Roof rafters, the roof framing members running from the wall plate to the roof ridge, must be properly spaced.
For one, they directly impact joist and ceiling spacing and the integrity of the roof deck. But, more importantly, they’re the sole framing members responsible for distributing roof weight to the walls.
Fortunately, many roofers and homeowners already understand that 16 and 24-inch on-center are the standard rafter spacing measurements. But the big dilemma is choosing one between the two. Which is better? This guide explains everything you need to know to make an informed decision.
Do you use 16 or 24 rafter spacing?
Both options are practical. However, 24-inch on-center spacing exerts about 50% more weight on each rafter than 16-inch on-center spacing. Also, the rafters must be deeper (thicker), of a higher grade/quality, and from a stronger wood species. But 24-inch on-center spacing requires 50% fewer rafters, thus significant cost savings.
What’s Rafter Spacing?
Rafter spacing refers to the horizontal distance between two parallel rafters on a roof. A rafter is a diagonal framing member on pitched roofs running from the apex of the roof to the roof plate or the eaves. Alternatively, it’s a near-horizontal member that doubles up as a roof joist on flat roofs.
Rafter spacing is measured “Off Center” (OC). This means measuring from the first rafter’s center to the next rafter’s center.
Factors Affecting Roof Rafter Spacing
Rafter spacing depends on multiple factors. We’ll only consider the six most important ones.
Rafter Type
Rafters are typically made from timber or steel. Wood is the material of choice for residential roof construction. Meanwhile, steel offers better strength properties for industrial applications.
The type and grade of the timber matter a lot. For instance, weaker softwood lumber, such as spruce, necessitates more rafters per square than a hardwood like oak. Similarly, you need more rafters when using lower-grade lumber.
Rafter Size
Besides material and grade, consider each rafter’s size. Size here primarily refers to the dimensions that impact the thickness, thus the width and depth. Though important, we’ll leave out the length for now.
Common rafter sizes are 2×4, 2×6, 2×8, 2×10, and 2×12. However, most roofers use 2×6 and 2×8 rafters.
Generally, you need thicker dimensional lumber (2×8 or higher) for wider-spaced rafters, especially at 24 inches on-center. Meanwhile, 2×4 and 2×6 rafters are ideal for closely-spaced rafters.
Rafter Span
Rafter span is the rafter projection horizontally from the ridge to the wall. So, it’s not the rafter’s diagonal length. Instead, it’s measured parallel to the ceiling or floor.
The span directly impacts rafter spacing. Generally, the shorter the span, the wider the maximum allowable rafter spacing.
Unfortunately, you cannot change the span after building the structure. Instead, you can only adjust the rafter spacing to match the span.
Roof Load
The two main roof load categories are live load and dead load. Live lead refers to temporary weight factors affecting the roof, including wind, snow, and a contractor on the roof.
Meanwhile, dead load refers to permanent roof load factors, like the shingle and decking weight.
Naturally, the greater the load, the more support you need to prevent a sinking roof. Therefore, you need a closer rafter spacing.
Roof Truss and Joist Properties
The best roof trusses and joists help to evenly distribute the roof load to the walls and foundation for greater structural stability. Thus the design, size, spacing, and physical properties of your trusses and joists are critical.
For instance, you need steel trusses for industrial applications or extra-heavy roof systems. Otherwise, you may need thick rafters spaced closely.
You can find roof joist spacing requirements from the IRC or IBC. Meanwhile, speak with your structural engineer about truss design.
Type and Properties of the Roof Sheathing
Three sheathing properties are important when calculating rafter spacing – material, size (thickness), and grade.
The two main roof sheathing materials are OSB and plywood. OSB is stronger, accommodating wider rafter spacing. However, the thickness changes everything. For instance, thicker plywood can carry more load, thus allowing wider rafter spacing.
Finally, roof sheathing materials are graded depending on their quality. Consider the highest grade if you wish to space your rafters wider.
What is the Most Common Spacing for Rafters?
Standard roof rafter spacing measurements are 12, 16, 19.2, and 24 inches on-center.
However, the most common spacing is 16 inches OC, followed by 24 inches OC. The other two are used in special circumstances, such as extreme climates, sheds, and barns.
16 vs. 24-Inch Rafter Spacing
Now we know the primary factors affecting rafter spacing. So, we can determine when to space rafters at 16 inches on-center and when to space them wider, at 24 inches on-center.
When to Space Rafters 16-Inch On-Center
The following are scenarios when you can space rafters 16 inches on-center with outstanding outcomes.
- When using 2×6 inch rafters spanning up to 14 feet 8 inches
2×6 rafters are the most common rafter size, applicable in many projects. Therefore, we wouldn’t be surprised if you’ve already ordered a few.
If so, you can space the rafters 16 inches OC, provided the rafter span doesn’t exceed 14 feet and 8 inches. Consider thicker rafters for wider spans or spacing.
- When using 2×8 inch rafters spanning up to 19 feet and 6 inches
2×8 rafters are thicker and ideal for heavier roofing materials like tile and slate. However, the correct span and spacing are critical to achieving a structurally sound roof.
We recommend spacing 2×8 rafters 16 inches on-center if the rafters span no more than 19 feet and 6.0 inches. The spacing works like a charm if the 2×8 rafters span 15 to 19 feet.
- When using 2×10 inch rafters spanning up to 24 feet 8 inches
2×10 rafters are perfect if you live in a snowy climate. They are strong enough to withstand 20 pounds per square foot of live load and another 15 pounds per square foot of dead load.
However, if you wish to space them 16 inches apart on-center, the rafters must span at most 24 feet 8.0 inches. So ideally, you should aim for a 16 to 24-foot span.
- When using 2×12-inch rafters spanning 30 feet
Heavy load applications combining substantial live load and significant dead load often call for 2×12 rafters. This is also the perfect rafter size for wide spans up to 33 feet if the rafters are spaced 12 inches apart on-center.
However, if you wish to space them 16 inches apart on-center, 2×12 grade-1 southern pine rafters can span up to 30 feet at most.
When to Space Rafters 24 Inches On-Center
The following are common roofing scenarios when you should space rafters 24 inches on-center.
- When using 2×6 rafters on a 12 feet 8.0-inch span
We’ve seen 2×6 rafters span up to 14 feet 8 inches when spaced 16 OC. But what if you’re working on a smaller roof with rafter spans under 12 feet?
In that case, you can space the rafters wider, up to 24 on-center. Specifically, 2×6 rafters span up to 12 feet and 8.0 inches when spaced 24 inches on-center.
- When using 2×8 rafters spanning up to 18 feet 6 inches
Again, a 16-inch on-center spacing is perfect if you wish to span about 20 feet (19 feet 6 inches, to be specific) using 2×8-inch rafters. But what if it’s a smaller house with a shorter span? You can space the rafters wider without compromising the roof’s structural integrity.
We recommend spacing 2×8 rafters at 24 inches on center if the span is 18 feet 6 inches or shorter.
- When spacing 2×10 rafters at a maximum span of 21 feet 3.0 inches
Whereas a 16-foot on-center spacing works best for 2×10 rafters spanning 24 feet 8 inches, a 24-inch on-center spacing allows you to use the same rafters wider apart, assuming a shorter span.
Specifically, you can space 2×10 rafters 24 on-center if you only need to span 21 feet. Consider thicker rafters for wider spacing.
- When working with 2×12 rafters spanning 25 feet 10 inches
Finally, a 24 on-center spacing makes sense when roofing with 2×12 rafters that must span about 25 feet.
A 16 on-center spacing allows you to span up to 30 feet using the same size rafters. However, a wider spacing (24 inches on-center) puts extra weight on each rafter. Therefore, they must span a shorter distance.
Porch Roof Rafter Spacing: 16 or 24 on Center?
Porch roof rafters can be spaced 16 or 24 OC. Both measurements work well. However, the choice comes down to load variables, the type of rafters, rafter span, and the local building codes.
For instance, 16 OC is the better choice in colder climates with substantial snow loads in the winter. It holds the extra weight better with a reduced risk of buckling.
Also, 16 inches OC spacing makes more sense when using thinner rafters, such as 2x6s and 2x8s. Finally, choose 16 OC spacing for longer spans.
On the other hand, a 24 on-center roof rafter spacing works better in low-load climates, such as in the south, where the weather is sunny most of the year. It’s also better when using thicker dimensional lumber, such as 2x10s and 2x12s, or working with shorter spans.
Finally, always check with your local building department to find out what the law says. Alternatively, follow International Residential Code (IRC) or International Residential Building Codes (IBC).
When to Use a Rafter Spacing Calculator
We advise finding reputable rafter calculators if you don’t have the time to visit the local building department or scrutinize span tables.
Industry professionals develop rafter calculators to help roofers and contractors quickly determine the number of rafters needed for a specific project. But they also work reverse, allowing you to determine the correct rafter spacing under specified conditions.
You begin by providing critical data, including the rafter size, type of roof (e.g., shingle or metal roofs), roof design, rafter span, etc. Then the rafter calculator determines the appropriate rafter spacing.
16 or 24 Rafter Spacing FAQs
What size rafters do I need for a 24-foot span?
Per IBC guidelines, you need 2×12 rafters for a 24-foot span. This means the rafters should be two inches wide and 12 inches deep. Space the rafters 16 inches on-center for a structurally sound roof. Alternatively, consider 2×14 rafters if you live in a region prone to strong winds, storms, or heavy snowing.
How far can a 2×8 rafter span 24 OC?
2×8 roof rafters can span a maximum of 18 feet 6.0 inches when installed 24 inches on center. This industry standard is based on grade-1 southern pine lumber rafters and roof pitches not exceeding 3/12. Also, it assumes a live load not exceeding 20 lbs/square foot and a dead load not exceeding 15 lbs/square foot.
What’s the ideal 2×6 rafter spacing
The ideal spacing for 2×6 rafters is 16 or 24 inches on-center. However, the actual spacing depends on various factors, notably the rafter span. For instance, space the rafters 12 inches apart on-center for a 16-foot span. Otherwise, the roof load and external pressures can exert too much weight on the framing.
What is the Maximum Span for a 2×6 Rafter?
The maximum span for 2×6 rafters is 16 feet 5 inches, which only applies when the rafters are installed 12 inches apart on-center. Otherwise, the maximum allowable span is 14 feet 8 inches for 16-inch rafter spacing and 12 feet 8 inches for rafters installed 24 inches apart on-center.
Summary
Now you know the difference between 16 inches on-center and 24 inches on-center rafter spacing. Both are practical. Indeed, they’re the most common rafter spacing measurements.
However, 16 OC spacing involves more rafters, thus accommodating thinner dimensional lumber, longer spans, and weaker sheathing.
24-inch on-center spacing is more economical but requires thicker rafters of higher grades.