6 inches (152.4mm) on center For roofs having snow loads in excess of 40psf (18.2kg/sq.m). Thats because the outer two support members are shared, thus counting as one or half + half. Purlin spacing is an important factor to consider when designing a corrugated metal or fiberglass roof. When spacing metal roof purlins for long spans during light-frame constructions, contractors and engineers should offer good judgment. The weight also matters. Rafter spacing is based on the width of the metal roofing panels. Purlin roof Insert two equally spaced nails into the rafter. Purlin Cleats Spaced to Accept Fibre Cement or Steel Roof Sheets . Heres what you need to know about each option. Roof purlins are overlapped when placed on edge. You need at least two, one at each end, for a rectangular or square room, though many projects require several trusses to form the shape of the roof. They come in the letter Z shape, with an inclination of less than 90 degrees, making them a wonderful choice for giving a roof shape and structure. The spacing of metal roof purlins is another important detail that you need to consider before choosing your system. As a result, most roofing experts recommend spacing them about two feet apart. :) With years of experience in the roofing industry I was able to join the team here at RoofCalc and help people like you with your roofing questions. If it's metal, then spacing the 2x6's to four feet is plenty strong enough. The most current IRC, published in 2021, along with previous editions, provides rafter span and spacing tables for different variables in Chapter 8. For instance, a 26-gauge galvanized steel roof weighs 150 lbs per square. Determine spacing and span direction The following factors should be considered when determining roof purlin spacing: General rule of thumb: The maximum distance between steel purlins should be about 1200 mm. It is a well-known fact, that assembled systems of profiled sheeting and purlins show significant . The outer board or fascia isnt a rafter and may be of 1 thick lumber, or of the same 2-by material as the rafters. In case the roof spans over 50 feet(15.24m), then the purlins should be designed in pairs by using a single higher purlin at the center of the span to support two lower purlins spaced 30 inches (762mm) apart. Answered by LCD: You need to check the installation instructions from the manufacturer - depending on whether the purlins (firring strips) are 1x or 2x material and how wide, thickness (metal gage) of roofing, steel or aluminum, type of seams, design wind and snow loading, whether designed to be walked on or not, etc I have seen it vary from as Quality is also important. For instance, shingles weigh up to 240 lbs per square (100 square feet). Dont hesitate to place the purlins closer if youre worried about the risk of winter ice or strong winds. Cleats are metal plates used to secure purlins to the roofing structure. Unfortunately, decks and roofs are heavy. Ideally, consider 28 rafters (two inches wide and eight inches deep) for 16-foot spans. The most common method employed is laying the purlin flat (typically a 24) and nailing to the truss with two 20D galvanized ring shank nails. 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. 40ft x 12 = 480 24 on-center = 20 rafters + 1 = 21 rafters per side x 2 = 42 rafters. Fiberglass purlins typically use an 18 on-center spacing while metal purlins normally have 24 on-center spacing. Lap metal profiled sheets by one profile (when choosing Box Profile or Tile Form) and one and a half . Open purlins are also easy to cut and customize, so you can get the exact size that you need! Staggering purlin row splice locations by halving the dimensional lumber on every other row is good practice. The standard rafter spacing for metal roofs is 16 or 24 inches on-center. 30ft x 12 = 360 24 on-center = 15 rafters + 1 = 16 rafters per side x 2 = 32 rafters. Rafter Spacing for Metal Roof Metal roofing, typically steel or aluminum, comes in a variety of profiles and compositions and may be referred to as tin, steel, or corrugated metal. Single cleats (accepting two bolts) are standard. You need a strong framing structure to carry such weight without sagging or crumbling for decades. You may also use available rafter spacing calculators online, where you can personally select the different dimensions for your roofing frame. A good purlin will be made specifically for what you need; if they are too tall or too short then this could cause issues later on down the line. In this guide, well explain rafter spacing, code requirements, roof rafter span tables, and rafter spacing calculators. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and both the ancient post-and-beam and newer pole building timber frame construction methodologies. You can select Max Span or Span Options, wood species, dimensions, grade, member type, deflection, spacing, live load, dead load, service conditions, and whether its incised lumber or not. For instance, the most common joist sizes are 26 and 28, though 210, 212, and 214 roof joists are common. You can achieve this by placing the purlins at the trusses nodes rather than on the members for even distribution and a straight roof. Galvanized and aluminum roof purlins typically have a spacing of 1.0 to 1.2 meters. However, when you actually lay them out things change a little but on you due to the fact that the gable trusses are nailed to the exterior of the posts, while the commons are notched into the post for full bearing in between. Remember that using this rafter spacing calculator may provide an accurate estimate. If the weight of the rafters of smaller pergolas wont cause the beam to droop, 12 spacing is recommended. At 16 centers, there are three support members every four feet versus two when spaced at 24 centers. For instance, Douglas fir is the standard wood rafter material in the US, UK, Australia, and Canada. Collar Ties Collar ties connect the upper ends of opposing rafters. However, the best spacing depends on many factors, including the type of metal roofing. Fortunately, you dont have to stress over rafter spacing anymore. Become more informed about the standard building codes for roof rafters to ensure its approved. There are numerous rafter spacing calculators available, but our preference is the American Wood Councils online Span Calculator. So, you can space the rafters wider. Rafters at 24 centers with strapping to support metal roofing are less expensive than those at 16 centers covered with OSB or plywood and metal roofing. A lean-to rafter can also be used to make a shed-style roof on a variety of buildings too, providing a single roofline with a high wall and a lower wall supporting the rafters. A good purlin is one that has been made to fit your given roofing needs. The width of the building or distance from the wall to the support beam determines the rafter span, spacing, and slope. In order for the metal sheets to be easily fixed to the purlins they should be at least 50mm wide. In addition, ice dams add significant weight in winter. Roofing joists also join the buildings opposite walls to support the floor above and the ceiling by transferring the roofs weight to the vertical columns and into the foundation. A rafter is an important structural component for roof construction, and if youre planning for a project, you might be wondering about the proper rafter spacing and sizing. The standard purlin spacing for metal roofs is four to six feet. These loads are classified as live or dead in roofing circles. In the case of double-span purlins, you may have a single purlin running the entire span (double unlapped) or two purlins overlapping over central support to provide continuity (double lapped). Some of our partners may process your data as a part of their legitimate business interest without asking for consent. So the first thing you need to do is convert 30 ft to inches. You should consider closer purlin spacing for greater roof strength if youre building in a location prone to heavy snow loads or high winds. The function of the purlin of a roof is to act as added support for the roof deck . Purlin spacing for corrugated roofing is something that should be given consideration when designing a roof. Rafters or roof joists, or even studs and floor joists at 19.2 on-center use 5 structural members over 8-feet instead of 6 pieces at 16 centers or 4 at 24 centers. In most roofs though, the metal roofing is flimsier and is supported by and fastened to a plywood or OSB deck, or to strapping fastened to the rafters. Rafters are roof framing members that run from the ridge or hip of the roof to the wall plate, eave, or downslope perimeter. It may be done using rafter calculators if you want a more straightforward method. Purlin Cleats Spaced to Accept Fibre Cement or Steel Roof Sheets . Generally, you need thicker rafters (28, 210, or higher) for wider spacing. A gap that isnt much different from the 14.5 between trusses at 16 centers. The design of the porch roof is a factor too. Such roofs have three support members every four feet. Z purlins are the stronger of the two purlin types. But we must explain a few more things to drive the point home. They provide support for the roof by transferring loads onto the rafters or trusses below them which can then distribute these forces further down through their own structure. Use the structures supporting outside wall length in inches and divide it by the on-center spacing between parallel rafters, and then add 1 for the 0 rafter. So, never confuse purlins for rafters. When laying the purlins flat should the seams be staggered or can all the seems lay on the same truss? Youll note that this is slightly larger than the recommended spacing for 29 gauge metal roofs. However, youre at liberty to find a spacing pattern that works best for your project. However, some roofers discourage homeowners from it, especially in heavy-load regions. 2023 Todays Homeowner Media. The purlins are horizontal beams which transfer the loads to either walls or columns. You have to consider live, dead, wind, and snow loads, select the correct species, grade, and dimension of lumber for the rafter span, and decide if there will be a ceiling or loft supported by the rafters. Rafter spacing is the measurement of the distance between the roof joists, also known as the parallel rafters that extend from the ridge or the hip to the wall plate, eave, or downslope perimeter. its a good idea to alternate slice locations by halving the dimensional lumber every other row. A metal roof must also allow for some overlap on each side of the panels, which will reduce the effective width of the panel. For instance, you can space your purlins less than a foot apart if using lighter roofing sheets and wider for thicker sheets. This not only causes damage to your roof in the form of UV degradation but it wastes energy on cooling costs as well. Then put your 2x4 purlins on the flat every 2 feet and attach your metal to the purlins. They are laid side by side in a series to provide base support for roof decks. The standard spacing for aluminum and galvanized roof purlins is 1.0 to 1.2 meters. Rafter Wood: What Wood is Used for Rafters? Lightweight roof and wall structural systems are widely used for cladding of steel portal frames. Wider panels allow for greater spacing between rafters. However, cedar, pine, and spruce are applicable in low-load applications. Metal roof purlins are the members of a metal panel system that run perpendicular to the panels. The same applies to thicker decking. The nails should be 20D ring shank nails, and there should be minimal spacing between the nails. These standard rafter spacing measurements apply to standard roofing projects. However, it all comes down to the project requirements. Higher snow loads and larger spans will require purlins to be spaced closer together than 24 on center in the drift area. A sheathed roof deck provides ample support for the metal roofing and is an additional barrier to insects and weather. This explains why Z purlins are preferred for large industrial and agricultural projects. 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. The following tips can be considered; That is, if the number of purlins in a span exceeds 5, then the center-to-center spacing should not be more than 30 inches. Example: In the rafter span table below, the highlighted cell (13-0), indicates that 2" x 8" Douglas Fir rafter, with a grade of #2, spaced 24" apart, can have a maximum span of 13 feet - 0 inches (13-0) if designed for a live load of 20 psf, and dead load of 20 psf. Steel Frame Building 30'x20'x10'. The purlins shouldnt be placed close to each other to prevent the roof from bending or the shape from being distorted. Heavier metal roofs dont need as much support as lighter alternatives. The rafter spacing used for metal roofs is commonly set by the metal panel manufacturer to ensure it's suitable for the dimensions of the roof structure. When using 29 gauge metal roofing, your purlins should be 1.2 meters apart when using 0.7mm roofing sheets and no more than one meter apart when using 0.5mm roofing sheets. However, you can also space your rafters 12 inches or 19.2 inches on-center. There you have it. A roofing joist is a parallel horizontal member or beam that runs across an open space, which can either be laid pitched or flat, exposed or concealed, depending on the construction. Well discuss 16 and 24 spacing, how to calculate the number of rafters required, 19.2 spacing, and the spacing when using metal roofing and for lean-to and porch roofs. However, this can be customized depending on what you need! Rafters that are strapped are either at 16 or 24 centers with strapping placed every 8 to 12 on-center to provide support. 3. Rafters serve two main purposes. You need a stronger roof frame to support heavier live loads. A few roofers use 12 inches or 19 inches on-center spacing. The standard roof joist span is 20 feet. Rafter beams are exposed horizontal structural elements you see in the ceiling designed to support the weight of the buildings roof, transferring the load to a vertical structural part. Although its only one rafter saved every eight feet, you can save significantly on nails, insulation, and wiring. Purlin spacing calculator - The calculator uses the ridge length for the length of the purlins. Finally, spacing roof rafters 16 inches on-center allows you to use slimmer dimensional lumber. However, switching to 24 centers also decreases the number of rafters required by up to 50%. The cleat bolts go through the cleats and the wood/metal purlins. Meanwhile, the standard rafter size is 24 for areas with low roof loads and 26 for regions with heavy roof loads. Alternatively, you can space the rafters closely to distribute the load. Apex 0 mm. The rafter spacing will be set by the steel panel manufacturer to provide a structurally sound roof. As you can imagine, you need stronger, thicker rafters to cover longer spans to minimize the risk of sagging. However, there are two more types of purlins spans double-span purlins and continuous span purlins. Many of the metal purlins are used to hold rafters at specified distances apart. The two main purlin styles are C purlins and Z purlins. How to Calculate Roof Rafters: How Many Rafters Do I Need. However, the structural engineer may determine other roof rafter spacing measurements, and these may vary depending on the contributing factors affecting it. Rafter spacing for metal roof. However, for closely spaced rafters, you can use thinner lumber boards (24 or 26). Rafters are members of the roofing structure that run perpendicularly from the top of the walls to the roof ridge. Many roofers join rafter feet with ceiling joists for a more rigid structure. The standard roof rafter spacing is 16 inches or 24 inches on-center. So you have rafters running vertically at some spacing then purlins running horizontally at some spacing on top of the rafters and the metal panels screw in to the purlins. There are many different types of roof purlins based on material and styles. The standard rafter spacing for metal roofs is 16 or 24 inches on-center. Apex 0 mm. When planning for roof construction, its important to know the standard rafter spacing and sizing, lumber sizes, and the estimated number of rafters needed to ensure proper structural building. Heres what you need to know, including how to install them. In addition, its necessary to know rafter spacing because, as part of the engineering calculations, it helps determine and form the required roof structure that meets the building codes while holding up the roof load and roof weight. We and our partners use data for Personalised ads and content, ad and content measurement, audience insights and product development. The manufacturers of the steel roof truss plates (also referred to as gussets or Gang-nails), provide the engineering design for pre-fabricated wood trusses. In addition, there are different types of lumber that can be used as sheathing boards; if they are plywood boards instead of regular wood board then there should be at least 3. For instance, you can use 24 rafters for 10 and 12-foot spans without issues. C-section purlins can span up to 20 feet and dont require drilling. Typically, though, the closer the rafters are together, the more support provided to the sheathing and roofing finish, the further the rafters can span, and the stronger and stiffer the roof. Kiln-dried Douglas fir is the preferred wood species for rafters on standard building projects as its lightweight but strong. Unfortunately, even Douglas fir doesnt make the grade for clay and concrete tile roofing. However, spacings smaller than a foot and as wide as five feet are common for weak purlin/roof materials and strong roof/purlin materials, respectively. For instance, you need purlins every four inches for a very light roof covering but can space your purlins up to four feet apart if you use a very heavy roof covering. Theres also a 24 lumber, but its not commonly used in building. They are typically made from hardwoods for strength and durability. Answers in 5 seconds In just 5 seconds, you can get the answer to your question. Rafter spacing for a porch roof may vary depending on the various factors that affect it, including wood species, grade, load, deflection, purpose, porch roof design, and even climate. We want to hear from you! Thanks for the queston. Roof joist spacing has requirements or factors, including the rafter size, rafter span, thickness, wood species, grade, thickness, wind and snow loads, deflection, and location. Otherwise, the rafters may buckle and fracture under their weight. Given rafters are the primary framing members responsible for carrying and distributing the roof load, any sign of laxity can bring the roof down. (GS) Girt Spacing (default: 36). Roll out an underlayment, then install the flashings on the eaves of the roof. You should consider closer purlin spacing for greater roof strength if youre building in a location prone to heavy snow loads or high winds. All dimensions are in inches (millimeters). If youre not sure which type of purlins to use for a corrugated metal or fiberglass roof, consult with someone who has experience designing roofs. If unsure which spacing to use, discuss the design with your local building department or a Structural Engineer. For example, purlins should be placed eight feet from 24 struts and six feet away from 26 struts. Aluminum is approximately 2.5x lighter than the other two materials, enabling wider rafter spacing. 40ft x 12 = 480 (length in inches) 24 on-center = 20 rafters. So, it is always important to check with your local building department or other building experts for rafter or roof joist spacing requirements. Purlins are dimensional lumber that are fastened to the top chord of trusses connecting them together for bracing purposes. The exact space between two rafters matters little, provided you use the right rafter sizes and spacing. Spacers will ensure the beams are properly positioned and do not shift over time. Therefore, you can space your rafters closer but not wider than the building code states. Nevertheless, the following are a few industry guidelines to help you choose a good purlin spacing pattern for your metal roofing project. It may also depend on other factors, including the pitch or slope of roof, roof sheating, grade of timber, lumber size, wood species, snow load, live load, and rafter spacing. The best way to ensure your Purlins are set properly is to use Purlin spacers. Purlin spacing determines the size of your roofing material, as well as how much ventilation you have in your building. For that, we thank you for taking time to browse our articles and hope you find some helpful information to help you with your situation. If the weight of the rafters would be a problem to bear for the beam, its best to use 16 spacing. This means the shingles alone weigh up to 3,600 lbs, equivalent to 24 average-sized adult humans. The loads are first taken by the sheeting and transferred to the rafters. Purlins are spaced 24 apart on horizontal roofs. Alternatively, the maximum span is 83 for 16 inches OC and 73 for 24 inches OC spacing. Rafter 0 mm. A lean-to roof typically attaches to the side of another building and projects down and outward to be carried by a wall or on a beam supported by posts. . Industrial roof construction projects mostly require 212 wooden rafters or steel rafters. Therefore, a 12-inch joist spacing is ideal for locations with strong winds, storms, and heavy snowing. Staggering Splice Locations I'm not really good at this internet stuff. The following table compares the span for common #2 grade dimensional lumber of different species and the three typical spacings between rafters. Unfortunately, many DIYers think you can space them as you wish. On-center means measuring the distance from the first rafters center to the next rafters center. Fortunately, many local authorities follow the International Residential Code (IRC) or International Residential Building Code (IBC). Rafter spacing is usually based on span, the dimensional lumber used, and load factors, so its best to check with the experts before driving in nails or screws.