Questions on truss spacing and roof design

[Bobby Robbinett]

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.

[Bob Riefer]

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.

[Ric Jones]

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

[Steve Rozmiarek]

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.

[Maurice Mcmurry]

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.

[Steve Jenkins]

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.

[John K Jordan]

Yes, I’d 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 I’ve 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.)

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

I’m planning to put up a 24’ or 30’x76’ equipment shed soon. Probably go with 8x8 posts and same roof construction. However, I’ll 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 I’ve ordered trusses the truss designer provided options and details.

JKJ

[Jack Frederick]

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

[Jim Becker]

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.

[Ron Selzer]

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

[Bill Dufour]

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

[Lawrence Duckworth]

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.

[Jim Becker]

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...

[Lawrence Duckworth]

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.

[Ric Jones]

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

[Bobby Robbinett]

Yes, I’d 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 I’ve 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.)

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

I’m planning to put up a 24’ or 30’x76’ equipment shed soon. Probably go with 8x8 posts and same roof construction. However, I’ll 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 I’ve ordered trusses the truss designer provided options and details.

JKJ

John I am curious, how do metal trusses like this compare price and strength wise to a typical wood truss? I would probably be able to get by with an 8ft to 10ft spacing with metal trusses and the purlins like you mentioned. If I build it 180ft x 40ft then I could use two 20ft beam style trusses times 18 all the way down. How would you compare the two?

[John K Jordan]

John I am curious, how do metal trusses like this compare price and strength wise to a typical wood truss? I would probably be able to get by with an 8ft to 10ft spacing with metal trusses and the purlins like you mentioned. If I build it 180ft x 40ft then I could use two 20ft beam style trusses times 18 all the way down. How would you compare the two?

I don't know, never used them before. They are much smaller than the wood trusses. What got me interested was a guy down the hill who had a big equipment shed put up a couple of years ago. Someone he found recommended steel trusses and built the whole thing in way less time and for less cost than he imagined. (looks like he put them about 8' apart) The guy said they hauled them in from out of state somewhere. I don't know about the availability of the steel trusses now.

One web site I looked at said the steel trusses are more expensive but you typically need 5 times more wood trusses. https://www.americanpolebarns.com/post/steel-trusses-vs-timber-trusses-why-steel-trusses-are-a-no-brainer

[Eugene Dixon]

OP you might benefit from watching some RR Buildings YouTubes and/or Texas Barndominiums youtubes, although the latter hasn't posted anything in several months.

[Jim Becker]

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.

That sounds like a defective installation. If you've ever had to physically remove the stuff, you'll have found it really takes a lot of effort and at the junction between the foam and the structure, you literally have to cut it away with a blade.

[Jim Becker]

OP you might benefit from watching some RR Buildings YouTubes and/or Texas Barndominiums youtubes, although the latter hasn't posted anything in several months.

Yea, RR is the bee's knees when it comes to post frame structures...few builders go as far as Kyle and Greg go when it comes to straight and square for the same, too.

[Bobby Robbinett]

Got a quote from a guy in Louisiana on using steel trusses. He said I would need 19 of them, and they are 20ft long each that bolt together in the center for a 40ft span. They are going to run roughly $700 each x 19. Which is cheaper than wood trusses using a 4ft spacing on center per the Manards website. I need to contact some local truss companies and get some quotes. I built a couple of triplexes a couple of years ago and got the trusses locally at $185 each. Can’t recall the span or number that I used, but I am guessing they would be significantly cheaper than Manards.

I would of course much prefer to not have to sheeth the roof. That would save me a ton of money and since I am paying for this out of pocket and not getting a loan I need to save money where I can. I have a local company to get the metal roofing panels from that run $3.59 per foot with up to 55ft lengths. They might be able to get that price down even lower according to their salesman.

We don’t get much if any snow. We do get a lot of strong wind and rain though.

[John K Jordan]

...

I would of course much prefer to not have to sheeth the roof. That would save me a ton of money and since I am paying for this out of pocket and not getting a loan I need to save money where I can....

Some sort of insulation would be needed to prevent condensation in many areas. I've seen several metal-roofed buildings where moisture condensed on the underside and dripped into the building. Some recommend a bubble plastic insulation just under the metal roofing. A spray on foam should work. I used the OSB sheathing under the metal and synthetic roofing felt to help manage the condensation, for the strength when walking on the roof, and to add to the overall rigidity of the structure. For areas open under the roof I painted the bottom side of the OSB.

JKJ

[Jim Becker]

The bot-together metal trusses are a viable option and worthy of comparison. You mention wind and with your reference to Louisiana...I'm making an assumption here since you actual general location isn't displayed...makes for the importance of proper engineering for wind. Structures are stressed with different kinds of loads in different geographies and wind really has to be dealt with. It's ok to try and mitigate cost as much as possible, but one place you can't afford to cut corners is ensuring that that building remains standing over the years through heavy weather.