Another set of Pole Barn Questions

Tscott9330 · Feb 10, 2009

I have been reading a ton on pole barn construction, and have a few books on the way in the mail, but many of you have built these before, so I would like to ask a question.

I am in the process of designing a 40' by 60' pole barn as a shop. I am investigating the possibility of using attic trusses to allow for a room over the shop. I have not seen any pole barn type buildings with attic trusses. Is there a reason for this?

Here are some PDF's of my current plan.
View attachment Shop side view 40' x 60' attic truss Model (1).pdf
View attachment Shop side view 40' x 60' attic truss Model (2).pdf

These show the front of the building (60' side) and the end of the building (40' side). As you can see on the front of the building (60' side) my posts will be spaced a max of 12' apart to allow for doors and on the rear (not shown) the posts will be set at 10' spacing. Between the tops of the posts (around the perimeter) will be 2 - 2"x12" connecting the posts. These will be notched into the top of the posts on the front and back of the posts and will be lag bolted to the posts. The trusses I will have made will rest on these 2"x12" members. Assuming the truss company specs these trusses to be 2' OC, there will be 5 or 6 trusses supported by most spans.

Will the additional weight of an attic and the future crap in it be adequately supported by these 2"x12" members?

Is this why you do not see more attic type trusses in pole barns?

Would I be better off going with a more traditionally accepted type of construction (i.e. standard 2"x4" or 2"x6" framing methods)?

What do the experts think?

Tom

dodge man · Feb 10, 2009

I would want to know from the people who make the truss how much weight can be suppourted in the middle(your attic area). That area looks weak to me, but I'm just guessing.

RWolf · Feb 10, 2009

Not an expert by any means but to elliminate any loading problems you might look at doing a mesinine for storage. This way nothing is tied to the roof and the open area under the mesinine could be an office etc. Otherwise I think you will need some additional supports within the shop to support the attic. Check out some of the construction techniques on old horse barns.

dsb5610 · Feb 10, 2009

I am looking into building a shed as well and am interested in what others have to say about the attic. When I asked about this I was told I need to request the bottom chord be 2x6. Seems to me there needs to be a more scientific answer.

dsb

J_J · Feb 10, 2009

Tom,

Here are two that I found.

EddieWalker · Feb 10, 2009

The farther you free span the truss, the harder it will be to make it strong enough to support a load. In your first picture, it looks very weak.

The reason a second floor is so rarely done is because of how expensive the floor has to be. While it should be doable for an engineer to design a truss that will do this, it will be cheaper and easier to do it with posts or spanning the distance from wall to wall with I-Joists. 40 feet might be pushing it, so you'll have to see what's available and what its rated for.

Compare the cost of this to going bigger on the ground, and you'll see the reason it's how most people get the space they need.

Good luck,
Eddie

LarryD · Feb 11, 2009

While I'm no expert, I have been on projects where mezzanine floors were built and it is a pretty stout structure even for a 50PSF rating. I would love to see a truss structure rated for 50PSF that spans the distance you're planning just for reference.

I've got a friend that is just finishing up a mezzanine in a third of his typical pole barn and it really seems the most logical way to make extra storage. His building is 14' at the eve so the peak has to be at least 20'. He's got a lot of lumber in the floor and as Eddie said, it might be cheaper to just expand the footprint. If space is a limiting factor, a mezzanine might be the way to go.

framer · Feb 11, 2009

The cost per sq. ft. on attic trusses makes expensive lofts.40' truss maybe
16' clear span. Depending on pitch load design.Very heavy. Framer

Tscott9330 · Feb 11, 2009

Thanks guys. I think I am just going to go back with my first design with scissor trusses. I had planned a mezzanine on the right end wall. It would only be 9' deep, but a 9'x40' mezzanine would still hold a lot of junk. Other than that, does anyone have nay input on the design?

Tom

EddieWalker · Feb 11, 2009

Tscott9330 said:
...my posts will be spaced a max of 12' apart to allow for doors and on the rear (not shown) the posts will be set at 10' spacing. Between the tops of the posts (around the perimeter) will be 2 - 2"x12" connecting the posts. These will be notched into the top of the posts on the front and back of the posts and will be lag bolted to the posts. The trusses I will have made will rest on these 2"x12" members. Assuming the truss company specs these trusses to be 2' OC, there will be 5 or 6 trusses supported by most spans.

Will the additional weight of an attic and the future crap in it be adequately supported by these 2"x12" members?

Tom,

It sounds like you have the basics, but lack a few details and might be doing things a little excessive.

The size of the building is pretty good. I'm sure you've thought that through and it's what works for you. My shop is just 24x30, so that's what I work with.

You didn't say how tall your walls were going to be? Mine are 12 feet and I highly recommend that height. If you went lower, I wouldn't go below ten feet. My roll up door is 10x10 and that works great for getting just about anything into my shop.

The scissor trusses will give you more height in the middle, but they will cost you allot more money to build. When you price them out, be sure to also price out conventional trusses to compare the difference.

You said you want to put the trusses on 2ft centers. Why? This spacing is good for roof sheathing and then shingles, but for a metal room, it's a bit close and wasteful. From the picture, it looks like you have purlins on top of the trusses, which means a metal roof.

Spacing of the trusses is decided by the size of your purlins. Depending on code in your area and snow loads, the size of lumber and the spacing might be out of your hands. Here, we don't have snow and there's allot of variation on how it can be done. I personally like trusses on 4ft centers with 2x4x12 purlins on the flat on top of the trusses. Having them flat gives me a little more support and a bigger target to hit when screwing in the metal panels.

At six feet centers, you can turn the 2x4s on edge for your purlins and save some money on fewer trusses with the same number and price for purlins. The difference is flat or on edge. At 8ft centers, you go up to 2x6's for your purlins on edge. Then it just keeps going up from there.

I like my purlins to be every four feet. This gives me a very solid roof. My manufactures says I can go further apart, but this is what I like and how I do it. For the small increase in materials, it's worth it. I also like my purlins to be 12 feet long and I stagger them. First row starts at the end of the building, then the next one up goes in one truss and so on. 12 feet also gives me three trusses to attach to. 16 feet would be even better, but I've found that you have to use those 16 ft 2x4' pretty quickly or they tend to start to bow on you while sitting around on the job.

If you went with 8ft centers on your trusses,then you would have to use 16ft long purlins, but then of course, they would also be 2x6's with less tendency to bow on you then 2x4's.

You mentioned lag bolting your top plates to your posts. The notching is a good idea, but not really needed or necessary. Instead, use two or three half inch hot dipped galvanized carriage bolts. Three would be better, but I don't know your budget or code. Use screws to put them in place while getting them level and true. Leave the screws in there, but be sure to leave an open area for the bolts. This way it's easy to drill through all the wood at one time to install your bolts. Remember, nuts on the inside.

Eddie