Roof Rafter Spacing, Span, and Sizing Explained

The International Residential Code (IRC) 2021 specifies roof rafter spacings of 12", 16", 19.2", and 24".The common sizes for rafters include 2x4, 2x6, 2x8, 2x10, and 2x12. For a particular span, rafter spacing distance can vary based on the species of lumber, grade, and load conditions.

Rafters act as support for roofs of houses, sheds, porches, cabins, and garages.

Multiple loads have to be factored in, such as the live and dead loads, wind loads, and snow loads, to select the right size of the rafters for a specific project.

In addition to loads, choosing the right species, grade, span, and spacing for the selected roof rafters to function efficiently is vital.

Roof Rafter Spacing, Span, and Sizing Requirements

Rafter spacing refers to the measure of the distance between two consecutive parallel rafters (also known as ceiling joists).

Measure the distance between one rafter center and another adjacent rafter center.

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.

Spans of rafters are highly dependent on spacing, in addition to being affected by species of wood used, its grade, and loads.

Greater the distance between two rafters, the lesser length they will be allowed to span.

A Douglas fir-larch, SS grade 2x6 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	2x6	2x8
(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

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, its span also needs to be determined 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, 2x8 rafters have span lengths ranging from 13’ to 19’.

A 2x10 can span up to 23’ while a 2x12 rafter may span up to 27’.

Rafters Spacing (inches)	Species	Grade	2x10 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

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.

These parameters must be established before starting the construction of a roof.

The most common rafter sizes are 2x4, 2x6, 2x8, 2x10, and 2x12.

2x4 rafters are more commonly used in non-residential construction.

Loads, wood species, wood grade, the pitch of rafters, the slope of a roof, and the location of the rafter all factor into determining the final size of a 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.

The manufacturer determines the metal panel width, while 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 bigger and have larger self-weights, 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.

Roofs with metal sheathing or planks are usually spaced with rafters 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 2x6 rafter with an unattached ceiling can span spans up to 18’.

In contrast, the same rafter can span only up to 16’-4” with an attached ceiling.

Rafters Spacing	Species	Grade	2x6
(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

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. 35x12= 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.

19x2=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 and spacing of rafters to ensure the stability of roofs built with them.

Rafters come in various sizes, as mentioned above.

It is important to select the right size of rafters for them to span long enough to meet the requirements.

The span of a rafter increases with an increase in thickness.

A 2x8 rafter will span longer than a 2x6 rafter under the same conditions.

Spacing is another important factor that needs to be set before construction.

The span of the rafters is also related to the spacing provided between them.

As the spacing increases, the span decreases in measurement.

The International Residential Code (IRC) 2021 has guidelines and tables relating to rafters to help guide builders and engineers.

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?

Using different rafter sizes, a span of 12' can be achieved.

A Southern pine, grade #1 2x6 rafter with a 24” spacing can span up to 12’-9”.

While graded #2, Southern pine 2x8 rafter can span up to 13’.

How Long Can a 2”x8” Rafter Span Without Support?

A 2x8 rafter can span anywhere between 11’-3” to 23’-9” depending upon the wood species, the wood grade, the spacing provided, and the loads on it.

The maximum span of a Douglas fir-Larch 2x8 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.