Roof Truss Span

[Coyote]

I want to build a 20' x 40' pole barn, (20 deep and 40 wide), basically I'll have 4 - 10' wide "bays". Anyway, can I span the 20 feet with one truss? I'm thinking about a 24' truss set on poles 20' feet apart to achieve a 2' overhang. Roof will be tin. Make sense?

 

[MarEng]

My barn has 40' clear span steel trusses. I see similar type trusses at the local 'barn' store - you may be able to buy them pre-fabbed. They may also have pre-fabbed wood trusses - or look up trusses in the phone book - pre-fab wood trusses are readily available.

 

[firefighter9208]

I have a 30x40 shop and used wood trusses. I told the truss company that I wanted to span the 30'. They build the trusses for the span and I haven't had a problem the 8 years since the building was built. I say check with they truss company, but it shouldn't be a problem.

Chris

 

[EddieWalker]

Yes, 24 foot long trusses are pretty simple. Are you going to build them yourself, or have them built? Either way, be sure to calculate your design on the 20 foot span from wall to wall. The overhang isn't the important measurement.

Spacing your trusses for a metal roof will decide how many you have. I like to put them in on 4 foot centers, then use 2x4 purlins on there flat side. This makes them easy to attach and easier to find when attaching the metal panels.

You can go to 6 foot centers with 2x4's on edge, or 8 foot centers with 2x6's on edge. Spacing of the purlins is something you need to check first with your metal supplier. I like them every four feet.

What pitch are you going to use? 4:12 is what I like as it's steep enough too look nice, but not too steep to walk on. Even at that pitch, you have to have clean, white soled sneakers to walk on it. Dark soled shoes are harder and more slippery. Any dust at all will cause you to slide off the metal at a 4:12 pitch. If you go steeper, it just gets allot harder, to impossible to walk on it and do any work. I did one roof at 5:12 that was the absolute max that I could crawl up and down. Even then, I had more close calls then I want to remember.

Do you get snow? If so, ignore everything I've said as it doesn't apply to snow loads. For snow, you need an engineer to tell you how much load you can handle and how to design the roof.

Are you limited in size for your building? Will you have doors in your four bays? If so, then ten foot is kind of tight. I'd go 12 foot for each bay, and make the doors ten foot wide. That extra 8 feet in lenght isn't going to add very much to the overall cost of the project, but the extra space will be appreciated for the life of the building.

Eddie

 

[Coyote]

Thanks for the expert advice, I am torn between a simple shed type roof, (10' high in front; 8' high rear) or trusses. This is a do-it-yourself job, except I would have the trusses made. Either way I haven't prepared the pad yet so I'm still in design mode! Eddie, your detailed answers are amazingly helpful! Thanks again.

 

[dtd24]

We built an office building that had a 60' span.

 

[anojones]

We're getting ready to build a shop with a 30' span with trusses, I don't know how far you can go but our building code will only allow 36' clearspan for wood trusses. After reading a ton of pole barn threads (and as Eddie mentioned)- we will go with wider bays to allow for 12' wide doors- should keep me from hitting anything with the bush hog attatched.

 

[MrJimi]

These on my roof are 50 foot span with proper enginering anything can be done
50 foot over wall with a 2 foot overhang

 

[gemini5362]

I would take earlier advice and have the truss company engineer them for you. They have software that will tell you how long you need to go with the span. Even with metal ceilings I disagree with eddie a bit I would go on 2 foot centers. My garage is spanning 28 feet and my truss builder said I could go with 24 inch centers if I was useing metal. I went with 16 inch centers because I liked the extra strength. You might want to look at a website. Adams Truss, Inc. - Engineered Steel Building Trusses and see what their prices are on just the steel trusses. They are built on a ten foot center.

 

[flusher]

I want to build a 20' x 40' pole barn, (20 deep and 40 wide), basically I'll have 4 - 10' wide "bays". Anyway, can I span the 20 feet with one truss? I'm thinking about a 24' truss set on poles 20' feet apart to achieve a 2' overhang. Roof will be tin. Make sense?

Don't know if you need a building permit in your county for your pole barn.

Some counties require trusses to be engineered and the truss design package has to accompany the application for the building permit.

Also, some counties require that the trusses be fabricated by a licensed manufacturer.

 

[MrJimi]

Coyote, I noticed you live in NC. there must be some pretty good code restrictions there as you guys get the hurricanes just after we do here in Florida, be careful with what you build, my building inspector is a good guy, they wont let us put up something unsafe to blow on your neighbors property and do more damage.
I would not even think of a tin roof on 4 foot centers, building codes have changed lots with all these hurricanes we've had and Miami is even more strict with building codes

 

[keving]

Here's a 6/12 pitch.

 

[JerryG]

I want to build a 20' x 40' pole barn, (20 deep and 40 wide), basically I'll have 4 - 10' wide "bays". Anyway, can I span the 20 feet with one truss? I'm thinking about a 24' truss set on poles 20' feet apart to achieve a 2' overhang. Roof will be tin. Make sense?

Tom,
Are in the part of North Carolina where chickens or turkeys are raised commercially? If you are, you should be able to purchase used steel trusses at a very reasonable amount. The most popular sizes will be 34', 36', 40', 42', 43, 50, 52, and 60'. The 34' and 36' are the oldest and should be the least expensive. The 40', 42' and 43' are normal chicken house trusses and are still being installed. If you are where turkeys are raised you might be able to buy 50', 52', and 60'. If you plan to insure the building, you need to stay 43' or smaller unless your insurance company tells you that that specific width and manufacturer is insurable by them. The company of manufacture does make a difference to some insurance companies in the larger sizes.

 

[roermo]

good luck with your project, post photos as you go along.

My garage is 26 foot wide span and 34 deep. The door is in the front of the picture. Had the tursses made and the company did the load factors for me.

Again good have fun.

Roger

 

[EddieWalker]

Coyote,

You're getting allot of opinions on spans and spacings, which are all correct. The decision you need to make is how much extra money do you want to spend? Spacing your trusses closer together will not make a metal roof any stronger, that is all based on your purlins. What you use and how you install them.

Take a good look at the picture Kevin posted, plus go to his thread on his barn. That pole barn is about as well built as any I've ever seen!!! His trusses are on 4 ft centers with 2x4's flat for purlins.

Adding three times as many trusses will make it stronger, but it's to a degree that's insignificant. Sort of like pouring a concrete floor 8 inches thick when your parking your pickup and tractor on it. 6 inches is overkill, 8 inches is just throwing money away.

You are also getting advice that just about any span is possible. This is true, but it's possible at a cost. Just because it's possible to span a great distance, doesn't mean that's the most affordable way to cover the square footage. You can build a 2,000 square foot shop for $10 a foot pretty easily, or you can spend $30 to $40 a square foot for the same square footage. It's your money, you decide.

Eddie

 

[gemini5362]

I agree with eddie for most of what he said. You are getting a lot of opinions about what to do and they are all advice from people that have done it. If you have trusses built I would follow the advice of the guys that build it. I know that they have a financial incentive for having you buy more trusses but most honest ones are going to worry about safety more than making more money. They are also going to worry about their reputation. They can make trusses for a thousand shops and get a really good reputation. Let one shop roof cave in for whatever reason and that will give them a bad reputation that will be hard to get rid of.

 

[schmism]

My 9-5 is working as an engineer in the truss industry.

I routinly check designs with clear spans of 80+ feet.

Ag type trusses commonly come in 30-60' 4-8' OC single ply.

There are virtually no clear span limits, but note most truss tables (the tables trusses are built on) are only sold up to about 80'

a few exist in the 100' range.

a 120' clear span feild splice is possible, but at that point your starting to push the limit of what is economicaly justifiable fin wood vs steel for that kind of clear span.

Ive personally worked on a 6 piece 300' truss that was nearly 40' tall when it was all assembled (in the 3 layers of piggy backs)

most AG trusses will require large material for TC and BC material. TO make maxium use of the lumber, larger OC spaceing is used to offset the total cost of the building.

I recently came across a really cool 40x40 (although size is varriable) 1 sided "run in" machine shead, dual pitch roof with the entire front 40' clear span. (although they are often 3 sided)

shoot me a PM ill be happy to run some basic designs and shoot you some ballpark prices of what you could expect to pay.

IT should also be noted (perhaps it already was i only skimmed part of the repsonses) Is that a 4/12 20' span 2' OC with 2' overhang is a "standard" truss most real lumber yards will have on hand. Menards is one that i KNOW stocks that. (they are a customer of ours) a 24' truss with 2' cantalever both ends with no top chord overhang will NOT be a stock truss as the heal type is significatly diffrent. (note you can not set a 24' clear span truss 0" OH on 20' wide walls and call it ok)

some other fun things, i recently did a design for a friend that wanted "as large an overhang on the front as i can get" i said, 6'...8'... 10' what do you WANT. he thougth "large" ment 3'. he was shocked to find out how easy it was to speck out a truss with a 8' front cantalever (overhang)