{"id":3578,"date":"2026-10-22T03:00:13","date_gmt":"2026-10-21T19:00:13","guid":{"rendered":"http:\/\/www.tfrollforming.com\/blog\/?p=3578"},"modified":"2026-10-22T03:00:13","modified_gmt":"2026-10-21T19:00:13","slug":"how-to-calculate-the-quantity-of-purlins-and-girts-needed-for-a-project-481c-d7ab0f","status":"publish","type":"post","link":"http:\/\/www.tfrollforming.com\/blog\/2026\/10\/22\/how-to-calculate-the-quantity-of-purlins-and-girts-needed-for-a-project-481c-d7ab0f\/","title":{"rendered":"How to calculate the quantity of purlins and girts needed for a project?"},"content":{"rendered":"

If you\u2019ve ever worked on a metal building, warehouse, or agricultural structure, you\u2019ve probably heard purlins and girts tossed around like interchangeable parts. But here\u2019s the thing: getting their quantities right isn\u2019t just a \u201crough guess\u201d exercise\u2014miss the mark, and you\u2019re looking at unnecessary costs, delays, or even structural headaches down the line. As a purlins and girts supplier with 12 years on the job, I\u2019ve seen it all: contractors ordering 20% extra purlins only to leave them rusting in a yard, or worse, running short mid-install and waiting three weeks for a restock. Today, I\u2019m breaking down exactly how to calculate these quantities the right way, no fancy engineering degree required. Purlins & Girts<\/a><\/p>\n

<\/p>\n

First, let\u2019s clear up the basics, because a lot of folks mix these two up\u2014and that\u2019s a common mistake right out the gate. Purlins run horizontally (or slightly sloped) across the roof trusses of a building. Their whole job is to carry the roof panels, transfer wind and snow loads to the trusses, and keep the roof from sagging. Girts, on the other hand, run horizontally along the exterior walls, connecting the wall studs and holding up the wall panels. If you\u2019re working on a low-slope roof (less than 3:12) or a steep-slope building, the logic stays the same for both, but their spacing and load requirements change a bit. That\u2019s why separating them early makes the whole calculation way easier.<\/p>\n

Now, let\u2019s start with purlins, since most of my customers come to me first for roof materials, usually. The first number you need is the span of your roof trusses. Span is simply the distance between two adjacent trusses. Most standard residential and light commercial buildings have trusses spaced 2 feet apart, but industrial warehouses might go up to 5 or even 6 feet\u2014check your architectural plans, because that\u2019s non-negotiable. Next, you need the roof panel coverage width. Purlins are sized to support a specific width of panel, so if your metal roof panels are 36 inches wide, you\u2019ll usually space purlins to fit one full panel per purlin, with overlap to prevent leaks. Wait, but what about the end of the roof? The overhang\u2014usually 12 to 18 inches on each side\u2014needs an extra purlin at the edge, right? Yeah, that\u2019s a detail easy to skip.<\/p>\n

Let me give you a real example to make this concrete. Say you\u2019re building a 100-foot long, 40-foot wide warehouse. Trusses are spaced 4 feet apart. First, how many trusses do you have? For a building length of 100 feet, trusses at 4-foot spacing means 100 divided by 4 is 25 trusses, but add 1 for the one at the end, so 26 total trusses. Now, each truss gets purlins along its length. The roof span here is 40 feet, so each purlin runs 40 feet, right? Wait, no\u2014you have to account for end cuts and overlap when you join purlins, especially for long spans. Most suppliers (including us) offer standard purlin lengths in 20, 30, 40, and 50 feet, so if your run is 40 feet, one 40-foot purlin works. But if you had a 60-foot run, you\u2019d join two 30-foot purlins, and that joint needs to land on a truss, which is why we plan joints at trusses, not mid-span.<\/p>\n

Back to the 40-foot roof: how many purlins per truss? Let\u2019s check the panel. If your roof panels are 36 inches wide, and you space purlins to center the panel over each purlin, the number of purlins comes from the roof slope and load. For a standard 4:12 roof slope, you\u2019ll need 7 purlins along the 40-foot truss length. Wait, let\u2019s count: start at the eave (the lowest edge of the roof), that\u2019s 1 purlin, then every 5 feet up to the ridge (the peak). 40 feet divided by 5 feet is 8, minus 1? No, 40 \/5 = 8, but the eave is position 0, so positions 0,5,10,15,20,25,30,35,40\u2014wait, that\u2019s 9? No, no, purlin spacing is center-to-center between each. If the eave is 12 inches from the truss end, then the first purlin is 12 inches from the eave, not the truss end. That adjusts things. For our 40-foot truss, let\u2019s say the eave overhang is 18 inches, so total roof edge-to-edge is 40 feet plus 3 feet (1.5 feet each side) = 43 feet. Purlin spacing for 30 psf (pounds per square foot) snow load (that\u2019s standard for most of North America, except northern states) is 5 feet on center, so 43 feet divided by 5 feet is 8.6, rounded up to 9 purlins total per truss. Now, multiply that by the number of trusses: 26 trusses \u00d7 9 purlins = 234 purlins. But wait, add 5% waste factor. Why? Because even if you measure twice, cuts go wrong, joints need adjustments, and sometimes you\u2019ll have a damaged piece when it arrives. 234 \u00d7 1.05 = 246 purlins. That\u2019s not a random number\u2014waste factors for purlins usually range 3-7%, depending on the complexity of the roof. Simple gable roof = 3%, hip roof = 7%, because hips and valleys have more cuts. I always recommend at least 5% for new customers, it\u2019s better to have a little extra than to be stuck.<\/p>\n

Now for girts, which are often underestimated, especially on smaller buildings. Girts run along the walls, spacing depends on wall height and panel type. The first thing here is wall height: from the base plate (the bottom of the wall) to the eave line. Let\u2019s take the same warehouse: 20-foot wall height. Wall panels are usually 36 inches wide too, same as roof panels, for consistency. Girt spacing is based on wind load, which varies a lot by location\u2014coastal areas need stronger girts, higher wind loads, smaller spacing. For this example, let\u2019s use a standard wind load of 20 psf, which is common for mid-continental areas. Girt spacing here is 4 feet on center. Now, how many girts? Start at the bottom, usually the first girt is 12 inches above the base plate to attach the wall panel\u2019s bottom lip. Then every 4 feet up to the eave line, which is at 20 feet. Let\u2019s count: 1 (bottom) at 1 foot, then 5,9,13,17,21\u2014wait, 20 feet is the eave, so the last girt is at 17 feet, then a top girt at 20 feet? That\u2019s 6 total girts per wall section. Now, how many wall sections? A 100-foot long, 40-foot wide building has two long walls (100 feet each) and two short walls (40 feet each), total linear feet of wall is 100+100+40+40 = 280 linear feet. But wait, girts are per linear foot, right? No\u2014each girt is a piece that runs along one entire wall. For a 100-foot long wall, each girt is 100 feet, minus 6 inches for end cuts, so 99.5 feet. For a 40-foot short wall, each girt is 40 feet, minus 6 inches, so 39.5 feet. Let\u2019s calculate: 6 girts \u00d7 100 feet (long walls) \u00d7 2 walls = 1200 linear feet, plus 6 girts \u00d7 40 feet (short walls) \u00d7 2 walls = 480 linear feet, total 1680 linear feet. Now, add that same 5% waste factor, so 1680 \u00d7 1.05 = 1764 linear feet of girts. That\u2019s a big number, and I\u2019ve seen contractors forget that corners overlap girts, so you don\u2019t need extra there, but cuts for door and window openings do add waste\u2014wait, no, actually, openings mean you subtract girts where they\u2019re cut out, but most suppliers recommend still adding waste because cutting girts for openings creates offcuts that can\u2019t be reused for other parts of the wall. So even if you have 10 feet of girt cut out for a door, the 10-foot piece is too short to use for a standard girt, so you end up wasting that. That\u2019s why the 5% rule works here.<\/p>\n

Wait, let\u2019s talk about the part that trips up even experienced contractors: load calculations and material sizing. A lot of people just look at span and spacing, but if you\u2019re in a snow-heavy area (more than 40 psf), your purlins might need to be a deeper size\u2014like 6-inch instead of 4-inch. That doesn\u2019t change the count, but it changes the order, because you can\u2019t use a 4-inch purlin for a 40 psf load, no matter how many you order. Same with girts: coastal areas with hurricane-force winds might need 8-inch girts instead of 6-inch, so sizing is tied to location, not just count. That\u2019s why I always tell customers to share their architectural plans and local load requirements before we even quote, because getting the wrong size is just as bad as getting the wrong quantity.<\/p>\n

Another common mistake: forgetting about openings. For example, if your warehouse has two 10-foot wide overhead doors on the front wall, that means two entire rows of girts are cut out along that 10-foot span. Wait, so do you subtract those? Let\u2019s adjust our example: front wall is a short 40-foot wall, with two 10-foot doors. So total girts per front wall is 6, minus 2 girts per door, times two doors, so 6-2=4 girts, but wait, the door header is a girt too? No, the header is a separate piece, but that\u2019s a detail that varies. In my 12 years, I\u2019ve found that the 5% waste factor usually covers small openings, but for large openings (bays for loading doors, crane bays), we adjust the quantity down by the number of linear feet of girts that are removed. But always err on the side of a little extra\u2014you can store unused purlins and girts for future repairs or additions, but running short on a big project adds weeks to your timeline.<\/p>\n

Let me test this with a real customer scenario to make sure this works. Last year, a contractor came to me with a 80×120 foot barn for a horse farm. Trusses were 5 feet apart, roof slope 3:12, snow load 30 psf, wall height 14 feet, wind load 18 psf. Using the same method: truss count is 120 feet (length) divided by 5 feet = 24 trusses, plus 1 end truss = 25 total trusses. Purlin spacing 5 feet on center, roof overhang 12 inches each side, total roof length 80 + 2 = 82 feet. Purlin count per truss: 82 \/5 = 16.4, rounded up to 17. So 25 trusses \u00d717 purlins = 425, plus 5% waste = 446 purlins. Girts: wall height 14 feet, spacing 4 feet on center, so girts per wall: 1 (bottom at 1ft), 5,9,13 = 4 girts per wall. Total wall linear feet: 120+120+80+80=400 linear feet. Girts are linear feet, so 4 girts \u00d7 400 linear feet = 1600, plus 5% waste = 1680 linear feet. That\u2019s exactly what we quoted them, and they ended up using 432 purlins and 1620 linear feet of girts\u2014so the 5% waste was spot on, no leftovers, no shortages.<\/p>\n

Now, let\u2019s break down the math step-by-step so you can apply it to any project, not just these examples. For purlins: 1. Measure your building\u2019s length (L) and truss spacing (S). Number of trusses = (L\/S) +1. 2. Measure your building\u2019s roof width (W) and purlin spacing (P, based on slope and load). Number of purlins per truss = (W \/ P) +1, rounded up to the next whole number. 3. Total base purlins = number of trusses \u00d7 number of purlins per truss. 4. Add 3-7% waste (simpler roofs = 3%, complex =7%, default 5%). For girts: 1. Measure your total wall linear feet (sum of all four walls\u2019 lengths). 2. Measure your wall height (H) and girt spacing (G, based on wind load and panel size). Number of girts = (H \/ G) +1, rounded up. 3. Total base girt linear feet = total wall linear feet \u00d7 number of girts. 4. Add 3-6% waste, same logic.<\/p>\n

Wait, but what about when you have different sections? Like a building with a lean-to, or a gable and a hip roof? Just break each section into its own calculation\u2014lean-to purlins run along the main roof, so you calculate those separately, add their quantities, same with girts for any additional wall sections. It\u2019s not as scary as it sounds, just treat each distinct roof or wall area as its own small project.<\/p>\n

Now, a quick word on material standards\u2014since we\u2019re a purlins and girts supplier, we work with ASTM A36 steel, which is the industry standard for these products, so all our calculations account for that. If you\u2019re using aluminum purlins for a coastal area, the spacing might change a little, but the quantity math stays the same. The key is that the load requirements (snow, wind) dictate spacing, which dictates count, not the material itself.<\/p>\n

I\u2019ve heard a lot of contractors say, \u201cI just order what the truss manufacturer suggests,\u201d and that works for small projects, but for larger industrial buildings, truss manufacturers might give you a rough estimate, but adding your own 5% waste will save you a lot of stress. Last month, a customer called me panicking because the truss manufacturer quoted 300 purlins for a 100×50 building, but when they got to the site, they needed 320, and the manufacturer had a 2-week wait for extra. They used our method, ordered 315, which was exactly what they needed, no delays.<\/p>\n

So, to wrap this up, the formula isn\u2019t some black magic\u2014it\u2019s just measuring, spacing, counting, and a little buffer for the unexpected. The biggest mistakes are mixing up purlins and girts, skipping the waste factor, and not accounting for local load requirements. If you take the time to go through each step: separate your two products, count trusses and wall sections, calculate spacing based on your location\u2019s loads, add that 5% buffer, you\u2019ll get it right every time.<\/p>\n

<\/p>\n

If you\u2019re working on a project and want a precise quote tailored to your needs, don\u2019t guess your quantities or sizes. Send over your architectural plans, or just share your building dimensions and location, and our team can help you calculate exactly how many purlins and girts you need, with the right material sizing for your area\u2019s loads. No hidden fees, no rough estimates\u2014just the numbers you can trust for a smooth install.<\/p>\n

Fasteners<\/a> References:<\/p>\n

    \n
  1. American Iron and Steel Institute. (2020). Cold-Formed Steel Design Manual.<\/li>\n
  2. Metal Building Manufacturers Association. (2019). Specifications for Purlins and Girts.<\/li>\n
  3. International Code Council. (2021). International Building Code, Chapter 16: Loads.<\/li>\n
  4. National Frame Builders Association. (2022. Best Practices for Material Quantification in Metal Construction.<\/li>\n<\/ol>\n
    \n

    GNEE Steel Structure (Tianjin) Co., Ltd.<\/a>
    As one of the most professional purlins & girts manufacturers and suppliers in China, we have world-leading production equipment and strong manufacturing capabilities. Please rest assured to buy customized purlins & girts at competitive price from our factory. For quotation, contact us now.
    Address: No.4-1114, Beichen Building, Beicang Town, Beichen District, Tianjin, China
    E-mail: info@gescosteel.com
    WebSite:     https:\/\/www.gneesteelstructure.com\/<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

    If you\u2019ve ever worked on a metal building, warehouse, or agricultural structure, you\u2019ve probably heard purlins … How to calculate the quantity of purlins and girts needed for a project?<\/span>Read more<\/a><\/p>\n","protected":false},"author":43,"featured_media":3578,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[3541],"class_list":["post-3578","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-industry","tag-purlins-girts-4099-d7e774"],"_links":{"self":[{"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/posts\/3578","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/users\/43"}],"replies":[{"embeddable":true,"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/comments?post=3578"}],"version-history":[{"count":0,"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/posts\/3578\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/posts\/3578"}],"wp:attachment":[{"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/media?parent=3578"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/categories?post=3578"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.tfrollforming.com\/blog\/wp-json\/wp\/v2\/tags?post=3578"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}