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Thread: Questions on truss spacing and roof design

  1. #1

    Questions on truss spacing and roof design

    So I have been kicking around the idea of either building a second shop or a larger shop and vacating the one that I am in. If I build a larger shop it will be 180ft x 40ft. I would use 40ft long trusses for the roof. My question is, what would be the largest spacing between trusses I could get away with with a 180ft long x 40ft deep shop? It will not be subject to local code as it will be out of town in the country. Will be going with a metal roof and spray foam on the bottom side.

    My second question is would it be necessary to install decking over the trusses to give the metal roof panels a solid base or could I attach the roof metal directly to the trusses? I plan to have a roofer do the install, I am just curious and in research mode at the moment.

    Shop will be in the south where temps get high but we still get at least one decent snow every year or two. Shop would be heated and cooled with spray foam insulation on the roof deck and cellulose in the walls.

  2. #2
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    There's a ton of options for truss spacing - the truss manufacturer will beef up / design accordingly.

    That said, they will have some "standards" to suggest, which will likely range from 2 foot spacing (typical for attic storage strusses) to 8 foot spacing.

    The trusses are usually connected with 2x material (again, matching whatever specs the trusses call for), and then the steel goes on above that.

    Most regions have a typical pitch that works with the prevailing snow load expectations. 4 pitch is about as "flat" as many areas get, and 6 pitch is common too.

    Then of course, you have to consider snow slide off risk... some snow jacks affixed around the eves is well worth it with steel roof.

    I consider gutters a must-do item.
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  3. #3
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    Truss spacing and or roof loads are based on building codes for your geographical area, simple call to a truss manufacturer with a few questions and they can reveal what is required locally, one must take into account if roof is loaded, if truss bottom cord is for load bearing, or if truss bottom cord is to bear on plate or top cord to bear on plate.
    also must consider decking material.

    as for the foam of underside of deck , heck yeah, i[m in phoenix, and my shop has 5 inches of foam on underside of roof, been up to 110 a few times already and shop stays in mid 80s if closed up. walls have r19, a few windows only weak spot is i opted for a roll up door, and they don't generally insulate well. but i can live with that, its on a shade patio and gets no sun.
    good luck, sounds like a great project
    rj in az

  4. #4
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    Like the guys above said, the truss manufacturer will design the roof. When you order trusses, they will engineer the roof system to your local requirements. Living in the country probably only means no building inspector, the truss manufacturer will still use code to design your roof though. It will probably be 8' spacing with 2x6 purlins if you are building a pole shed. If you are framing the walls, you can use more lighter trusses, 2' OC. Purlins will probably be hung on hangers on the side of the trusses. Adding sheeting makes a much stiffer building but is something you will have to know if you want to do prior to truss design. I'd recommend sheeting if you plan on sprayfoam. If you have to replace a panel or a roof that has been foamed without sheeting, you will completely agree.

  5. #5
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    I do not know your location. In the mid west Menard's is helpful. They have an interactive truss designer built into their web-site. They will even do custom engineering for free. you can buy your custom trusses online from them. Some other truss manufactures and lumber yards will do the design too.
    Last edited by Maurice Mcmurry; 06-21-2022 at 9:24 AM.
    Best Regards, Maurice

  6. #6
    Join Date
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    My shop is 40x70 with a metal roof. Trusses are 10’ apart. Purlins run lengthwise welded to trusses and are 3-4 feet apart with the roof attached directly to them.
    Steve Jenkins, McKinney, TX. 469 742-9694
    Always use the word "impossible" with extreme caution

  7. #7
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    Yes, Id sit down with the truss manufacturer for recommendations. On my shop I used 4 spacing, 2x4 purlins, a layer of OSB sheathing, and felt underlayment with metal roofing on top. I chose 5v galvalume roofing in a heavy gauge (I forget what) with a good ridge vent, of course. Lots of bracing inside. My building was only 62 long. I built with 6x6 posts every 10 around the perimeter with double 2x12 beams, (several posts in the center inside interior walls, 2x6 studs on 16 centers, 2x4 girts, with smartsiding over OSB, 1/2 ply on interior walls. BTW, most trusses Ive seen were 2x4. I opted for 2x6 top and bottom chords and 2x4 web. The whole building is very solid. The biggest effort was setting the 16 6x6 posts and putting up the beams. (I got someone to help with that.)

    Ive seen barns put up with 10 between heavy trusses. I thing they used 2x6 perlins.

    Im planning to put up a 24 or 30x76 equipment shed soon. Probably go with 8x8 posts and same roof construction. However, Ill go with steel trusses on this one. They have built-in channels to hold 2x6 perlins; those I want to use provide a lot of interior height down the center of the building - nice for excavator, etc.

    There are lots of books on building design and I found a lot of recommendations on the internet. Again, whenever Ive ordered trusses the truss designer provided options and details.

    JKJ

  8. #8
    Will you add solar to this roof? Consider that in your discussions with the truss folks.

  9. #9
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    Typical spacing for a post frame building, which is likely the most economical way to get the size you want, vary between 4' and 8' for truss spacing. This is purely an engineering question and has to be handled by a professional.

    Typical roof construction for a post frame building does not get sheathed. Rather, there are purlins attached to the trusses horizontally, sized appropriate to the expected load (again, engineering) and typically spaced 2' OC. The metal roof panels get fastened to that. The exception would be if standing seam roofing is selected and that does go over sheathing.

    You can do as much of the construction yourself as you feel comfortable to do, but you really do need to have the building engineered so it is designed correctly for both longevity and to meet building codes.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  10. #10
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    Bobby
    use this search string on goggle: buildings on newagtalk.com
    guys all the time posting about buildings
    good luck
    Ron

  11. #11
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    Wind load is a big impact on such a large roof. Any seismic concerns?
    Bill D

  12. #12
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    Quote Originally Posted by Bobby Robbinett View Post
    Will be going with a metal roof and spray foam on the bottom side.

    I don't think that stuff will stick to the backside of the metal decking for very long, also I'd check with the decking supplier to see if it'll cause problems with the sunny side colored finish. I believe eventually you will see the outlines of the girts and beams.

  13. #13
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    Quote Originally Posted by Lawrence Duckworth View Post

    I don't think that stuff will stick to the backside of the metal decking for very long, also I'd check with the decking supplier to see if it'll cause problems with the sunny side colored finish. I believe eventually you will see the outlines of the girts and beams.
    Oh, it will adhere just fine...
    --

    The most expensive tool is the one you buy "cheaply" and often...

  14. #14
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    Quote Originally Posted by Jim Becker View Post
    Oh, it will adhere just fine...

    for how long? My son had that crap sprayed in when he built his house (8,500 sq ft) . When he remodeled a few years later and opened up an exterior wall and found the stuff had shrunk away from the studs and osb. On a metal roof I think it's especially risky given the expansion contraction of a metal roof.....I know, I know this stuff is great if you get a crew that is well trained and the humidity and temperature are just right and everyone holds their head and tongue just right the stuff will even make childbirth easy,...but the backside of a metal roof??? I'd cover the ceiling with drywall so you couldn't see it when it's falling off, kind of if you don't see it moron deal.

  15. #15
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    probably used wrong type of foam which is what some contractors got away with

    lotsa metal buildings here in Az, have the foam directly on underside of roof metal decking. its awesome, for heat and ac imo.

    i have it on my shop (albeit wood frame) today it was 103 and without ac on it was about 85 degrees inside, cranked the mini split to 75 an an hour later it was 78 degrees, almost chilly. lol
    good luck, dont scrimp on frame and insulation, do it right, and never look back, then next is the electric, plan it and you can never have too many outlets or lights.
    rj in az

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