |
|---|---|
| 12” | >26ft* |
| 16” | 25ft 2in |
| 19.2” | 23ft |
| 24” | 20ft 7in |
*Note: the IRC only accounts for rafters up to 26ft in length. Any rafter span above that will require guidance from a structural engineer and special approval from code enforcement to determine the maximum span.
Measuring the right size roof rafters means considering all of the factors that affect the load a roof structure can handle and adjusting spacing and maximum board length in accordance with the IRC roof rafter tables.
The length of your roof rafters is predetermined based on your structure’s footprint and the calculated roof pitch, so it’s a good idea to start there. You’ll also need to know the expected live and dead loads, which a structural engineer will determine. These will also be fixed measurements.
Once you know the length of the rafters and the loads they need to support, you can choose your wood species and board width and height based on what the IRC allows for your lengths and loads. Finally, you can choose the proper rafter spacing using the appropriate tables in the IRC guidelines.
Five primary factors influence roof rafter size, all of which change as the others do. Under any given load, a variety of board widths, heights, lengths, and species will satisfy local building codes based on these factors.
The dead load is the weight of the structural components your rafters need to support, which almost always includes roof decking, underlayment, and roofing materials. If you have a ceiling attached to the underside of the rafters, the weight of the drywall or other finishing material will make up an additional portion of the dead load.
In most cases, you’ll need a structural engineer to determine the dead load. As the dead load increases, the maximum rafter length decreases, the necessary rafter height increases, or the rafter spacing decreases to compensate.
The live load is a changing load on your rafters, including snowfall and wind forces. An engineer will determine the live load that your rafters will need to support.
Similar to a dead load, as a live load increases, there must be compensation in the form of decreased rafter spacing, shorter rafter lengths, or larger rafter heights, like using two-by-tens instead of two-by-eights.
The density and strength of the wood you choose affect rafter sizes, as stronger wood species allow for longer rafters, wider spacing between rafters, or smaller board heights under given loads.
Douglas fir is the most common wood in construction and the strongest of the four mentioned in the IRC.
Rafter length is often fixed based on your structure’s dimensions and roof slope, but the length of the boards can determine the height and width you need, as well as the species and appropriate spacing between the rafters.
As the size of your roof increases, so too does the rafter length, and as the rafter length increases, you’ll need to compensate elsewhere to maintain proper support for your expected loads. That can come in the form of using larger boards—two-by-twelves instead of two-by-tens, for example—a stronger wood species, or reducing the spacing between your rafters to a minimum of 12 inches.
For most residential applications, roofers will cut roof rafters from two-by-ten boards, but the IRC dictates that the following board dimensions are acceptable:
Two-by-four boards
Two-by-six boards
Two-by-eight boards
Two-by-ten boards
Two-by-twelve boards
The strength of the board increases in proportion to its height, which means using smaller boards requires compensation elsewhere to maintain strength. That can include reducing the length of the rafters, spacing the rafters closer together, or using a stronger wood species.
Finally, rafter spacing will affect the maximum allowed length of your rafters, as well as the board height. You can place rafters at distances of 12 inches, 16 inches, 19.2 inches, or 24 inches apart on center.
As rafter spacing increases, you’ll need to add strength using another factor, which can include choosing a stronger wood species, decreasing maximum rafter length, or increasing the height of your rafter boards.
If you’re building a roof for a new structure and discover you’ve purchased the wrong size rafters, your best option is to return them, if possible, and buy the correct ones. Alternatively, you can compensate for the wrong size by changing one of the factors discussed above.
For example, if you’ve mistakenly purchased two-by-sixes instead of two-by-tens, you can consult the IRC to see what other factors you can change to remain in compliance. If possible, decreasing the maximum length of your rafters or reducing the spacing between the rafters can compensate for the incorrect board height in some cases. It’s a good idea to consult a structural engineer to make sure you’re in compliance with respect to expected loads, too.
If you’re completing a renovation project that may add to the load on your rafters, like converting an attic to a living space, it’s possible that the existing rafters aren’t suitable. In that case, you should contact a roofing company near you to see what your options are for moving forward.
