Rafters act as support for roofs of houses, sheds, porches, cabins, and garages.
The International Residential Code (IRC) 2021 specifies roof rafter spacings of 12″, 16″, 19.2″, and 24″.The common sizes for rafters include 2×4, 2×6, 2×8, 2×10, and 2×12. For a particular span, rafter spacing distance can vary based on the species of lumber, grade, and load conditions.
Roof Rafter Spacing, Span, and Sizing Requirements
Rafter spacing is the distance between the centers of two consecutive parallel rafters.
The typical on-center spacing provided in the International Residential Code (IRC) 2021 includes spacings of 12”, 16”, 19.2”, and 24”.
Specifically, 10.5″, 14.5″, 17.7″, and 22.5″ are the actual spacings.
Rafter spacing is an essential parameter of roof design.
Once the rafter spacing is determined, the number of total rafters and their span can be calculated.
A greater distance between two rafters allows them to span a shorter length.
A Douglas fir-larch, SS grade 2×6 rafter spans 18’ with 12” spacing, while it only spans 14’-4” with 24” spacing.
These tables are for dead loads of 10 psf and live loads of 20 psf with unattached ceilings.
| Rafters Spacing | Species | Grade | 2×6 | 2×8 |
|---|---|---|---|---|
| (inches) | Spans (ft-in) | Spans (ft-in) | ||
| 12 | Douglas fir-larch | SS | 18-0 | 23-9 |
| 12 | Douglas fir-larch | #1 | 17-4 | 22-5 |
| 12 | Douglas fir-larch | #2 | 16-10 | 21-4 |
| 12 | Hem-fir | SS | 17-0 | 22-5 |
| 12 | Hem-fir | #1 | 16-8 | 22-0 |
| 12 | Hem-fir | #2 | 15-11 | 20-8 |
| 12 | Southern pine | #1 | 17-0 | 22-5 |
| 12 | Southern pine | #2 | 15-7 | 19-8 |
| 12 | Spruce-pine-fir | #1 | 16-3 | 21-0 |
| 12 | Spruce-pine-fir | #2 | 16-3 | 21-0 |
| 24 | Douglas fir-larch | SS | 14-4 | 18-10 |
| 24 | Douglas fir-larch | #1 | 12-6 | 15-10 |
| 24 | Douglas fir-larch | #2 | 11-11 | 15-1 |
| 24 | Hem-fir | SS | 13-6 | 17-10 |
| 24 | Hem-fir | #1 | 12-4 | 15-8 |
| 24 | Hem-fir | #2 | 11-7 | 14-8 |
| 24 | Southern pine | #1 | 12-9 | 16-2 |
| 24 | Southern pine | #2 | 11-0 | 13-11 |
| 24 | Spruce-pine-fir | #1 | 11-9 | 14-10 |
| 24 | Spruce-pine-fir | #2 | 11-9 | 14-10 |
Rafter Span Table with Different Spacing (IRC 2021)
The span of a rafter is the horizontal length it covers from the point of its placement to where it ends.
Just like the size of a rafter, you should determine its span beforehand to avoid any failure later.
Many factors influence the span of rafters; hence they vary significantly.
Rafters of a particular wood species can have different spans if their size is different.
Similarly, rafters of the same size can have varying spans due to the difference in wood species.
For example, 2×8 rafters have span lengths ranging from 13’ to 19’.
A 2×10 can span up to 23’, while a 2×12 rafter may span up to 27’.
| Rafters Spacing (inches) | Species | Grade | 2×10 Spans (ft-in) |
|---|---|---|---|
| 12 | Douglas fir-larch | #2 | 26-0 |
| 12 | Douglas fir-larch | #3 | 19-10 |
| 12 | Hem-fir | #2 | 25-3 |
| 12 | Hem-fir | #3 | 19-5 |
2×10 Rafter Span Table (IRC 2021)
Note that these values are for unattached ceilings with a 180-degree deflection.
You can also calculate the rafter span with the American Wood Council’s online calculator.
The size of a rafter is a measure of its width and depth, and the most common rafter sizes are 2×4, 2×6, 2×8, 2×10, and 2×12.
2×4 rafters are more commonly used in residential construction.
To determine the final size of a rafter, consider various factors such as loads, wood species, wood grade, the slope of a roof, and the location of the rafter.
Rafter Spacing for Metal Roofs
Metal roofing for houses commonly uses aluminum or steel sheets.
The common spacing for the rafters used in metal roofing is 16” or 24” in the center.
While the manufacturer determines the metal panel width, the length depends upon the ridge-to-eave measurement.
You need to fasten the metal plates to the roof rafters to provide support.
Rafter Spacing for Pergolas
Rafter spacing in pergolas varies with its dimensions and aesthetics.
The standard spacing provided in pergolas ranges between 12” to 20”.
Spacings of 12″ and 16″ are most commonly used.
A spacing of 12″ is used when the dead weight of the rafters is not heavy enough to cause problems.
If the rafters are heavier, then 16” spacing is provided for rafters used in pergolas.
16” Spacing or 24” Spacing for Rafters?
A rafter spacing of 16″ is quite common in oriented strand board and plywood roofs.
Six rafters are placed every 8′ when 16″ spacing is used.
When compared to rafters with wider spacing, 16″ spacing indicates the use of a smaller lumber size and a lower wood grade.
In contrast, roofs with metal sheathing or planks typically have rafters spaced 24″ apart.
Providing 24” spacing between rafters causes each rafter to support 50% more load than 16” spacing.
In general, rafters spaced 24″ apart have larger dimensions and are made of stronger wood.
When is 19.2 Rafter Spacing Used?
Rafters made for engineered joists or trusses use 19.2” spacing.
It can also be used for dimensional lumber rafters, but the lumber depth and overall wood quality may need to be increased.
This saves cost compared to using 16” with six members over 8’, requiring fewer nails and lesser insulation.
Spans for Rafters with Attached Ceilings
Rafter spans with ceilings attached are relatively smaller than those without ceilings attached.
For 12” spacing and the SS grade of Douglas fir-larch, a 2×6 rafter with an unattached ceiling can span up to 18’.
In contrast, the same rafter can span only up to 16’-4” with an attached ceiling.
| Rafters Spacing | Species | Grade | 2×6 |
|---|---|---|---|
| (inches) | Spans (ft-in) | ||
| 12 | Douglas fir-larch | SS | 16-4 |
| 12 | Douglas fir-larch | #1 | 15-9 |
| 12 | Douglas fir-larch | #2 | 15-6 |
| 12 | Hem-fir | SS | 15-6 |
| 12 | Hem-fir | #1 | 15-2 |
| 12 | Hem-fir | #2 | 14-5 |
2×6 Rafter Span Table with Attached Ceiling (IRC 2021)
How Many Rafters Do You Need?
Calculating the number of rafters required for a specific project is important for accurate cost estimation.
This can be done either using rafter calculators (available online) or by manual estimation using IRC.
The first step in calculating the required number of rafters is to arrive at the roof measurement, including the roof length and overhang.
As a next step, determine the size and spacing of the rafters you intend to use.
Next, divide the length of the roof by the required spacing.
Then, add one and multiply the sum by 2 to cover both sides of the roof.
The new sum now will be the total number of rafters required in your project.
For example, consider a roof of length 35’ with a 24” rafter spacing.
Convert the length of your house from feet to inches,i.e., 35×12= 420”.
Now divide this length by the spacing, i.e., 24”. 420” ÷ 24″ on-center=17.5.
Round off the value and add 1 to it, 18+1=19.
Now multiply this value by 2 to get the total sum.
19×2=38 is the required number of rafters for a house spanning a length of 35’.
Conclusion
Rafters are important structural components designed to support roofs of houses, sheds, and similar structures.
It is essential to know the required size, spacing, and span of rafters to ensure the stability of roofs built with them.
A 2×8 rafter will span longer than a 2×6 rafter under the same conditions.
As the spacing increases, the span decreases.
The International Residential Code (IRC) 2021 provides guidelines for rafter construction.
Local codes and online calculators can also be used to determine the required size and spacing of rafters.
Frequently Asked Questions (FAQ)
What Rafter Size Do You Need for a 12’ Span?
You can achieve a 12′ span using different rafter sizes.
A Southern pine, grade #1 2×6 rafter with a 24” spacing can span up to 12’-9”.
While graded #2, Southern pine 2×8 rafter can span up to 13’.
How Long Can a 2”x8” Rafter Span Without Support?
A 2×8 rafter can span between 11’-3” to 23’-9” depending upon the wood species and grade, the spacing provided, and the loads on it.
The maximum span of a Douglas fir-Larch 2×8 rafter with 12″ spacing and acting under a 10psf dead load and a 20psf live load is 23′-9″.
What Rafter Spacing is Used for Porch Roof?
Porch roofs are typically spaced 16″ or 24″ apart.
If the porch is small and the rafters’ self-weight is not too great, 16″ spacing is used.
For larger porches and heavier rafters, 24″ spacing is commonly used.
