Roof Truss Span

[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)