However, cedar, pine, and spruce are applicable in low-load applications. In general, the rafter spacing for the porch roof is typically either 16 or 24 on-center. 2x15' Bays. If you have any worries about whether or not your purlin is performing well we recommend that you consult a construction professional. Purlin Cleats Spaced to Accept Fibre Cement or Steel Roof Sheets . This method is used for trusses designed to be closer together, from 2 on center up to a maximum of 5 on center. 3 inches (76.2mm) on center For roofs having snow loads less than 20 pounds per square foot (psf)(9.1kg/sq.m). A building that is 30 long with a gable roof will require 24 rafters at 16 centers and 32 at 24 centers. When laying the purlins flat should the seams be staggered or can all the seems lay on the same truss? Purlins get their support either from rafters or the walls of the building. The purlin spaces in between the panels in your corrugated metal or fiberglass roof can greatly affect how well it performs, and whether it needs to be replaced earlier than expected. Rafter spacing is based on the width of the metal roofing panels. A gap that isnt much different from the 14.5 between trusses at 16 centers. Purlin spacing for corrugated roofing is something that should be given consideration when designing a roof. The shorter the span, the smaller the required rafter size. This means they are installed horizontally if the roof slopes at a 90-degree angle. For instance, shingles weigh up to 240 lbs per square (100 square feet). Unfortunately, even Douglas fir doesnt make the grade for clay and concrete tile roofing. 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. Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner. Therefore, you need quite a few for your roofing project, spaced closer than standard purlins for the highest roof quality. The weight also matters. Rafter 0 mm. 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. Purlin spacing is an important factor to consider when designing a corrugated metal or fiberglass roof. In this case its advisable to use additional purlins. Live and snow loads vary from 10 to 70psf with dead loads of 10 and 20psf and deflections of L/=180 and L/=240. Spacing is typically 24 inches on center in low snow loads, and it is reduced depending on snow load and span. 20403 Views 6 Replies 4 Participants Last post by cibula11, Jun 7, 2017. Purlins are dimensional lumber that are fastened to the top chord of trusses connecting them together for bracing purposes. Along with other factors affecting the span, it includes grade, load variables, and wood species. Step 5 - Install roofing materials. In general, constructions often use the 4ft to 12 ft rafter spacing for a pole barn. A good purlin will fit together properly, resulting in even spacing throughout your structure. Typically, porch roof rafter spacing is either 16 or 24 on-center. So, you can see the importance of rafter sizes and spacing. For instance, what if you can only find 3/8-inch thick plywood? 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 following are additional rafter spacing tips to consider for other roof construction projects around the home; Roof purlins must be 1.2 meters apart or closer for 0.7mm sheathing and 1.0 meters for 0.5mm sheathing material. However, if the building codes allow it, you can always increase your spacing according to your requirements. So, you need closer rafter spacing for slates. 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. However, switching to 24 centers also decreases the number of rafters required by up to 50%. So the truss support extends past the post and the end truss sits on it just like all the other trusses. For very long spans, it is recommended that one end post every 200 feet (60.96m) be fitted with a bearing plate to support the purlins. Insert two equally spaced nails into the rafter. However, its a good place to start when working on purlin spacing. Consider the wood species and grade, rafter span, roof load, and your local building codes when deciding. Purlin roof 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. Engineers calculate this on a job site basis. Purlin noun. Can wood purlins break (as in the end of the board, not break as in failure) on center of a roof truss. However, the spacing should not exceed 1500mm for 0.7mmand 1000mm for 0.5, unless recommended by a roofing expert. Roof rafter spacing requirements include rafter span, live and dead loads on the roof, deflection, wind and snow loads, wood species, grade, rafter size and thickness, and whether a ceiling will be attached or not. For instance, do you live in a windy area? Measure from the center of the first to the center of the next. As a result, 19.2 rafter spacing is commonly used by contractors when using engineered structural members. User Input: Pitch* 5 11 15 22.5. Staggering purlin row splice locations by halving the dimensional lumber on every other row is good practice. The roof finish may also affect the spacing between roof joists. 12.5 Roof Pitch. Meanwhile, the roof span is the horizontal distance between two opposite wall plates. You need a strong framing structure to carry such weight without sagging or crumbling for decades. The standard distance for most buildings ranges from 12 up to 24. Then run 16 lengths from truss to truss until you get to the other end. 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. We will explore this issue further below. Heres what you need to know about each option. For this reason, weve developed this guide to explain rafter spacing requirements for metal roofs on residential homes. The exact spacing depends on your straps type, size, and spacing. The size is mainly concerned with the width and depth. A #2 S-P-F 24 at 16 centers can span 9-5 but only 80 at 24 centers with a live load or 20psf, dead load of 10psf, and deflection L/=180, so span and spacing are linked to other building factors. 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. The reason for this is that different types of roofing material requires different sized spacers in order to properly support it. Its also more resistant to unexpected live loads. Purlin span is like the length of the purlin. The actual spacing depends on wood type and grade and purlin thickness. 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. However, there are two more types of purlins spans double-span purlins and continuous span purlins. 4 inches (101.6mm) on center For roofs having snow loads between 20 and 40 psf (9.1-18.2kg/sq.m). Because of this, the rafter spacing can be greater. Rafter Wood: What Wood is Used for Rafters? A 16-inch on-center spacingnaturallyprovides a stronger, more rigid roof frame. Depending on your roofs structure and overall size, the sizes of lumber you need for your roof rafters will vary. 3. It all comes down to the type of project and the architectural preferences. . Related The greater the space between rafters, the shorter the distance they can span. Squaring Up Your Pole Building 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. In addition, the 24 lumber may span from 7 to 9 feet. The best way to ensure your Purlins are set properly is to use Purlin spacers. The width of the panel or sheet is commonly set by the manufacturer and the length by the ridge to eave measurement. Roofs sheathed in planks or strapped for metal or other roofing finishes commonly are set at 24 centers, and the planks or strapping span the gap between the rafters and provide additional stiffness and support. Typically, you can reduce the total number of rafters by 50%. A distance between one roof purlin and the next should not exceed 1.2 meters. Meanwhile, 24-inch on-center spacing is cost-saving. Metal roofs are also flimsier and generally lighter, so the rafters need less weight to support or bear. The rafter spacing for metal roofing often needs to be set to support the metal panels directly fastened to the rafters. Generally, most roofers arrange purlins depending on the roof slope. However, we understand the confusion given that separating joists from rafters is nearly impossible in low-pitched roofs. Steps to roof purlin design 1. It is estimated from the on-center or the given dimension of the distance between one rafters center to the center of the next rafter. The purlin are ultimately used to fasten the roof steel providing a diaphragm effect, along with the siding, when properly engineered and installed. Others fasten purlins (or other strappings) to the rafters and fasten the roofing material directly onto the purlins. The support system for the roof is comprised of ridgepole and rafters or purlins. Choose a timber material and find material properties ( k m o d, f c .0. k, f m. k, M) Assume the width w and height h of the cross section. Given rafters are the primary framing members responsible for carrying and distributing the roof load, any sign of laxity can bring the roof down. Attach the roofing panels starting from the eaves, overlapping them by about 3-4 in (7.6-10.2 cm), and working your way to the peak of the roof. Single cleats (accepting two bolts) are standard. It includes wood species, grade, span options, max span, deflection, dead load, live load, spacing, and other service conditions. Indeed, you can be penalized if a home inspection reveals improper rafter spacing. They are butted together when laid flat Answers in 5 seconds In just 5 seconds, you can get the answer to your question. This not only causes damage to your roof in the form of UV degradation but it wastes energy on cooling costs as well. If you need the trusses on edge and want the fasteners to line up then hang the purlins between the trusses. 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). When positioned, the distance between one roof purlin to the next should be no more than 1.2 meters when using 0.7 mm-thick sheeting and 1.0 meters when using 0.5mm-thick sheeting. Thats because the outer two support members are shared, thus counting as one or half + half. In the order shown above, purlins laying flat ( use 2 20D ring shank nails per connection) for trusses with minimal spacing, hung purlins for wide span trusses (LU26 hangers for 26, LU28 hangers for 28 etc), and purlins on edge (overlapped not butted) for mid range to wide span trusses (8 or 9 spacings), installed with 1 60D galvanized ring shank nail. Standard industry roof rafter spacing is 12, 16, 19.2, and 24 on-center, however other spacing may be used by a Structural Engineer. Open purlins are also easy to cut and customize, so you can get the exact size that you need! A sheathed roof deck provides ample support for the metal roofing and is an additional barrier to insects and weather. Simply cut a 16 length to fit the first 8 span, say with a 66 you would cut it down to 7 10. Lightweight roof and wall structural systems are widely used for cladding of steel portal frames. They are installed in the same way as metal roofing and stand 2 by 4 feet tall. The rafters sit on top of your purlins and transfer the load from above to your structure below. However, the opposite is true, especially for the main home. We recommend 8-12 inch spacing on-center for a strong roof. Their purpose is to prevent uplift. So, never confuse purlins for rafters. Many people use OSB or plywood for roof sheathing or decking. Finally, you need the highest decking and lumber grade for 24-inch on-center roof rafter spacing. However, the best spacing depends on many factors, including the type of metal roofing. So, you can check theIRC websitefor guidelines. Calculate Go Back. They are covered in drywall or siding so you cannot see them, but they help distribute loads through a buildings walls and ceiling! The relationship is straightforward. 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. If not verified increase the width or height of the beam and do the calculation again. Dont hesitate to place the purlins closer if youre worried about the risk of winter ice or strong winds. In this guide, well explain rafter spacing, code requirements, roof rafter span tables, and rafter spacing calculators. Too high a snow load means the purlins fail regardless of how close together you put them because a 24 or 26 flat can only span a certain distance between trusses as snow loads increase. The more space there is between them, or rafter-to-purlin spacing, the less. There are three typical methods used to fasten purlins to the top chord of trusses. The design of the porch roof is a factor too. Alternatively, you can space the rafters closely to distribute the load. The standard purlin spacing for metal roofs is four to six feet. The rafter or roof joist spacing for metal roofing may need to be set to support rigid metal panels designed and engineered to fasten directly to the rafters. The best rafter spacing for a metal roof is 16 inches or 24 inches on-center. Thats because all rafters are two inches wide when installed. For instance, concrete tiles weigh up to 1,100 lbs/square (100 square feet). If youre using wood boards as sheathing, then there should be at least 2 between them; if they are plywood boards instead of regular wood board then there should be at least 3. In addition, ice dams add significant weight in winter. Code requirements vary based on different climate conditions in different geographic regions. Here is a list of recommended rafter spacing distances for metal roof purlins: 3 inches (76.2mm) on center - For roofs having snow loads less than 20 pounds per square foot (psf) (9.1kg/sq.m). The standard rafter spacing for metal roofs is 16 or 24 inches on-center. The two main purlin styles are C purlins and Z purlins. After choosing the different dimensions, you can click on the Calculate button, which provides helpful information about rafter spacing. Rafter spacing depends on a variety of variables and engineering calculations for structural integrity. 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. Collar Ties Collar ties connect the upper ends of opposing rafters. The standard roof joist span is 20 feet. For instance, the most common joist sizes are 26 and 28, though 210, 212, and 214 roof joists are common. I-truss rafters typically have a gap of 15.7 between parallel flange edges when spaced at 19.2 on-center. At 16 centers, there are three support members every four feet versus two when spaced at 24 centers. Using a combination of rafters and purlins is common. 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. Purlin Spacing for Metal Roof Sheets. 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. Calculating the number of rafters needed for a project can be done with some rafter calculators, but it isnt impossible to do with paper and pencil or a standard calculator. 40ft x 12 = 480 24 on-center = 20 rafters + 1 = 21 rafters per side x 2 = 42 rafters. The same applies to thicker decking. If the number of purlins used in a span is more than 5, then the center-to-center spacing should not exceed 30 inches (762mm). A good purlin is one that has been made to fit your given roofing needs. Purlin spacing is important because it determines whether or not you are using the right purlin board. 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 rafter spacing used for metal roofs is commonly set by the metal panel manufacturer to ensure its suitable for the dimensions of the roof structure. Many building codes and metal roof purlin manufacturers recommend the following: Metal purlins shall not be more than 30 inches (762mm) on center when supporting steel framing and 38 inches(965.8mm) when carrying sheathing only. 1. In general, the distance between one roof purlin to the next should be no more than 1.2 meters when using 0.7 mm-thick sheeting and 1.0 meters when using 0.5mm-thick sheeting. 2. Please distinguish it from the rafter length, measured diagonally along the rafter. The post spacing on the ends are inset by 1 1/2 so the total building width and length will work out after nailing the skirt board, girts, and truss supports to the outside of the posts. Their programs will allow for trusses to be placed on 12 foot or even 16 foot centers, and their engineers will place their engineer's seal on the drawings to verify. Wood species, grade, deflection, and loads all affect the span. The International Residential Building Code (IRC), which is the basis of most North Americas building coide, identifies these standard industry measurements. Remember, the outer two support rafters are shared, so only count as one. It is a well-known fact, that assembled systems of profiled sheeting and purlins show significant . So the big question is, how far apart should purlins be for a metal roof? Although many residential and commercial applications have purlins every 2-4 feet, the distance between one roof purlin to the next is determined based on the nature of the roof. There are many things to consider when you choose your spacing and distance, and we will go through them in this post! They should be installed on every other rafter in the upper third of the roof. It runs perpendicular from the walls to the ridge with rafters, while with purlins, it runs to the ridge in parallel. Plus, you have to select the correct rafter spacing to span the distance based on all the factors that affect your design. Paul, Good judgment should be used when spacing metal roof purlins for long spans on light-frame construction. Several factors influence rafter spacing. The reason is that 26-gauge metal roofs are a lot heavier than their 29-gauge counterparts. 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. What is the pitch of your lean to going to be? The span dictates the rafter depth and spacing based on the wood species and grade used, load variables, and deflection. We and our partners use cookies to Store and/or access information on a device. Designing and building a roof for a shed, porch, cabin, garage, or home can be a daunting task. :) 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. We are an engineering and design company providing pole barn kits nationwide from small home projects to large engineered commercial buildings. It is measured from the center of one rafter to the center of the next, also known as on-center, which makes it easier to determine the number of rafters based on the buildings length. Spacing roof rafters 16 inches on-center is a common practice for sheathed roofs. I see them usually overlapped. Therefore, you can space thicker rafters wider without adverse consequences. A shorter rafter spacing is recommended if you discard the decking. Numbers 20 and 22, U.S. Standard gage, are generally used for . The standard spacing for aluminum and galvanized roof purlins is 1.0 to 1.2 meters. Rafters serve two main purposes. Specifically, 7/16 OSB and 5/8 plywood are the standard sizes. Either if they are on edge or flat, so you can have a seamless row. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls, steel beams etc. The first thing to do is find the measurements, including the roofs length and overhang or rafter spacing. 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. We also briefly explain rafter spacing requirements for various structures, including porches, pergolas, and sheds. First, avoid the spacing in regions with extreme weather. It should be noted that some roofs have an overhang at the gable ends known as the rake. . . Greater space and span often require rafters of greater depth than shorter spans or narrower spacing. Rafter and roof joist spacing is only one part of a complex roof design that works together to form a structure that meets building codes and can hold up the weight of the roof and roof load. Introduction. Ring shank nails: Use two nails per connection for trusses. Do you have snow? Verify the beam for bending (with compression or tension). Additional factors include slope, eave and rake overhangs, roof tie-downs, rafter support bracing, collar ties, ridge connection, roof openings, ventilation, and roof finish. In general, rafter sizing refers to the thickness and depth range of the specific lumber used for the roof rafter project. We suggest 5 on center trusses with flat purlins to be restricted to 30# ground snow loads or less in most instances. We want to hear from you! It typically bridges the gable wall and attaches to the end rafter at each gable. 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. Meanwhile, rafters are diagonal in pitched roofs. Continue with Recommended Cookies. Above all, timber is readily available. In addition, the standard roof rafter spacings commonly used for most building structures are 16 and 24. This explains why Z purlins are preferred for large industrial and agricultural projects. Lastly, multiply the number of rafters by 2 to get both sides of the roof or the roofs slopes. It provides better support for the decking, preventing premature sagging. Purlins for Shed or Run-in (RP): The calculator returns the number and length of purlin rows for the roof, and. After doing this, add 1 for the 0 rafter. Cleats are metal plates used to secure purlins to the roofing structure. Rafter ties prevent the weight of the roof from spreading the tops of the walls and causing the ridge to sag.