Did you know that there are standard guidelines for roof rafter spacing? Unfortunately, many DIYers think you can space them as you wish. However, the opposite is true, especially for the main home. Indeed, you can be penalized if a home inspection reveals improper rafter spacing.
For this reason, we’ve developed this guide to explain rafter spacing requirements for metal roofs on residential homes. We also briefly explain rafter spacing requirements for various structures, including porches, pergolas, and sheds.
Best metal roof rafter spacing
The standard rafter spacing for metal roofs is 16 or 24 inches on-center. However, you can also space your rafters 12 inches or 19.2 inches on-center. The exact figure depends on many factors, including the type of roof structure, roof design (style), etc. Generally, the smaller the rafters, the closer the spacing.
What are Rafters?
Rafters are roof framing members that run from the ridge or hip of the roof to the wall plate, eave, or downslope perimeter. They are laid side by side in a series to provide base support for roof decks.
Although steel is preferred in industrial construction, dimensional lumber is the best rafter material for residential homes. It’s eco-friendly, energy efficient, beautiful, and easy to install. Above all, timber is readily available.
Rafters serve two main purposes. First, they provide a platform for decking installation. But more importantly, they distribute the roof load down to the walls and the foundation for a structurally sound home. Many roofers join rafter feet with ceiling joists for a more rigid structure.
What’s Rafter Spacing?
Rafter spacing is the shortest distance between two parallel rafters measured “on-center.” Alternatively, you can determine the rafter spacing by measuring the distance between two ceiling joists on-center.
“On-center” means measuring the distance from the first rafter’s center to the next rafter’s center. The same applies if measuring the distance between ceiling joists. Measure from the center of the first to the center of the next.
Importance of Rafter Spacing
Why is rafter spacing important? Because the wrong spacing leads to a weak roof, thus, a weak house.
The relationship is straightforward. Rafters support decks, which support the shingles or panels. Unfortunately, decks and roofs are heavy.
For instance, shingles weigh up to 240 lbs per square (100 square feet). Yet, a standard roof is 1,500 square feet. This means the shingles alone weigh up to 3,600 lbs, equivalent to 24 average-sized adult humans.
Meanwhile, a sheet of 7/16 OSB decking weighs 43.8 pounds, and you need about 50 per for a standard roof, adding 2,000+ lbs to the roof.
You need a strong framing structure to carry such weight without sagging or crumbling for decades.
So, you can see the importance of rafter sizes and spacing. Given rafters are the primary framing members responsible for carrying and distributing the roof load, any sign of laxity can bring the roof down.
Remember also that the roof is constantly up against winds and storms. In addition, ice dams add significant weight in winter.
These loads are classified as live or dead in roofing circles. A live load refers to any temporary weight-adding factor, such as a contractor on the roof, wind, ice, or standing water. Meanwhile, a dead load is any weight-bearing feature permanently stationed on the roof, notably the shingles and decking.
Rafters rely on their individual and collective “strength” to carry the loads and counter additional pressures, and spacing is critical for the “collective” strength.
Factors Affecting Roof Rafter Spacing
Several factors influence rafter spacing. However, the six most important factors are as follows;
Rafter Size
Rafter size is one of the most important factors when determining the right rafter spacing. The “size” is mainly concerned with the width and depth. Meanwhile, the span size address all rafter length concerns.
Generally, you need thicker rafters (2×8, 2×10, or higher) for wider spacing. However, for closely spaced rafters, you can use thinner lumber boards (2×4 or 2×6). Industrial roof construction projects mostly require 2×12 wooden rafters or steel rafters.
Span Size
The rafter span is the horizontal distance between the supporting wall and the center of the roof. Please distinguish it from the rafter length, measured diagonally along the rafter. Instead, the span is measured horizontally or parallel to a flat floor.
As you can imagine, you need stronger, thicker rafters to cover longer spans to minimize the risk of sagging. Alternatively, you can space the rafters closely to distribute the load. Otherwise, the rafters may buckle and fracture under their weight.
Wood Species and Grade
Kiln-dried Douglas fir is the preferred wood species for rafters on standard building projects as it’s lightweight but strong. Moreover, it’s less knotty than other softwoods. It’s also readily available and economical. However, cedar, pine, and spruce are applicable in low-load applications.
Unfortunately, even Douglas fir doesn’t make the grade for clay and concrete tile roofing. For instance, concrete tiles weigh up to 1,100 lbs/square (100 square feet).
To bear the weight, you’d need to space the rafters less than a foot apart. Thus in such cases, you may opt for stronger hardwoods like oak to maintain a sensible and economical rafter spacing.
Roof Sheathing
Many people use OSB or plywood for roof sheathing or decking. Specifically, 7/16 OSB and 5/8 plywood are the standard sizes. But what if you cannot find sufficiently strong plywood?
For instance, what if you can only find 3/8-inch thick plywood? In that case, you must space the rafters closer. The opposite is true for thicker plywood.
The type of OSB or plywood is also important. For instance, hardwood plywood is stronger. So, you can space the rafters wider. Meanwhile, some softwood plywood sheets need to be stronger, requiring closer roof rafter spacing. Quality is also important.
Roof Load
Different roofs support different amounts of load. For instance, we’ve seen that concrete tiles are among the heaviest roofing materials. Thus, you need more rafters to support a concrete tile roof. The same applies to slate, which weighs up to 1500 lbs per square.
On the other hand, metal roofs are very light. For instance, a 26-gauge galvanized steel roof weighs 150 lbs per square. So, you can do with wider-spaced roof rafters.
You also must consider the expected live load. For instance, do you live in a windy area? What about snow? Does the area experience heavy snowing? You need a stronger roof frame to support heavier live loads.
Building Codes
Finally, always follow your local building codes. Fortunately, many local authorities follow the International Residential Code (IRC) or International Residential Building Code (IBC).
So, you can check the IRC website for guidelines. Alternatively, check your local building department’s website or visit the offices.
Building codes focus on maximum roof rafter spacing depending on expected live and dead load. Therefore, you can space your rafters closer but not wider than the building code states.
How Far Apart should the Rafters Be?
The standard roof rafter spacing is 16 inches or 24 inches on-center. However, you can also space your roof rafters 12 inches or 19.2 inches on-center.
These standard rafter spacing measurements apply to standard roofing projects. For instance, Douglas fir is the standard wood rafter material in the US, UK, Australia, and Canada. Meanwhile, the standard rafter size is 2×4 for areas with low roof loads and 2×6 for regions with heavy roof loads.
The exact space between two rafters matters little, provided you use the right rafter sizes and spacing. That’s because all rafters are two inches wide when installed.
What’s the Best Metal Roof Rafter Spacing?
The standard rafter spacing for metal roofs is 16 or 24 inches on-center. However, the best spacing depends on many factors, including the type of metal roofing.
For instance, is it aluminum, tin, or steel? Aluminum is approximately 2.5x lighter than the other two materials, enabling wider rafter spacing. Also, is it a standing seam roof with concealed fasteners or a corrugated roof with exposed fasteners?
The weight also matters. For instance, metal slates are heavier than metal tiles or shingles. So, you need closer rafter spacing for slates.
Another critical consideration is whether you intend to fasten the shingles directly to the rafters. Most roofers install decking over the rafters and lay the roofing material over the decking (after installing an underlayment).
Others fasten purlins (or other strappings) to the rafters and fasten the roofing material directly onto the purlins.
A shorter rafter spacing is recommended if you discard the decking. We recommend 8-12 inch spacing on-center for a strong roof. The exact spacing depends on your straps’ type, size, and spacing. Closer strapping spacing allows for wider roof rafter spacing.
16 vs. 24-Inch On-Center: Which is Better?
This is one of the biggest dilemmas among roofers. A 16-inch on-center spacing naturally provides a stronger, more rigid roof frame. However, it gobbles up almost twice as many rafters as 24-inch on-center spacing.
Meanwhile, 24-inch on-center spacing is cost-saving. However, some roofers discourage homeowners from it, especially in heavy-load regions.
So, which is better? It depends on various factors. The following are the pros and cons of each option to help you make an informed choice.
16-Inch On-Center Rafter Spacing Pros and Cons
Spacing roof rafters 16 inches on-center is a common practice for sheathed roofs. It provides better support for the decking, preventing premature sagging. It’s also more resistant to unexpected live loads.
Such roofs have three support members every four feet. Contrastingly, a 24-inch on-center spacing only has two support members every four feet. That’s because the outer two support members are shared, thus counting as one or half + half. More support members per unit area mean better roof load support.
Finally, spacing roof rafters 16 inches on-center allows you to use slimmer dimensional lumber. For instance, you can use 2×4 and 2×6 boards without any qualms. Additionally, the spacing is more forgiving on lumber grade.
For example, whereas you need construction-grade (grade-1) lumber for wider spacing, you can use standard-grade (grade-2) lumber if you space the rafters 16 inches apart on-center.
24-Inch On-Center Rafter Spacing Pros and Cons
A 24-inch on-center roof rafter spacing is desirable if you wish to minimize the number of rafters. Typically, you can reduce the total number of rafters by 50%.
Moreover, 24-inch on-center roof rafter spacing works like a charm in fair climates with no storms, snowing, or strong winds. They safely accommodate roofer weight and easily resist sagging.
However, they come with a few challenges worth noting. First, avoid the spacing in regions with extreme weather. For instance, a 24-inch on-center roof rafter spacing in the north is asking for trouble.
The heavy snow load in winter will bring down the roof within months. Also, the spacing is ill-suited for regions that experience heavy storms.
Secondly, you should strongly consider thicker decking and dimensional lumber. Thicker dimensional lumber supports more weight individually.
For instance, a 2×8 roof rafter supports twice as much load as a 2×4 rafter. Therefore, you can space thicker rafters wider without adverse consequences. The same applies to thicker decking.
Finally, you need the highest decking and lumber grade for 24-inch on-center roof rafter spacing. For instance, grade-C plywood sheets with visible knots are a no-no for 24-inch on-center rafter spacing. Instead, find grade-A plywood sheets with no visible flaws.
What about 19.2-Inch Rafter Spacing?
A 19/2-inch on-center rafter spacing is the perfect compromise for many roofers and homeowners. It offers the best of both worlds, consuming fewer rafters without weakening the roof structure.
Rafters spaced 19.2 inches apart on-center have five structural members over eight feet instead of four (at 24 inches on center) or six at 16 inches on center. Although it’s only one rafter saved every eight feet, you can save significantly on nails, insulation, and wiring.
Above all, 19.2 inches on-center rafter spacing is perfect for engineered i-joists and truss rafters.
How to Calculate Roof Rafter Spacing
Fortunately, you don’t have to stress over rafter spacing anymore. The following are three quick ways to determine the appropriate spacing of roof rafters without spending a dime.
- Follow the standard guidelines: The first option is to follow the standard industry figures, i.e., 16 or 24 inches on-center. Use 16 inches on-center spacing if you live in an area with severe weather or when roofing with heavy roofing materials like slate or tiles. Meanwhile, use 24-inch on-center spacing for regular asphalt and metal roofing in normal climates.
- Use a rafter spacing calculator: A few professionals have designed modern online calculators that quickly calculate the rafter spacing if you provide certain information. For instance, you may need to provide the rafter length, roof span, and rafter size (dimensions). Then the calculator uses international codes (IRC and IBC) to determine the appropriate roof rafter spacing. Most roof rafter calculators are free to use.
- Let your contractor handle it: We always recommend professional roofing as it gives you peace of mind. Moreover, professionally built roofs ultimately justify the cost thanks to fewer repairs. Above all, your roofer knows rafter spacing requirements like the back of their hand. So they’ll handle it without asking.
Metal Roof Rafter Span Requirements
We’ve already highlighted the role of the rafter and roof span in determining the right rafter spacing. But we must explain a few more things to drive the point home.
The Rafter span is the horizontal distance between the supporting wall (or the wall plate) and the center of the roof. Meanwhile, the roof span is the horizontal distance between two opposite wall plates.
The easiest way to determine the correct rafter span is to check roof rafter span tables from reputable sources, such as the IRC or IBC. Nevertheless, the following guidelines apply to most residential roof construction projects;
- For 2×4 rafter sizes, the maximum span is 9.0 feet if the rafters are spaced 12 inches on-center (OC). Alternatively, the maximum span is 8’3″ for 16 inches OC and 7’3″ for 24 inches OC spacing.
- For 2×6 rafter sizes, the maximum allowable span is 14’6″ for 12 inches OC spacing, 13’0″ for 16 inches OC, and 11 feet 6’24” for 24 inches on-center spacing.
The maximum span increases for thicker rafters. For instance, you can go up to 19 feet using 2×8-inch dimensional lumber, 23 feet using 2×10 rafters, and 26 feet 9.0 inches using 2×12 rafters.
Metal Roof Joist Spacing Requirements
Roof joists, often confused with rafters, are horizontal framing members that run across an open space between one wall plate and the next. Meanwhile, rafters are diagonal in pitched roofs.
However, we understand the confusion given that separating joists from rafters is nearly impossible in low-pitched roofs. Indeed, rafters double up as joists in flat roofs.
For this reason, joist spacing is similar to rafter spacing. The standard roof joist spacing is 16 or 24 inches on-center, though 12 inches on-center and 19.2 inches on-center spacing is common.
Again, the closer the spacing, the sturdier the roof structure. Therefore, a 12-inch joist spacing is ideal for locations with strong winds, storms, and heavy snowing. It’s also the perfect choice for heavy ceiling loads or to convert the attic into a storage location.
Meanwhile, 24-inch joist spacing is best for fair-weather locations, especially for regular ceiling projects.
Roof joist sizes and spans are almost similar to rafter spans. For instance, the most common joist sizes are 2×6 and 2×8, though 2×10, 2×12, and 2×14 roof joists are common. The standard roof joist span is 20 feet.
Additional Spacing Tips for Roof Rafters
The following are additional rafter spacing tips to consider for other roof construction projects around the home;
- Lean-To Rafter Spacing for Metal Roof: We recommend 16 or 24 inches on-center roof rafter spacing for lean-to-object roof styles. Consider the rafter span, wood species, wood grade, and load variables.
- Porch Roof Rafter Spacing: The ideal rafter spacing for a porch roof is 16 or 24 inches on-center. However, the exact spacing depends on numerous factors, including the wood species/grade, roof load, deflection, roof design, and the local climate.
- Rafter Beam Spacing: A rafter beam is the exposed horizontal structure in the ceiling. It supports the weight of the building and transfers the load to vertical structures. The standard roof rafter beam spacing is 16 inches on-center. Never go above this mark.
- Rafter Spacing for Pole Barn: Roofers generally space pole barn rafters at 4-12 inches apart. It depends on the building specifications and the owner’s preference.
- Rafter Spacing for a Pergola: The standard pergola roof rafter spacing is 12-20 inches on center, though 12-16 inches OC is the most common range. The exact spacing depends on the size of the pergola and roof load.
- Rafter Spacing for Shed: You can space shed roof rafters at 16 or 24 inches on-center, depending on the rafter size and span. For instance, consider 16 inches on-center spacing for 2×6 or thicker dimensional lumber and 24 inches on-center for 2×4 rafters.
FAQs
How far apart should purlins be spaced?
Roof purlins must be 1.2 meters apart or closer for 0.7mm sheathing and 1.0 meters for 0.5mm sheathing material. The actual spacing depends on wood type and grade and purlin thickness. The industry standard is four to six feet, though most residential building purlins are spaced 3-4 feet apart.
What size rafters for a 16-foot span?
Ideally, consider 2×8 rafters (two inches wide and eight inches deep) for 16-foot spans. The shorter the span, the smaller the required rafter size. For instance, you can use 2×4 rafters for 10 and 12-foot spans without issues. However, you need 2×8 or thicker rafters for wider spans.
Summary
The best rafter spacing for a metal roof is 16 inches or 24 inches on-center. A few roofers use 12 inches or 19 inches on-center spacing. However, 16 and 24 are the industry standards. Consider the wood species and grade, rafter span, roof load, and your local building codes when deciding.