Rafter Size for 16ft span

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Old 07-21-10, 05:13 PM
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Rafter Size for 16ft span

Hello,

I am building a 16 x 24 home workshop with shed roof and 3/12 ptich. The roof material will be metal and it is in Central Texas outside of the snow belt-so there are no issues with snow load.

I have been trying to size the rafters so I can have a clear span. The span tables are saying 2x10 on 24 oc will be fine, but the lumber yard as well as the "old hands/contractor fiends" are saying I can do 2 x 8 on 16 oc with no issues. Of course I am also getting the suggestion of trusses.

I am out in the county, so there is no building department/code enforcement to consult.

I am looking for any feedback/opinions.

Thanks,
-TMD
 
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Old 07-21-10, 06:06 PM
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Hi TMD and welcome to the forum.
I'm not the best at this, but can try to get you started. If I'm reading you correctly, you want a single slope roof with rafters running the 16' dimension. Will you want to overhang anything front and back or just end flush? I'm in snow country so we always use an overhang. Optional where you are. However, the 16 foot dimension is still going to need a rafter slightly over the 16' mark and those 18' or 20' boards can get expensive. You could of coures make the shed 15.5' or whae ever works.

As for load capacity, 2x8's 16" oc are not that far below 2x10's @24"oc. Will they hold, absolutely. With no live load like a deck full of party goers, all they have to hold up is themselves, the strapping, and the steel. Will they bounce when you walk up there, certainly will. Will the sag, a bit more than the 2x10's.

Now, lets go back to the start. Front wall and back wall heights? If they are both 8' and the attic area is of no real use, price up some trusses. Or make your own. Give us some more details on how you want to build this and what space you need inside?

Bud
 
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Old 07-21-10, 09:09 PM
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Originally Posted by Bud9051 View Post
Hi TMD and welcome to the forum.
I'm not the best at this, but can try to get you started. If I'm reading you correctly, you want a single slope roof with rafters running the 16' dimension. Will you want to overhang anything front and back or just end flush? I'm in snow country so we always use an overhang. Optional where you are. However, the 16 foot dimension is still going to need a rafter slightly over the 16' mark and those 18' or 20' boards can get expensive. You could of coures make the shed 15.5' or whae ever works.

As for load capacity, 2x8's 16" oc are not that far below 2x10's @24"oc. Will they hold, absolutely. With no live load like a deck full of party goers, all they have to hold up is themselves, the strapping, and the steel. Will they bounce when you walk up there, certainly will. Will the sag, a bit more than the 2x10's.

Now, lets go back to the start. Front wall and back wall heights? If they are both 8' and the attic area is of no real use, price up some trusses. Or make your own. Give us some more details on how you want to build this and what space you need inside?

Bud
Hello Bud

Thanks for the welcome.

You are correct in your understanding. I am looking to do a single slope roof--this is drive by aesthetic considerations from both my wife and me.

I am still at the design phase of the project and I am flexiable in my design considerations. I had not planned on using the attic space. My original thinking, when I was focused on using dimenisional lumber for roof rathers, was to have a cathederal ceiling with the installation of clere story windows along the "tall" wall--which faces North, for light.

Since this building is near my existing barn, which has a 2/12 pitch, I had planned on the building having a 2/12 pitch or a 3/12 pitch. If I have a 2/12 pitch, we are looking at a tall wall height of (to the top plate) of 10'8". If I go with the 3/12 pitch, we are at 12' even at the top plate.

I hope that helps
 
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Old 07-22-10, 04:15 AM
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Where are you located? Do you have snow? Do you have rain? If so a 3:12 is minimal. 2:12 may allow for too much capillary action with water at the edges. As Bud says, you will need to make a consideration for the flat side and it's rain/snow shedding properties.
 
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Old 07-22-10, 05:13 AM
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Here's a span calculator. Link There are others available just Google "rafter span calculator". It looks like 2X8 might be borderline depending on the grade and species used.
 
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Old 08-07-10, 07:11 PM
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a frame cabin

i have a rafter spacing question as well. i'm getting ready to build an a frame cabin. the wall/roof is going to be a 20/12 pitch. i'm going to use 1x4 tongue and groove, panel insulation, 2x4 2nd rafter, 5/8" plywood and asphault shingles. my question is this: if i use 4"x6" timbers for the main rafters for the wall/roof, can i space them at 4' oc?
 
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Old 08-08-10, 03:53 AM
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This is a duplicate post in the Carpentry forum and is a hijack anyway.
 
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