lean to for extra storage

[roxynoodle]

I would like to build a lean to on my corncrib to gain some extra storage. It would be probably 15' or 16' wide by 24' long. My question is that I was planning just to hang some joists (vertical) with purlins (horizontal) for the roof, which would be metal. I was thinking 2x6 joists with 2x4 purlins. I ran this idea past a friend who suggested strongly that I need either fabricated trusses or 2x12s for the joists. I don't see why the lean to needs to be stronger construction than the building it is attached to, which has 2x6 joists and old barn siding for purlins. I am planning 2x12s for hanging the joist brackets against the corncrib wall (bolted into the studs) and 2x12s at the other end for resting the joists on and nailing those into my posts. I was looking at a pitch of 3 feet high over the run of 15 or 16'. Any comments or suggestions would be appreciated.

 

[randy41]

i think snow load is your only loading concern and 2x6's on 24" centers should be sufficient. i would use 1x purlins as they are only there to be nailers.

 

[MikeD74T]

" I don't see why the lean to needs to be stronger construction than the building it is attached to"
One reason would be that snow falls evenly onto a roof but slides downhill compacting as it goes. The leanto will see a higher density snow load than the main roof. If you are attaching to the wall rather than extending the main roof rafters the leanto roof will also get shock load from snow falling in chunks from the main roof. MikeD74T

 

[ovrszd]

I would like to build a lean to on my corncrib to gain some extra storage. It would be probably 15' or 16' wide by 24' long. My question is that I was planning just to hang some joists (vertical) with purlins (horizontal) for the roof, which would be metal. I was thinking 2x6 joists with 2x4 purlins. I ran this idea past a friend who suggested strongly that I need either fabricated trusses or 2x12s for the joists. I don't see why the lean to needs to be stronger construction than the building it is attached to, which has 2x6 joists and old barn siding for purlins. I am planning 2x12s for hanging the joist brackets against the corncrib wall (bolted into the studs) and 2x12s at the other end for resting the joists on and nailing those into my posts. I was looking at a pitch of 3 feet high over the run of 15 or 16'. Any comments or suggestions would be appreciated.

Next spring I'm building a 12 addition down one side of my shop. My best buddy builds houses. I told him I was using 2x6 rafters on 2ft centers and covering with metal roofing. He said it wouldn't work, that it would sag. He asked about a ceiling and I said I was putting in a flat ceiling. He said to use 2x6 ceiling joists on 2ft centers to match the rafters, then truss them together and it would hold.

I was just like you Roxy. I just couldn't understand why I needed that much rafter to hold 12 feet when my truss rafters are spanning 30 feet. I'm using almost the same fall as you, 2ft in a 12ft run. He said a 2x6 rafter on 2ft center covered with metal will only span 8ft without sagging. He said a 2x8 would span 10ft and a 2x10 would span 12ft. But he said I could truss with 2x6s since I was adding a ceiling cheaper than using 2x10s. As was said above, it's all about snow load.

He makes his living doing this, so I'm gonna go along with it. When I argued about this with him he said "Fine, do it however you like, but don't (complain) to me when it sags". Then he looked at me with that look that I hate, as to say go ahead and make a huge mistake!!! I'm following his guidance, then if it sags I can blame it all on him!!!!

 

[randy41]

i was just looking at the roof over the deck on my house...it extends 10' out and is covered with metal roofing and the rafters are 2x8's. on 4' centers except for the middle one which is really 2 rafters about 7 1/2" apart. the purlins are 2x4's. it is attached to the house just below the house's roof. it has withstood some very heavy 8" wet snow without any sag...its been there for about 6 years.

 

[EddieWalker]

Roxynoodle,

How much snow do you get? With a pitch of less than 3:12 and probably closer to 2:12, you will have all your snow just sitting on that lean-to.

I've had good success with 2x6's on 4 foot centers for a 12 foot wide lean-to, but wouldn't use them for anything wider without a support post. Snow is the deciding factor here. If you have a thousand pounds of snow on your roof, it will sag in the middle? What about ten thousand pounds? It might snap and damage what you have stored in there, damage your buiding and maybe even hurt somebody.

There are several types of truss designs that will work for you that can be built from 2x6's and 2x4's that would be plenty strong, but again, it's all dependent on how much load you will have to suport.

You realy should listen to your buddy who builds these things. He's giving you good advice.

The spacing of your truss or beams will dictate what size purlins you use. Putting them on 2 foot centers will let you use very small lumber. Here in Texas where we don't get any snow, I build mine on 4ft centers and use 2x4's flat on top of the truss. If you go further apart, you will need larger lumber and it will have to be on it's edge.

Eddie

 

[schmism]

" I don't see why the lean to needs to be stronger construction than the building it is attached to"
One reason would be that snow falls evenly onto a roof but slides downhill compacting as it goes.

thats called sliding snow... theres also drift loads. that which is blown off of one side, over the ridge, and deposited on the other side.

for low slopes (generall less than 3/12) the design unballanced snow loads can be as high as .3 and 1.7

(meaning the leaward side sees a 70% increase due to the snow from the windward side)

see ASCE for more complete explantion.

 

[roxynoodle]

Well, the only addition my buddy put on her barn, she just used 2x4s for joists, though it's a small addition. She isn't an expert, I just asked her opinion and she went kind of nuts on me. Yes, I did have a barn come down but it was hit by 2 tornados. That was what I would call extraordinary! If a tornado hits this barn it will likely come down, too. There isn't much you can do about something like that.

Ok, the lean to will be facing west. With the wind we have out here I don't think much snow will stay on it. It would have to be a very heavy snow fall, which isn't real common. My pole barn has a 1:3 pitch and is in nearly the same location. That has trusses made of 2x4s. I guess I should check out other people's buildings around here to get a better idea of what is necessary.

Thanks for all your suggestions!

 

[rlk]

Roxy, 2 x 4 trusses are engineered to be strong. They are much stronger than a regular 2 x 4 or a 2 x 6 that is used as a rafter.

You should be able to call your inspections dept or a local builder and ask them if they would look in their span tables and tell you the requirement for your area. You don't have to identify yourself, just tell them what you are thinking, and ask their recommendation.

The building codes that I am familiar with have span tables for the snow and wind load for each area. Since I'm in North Carolina, my span tables will not be accurate for your area.

I seriously doubt the 2 x 6 you want to use will be strong enough to span 16 feet, especially with the low slope you will have. Now if you had a 12/12 pitch, you might could use 2 x 6 rafters.

Best of luck with your project.

Bob

 

[EddieWalker]

Roxynoodle,

Try this, get a 16 foot 2x6 and put it's ends on some blocks. Secure it so it wont flip on you and climb on it in the middle. Than jump on it a little and see what happens.

You won't break the board, but the amount of flex you get should give you some idea of how much sag you will have in just a short time after you build it. You can also see from the amount of movement you will have to understand how it will be impossible to secure your metal roofing material to it and have it last. The constant flexing of too small of lumber will lead to the failure of what's attached to it.

You also have to realize that putting a board across your span is a beam, not a truss or a joist. I know it's just a term, but what this is significant in how it will work. A truss is a series of triangles that tie multiple pieces of lumber together in such a way as to make it stronger than the individual pieces. A truss build out of 2x4's can easily span 30 or 40 feet, or even more.

A 2x6 just isn't strong enough for that amound of distance to support it's own weight without sagging over a short amount of time.

Eddie