I'm getting ready to cover a 16 x 20 patio with a 2/12 pitch and clear fiberglass roofing. My question is: can I use 2 x 6 - 16" o/c on a 17 ft. span or would I need to go to 2 x 8's?
Used to have a book that gives all that info but a lot of my books disppeared after I retired. Anyone give me a quick answer to this?
Al in SW WA
I'm not a structural engineer. However, if it was mine, I would use 2x8
24"c. The roof load is not very heavy but the big conceren would be up-lift and other wind loads. I assume that in SW WA you should not have any snow loads.Dave
Without question, use 2x8's, especially on a 17 ft span.
I could dig out the books and do the calculations, but frankly, it is more work than it is worth for the few pieces of 2x8 wood involved.
You might 2x10's on 24" centers, but don't think you would save much.
HTH
Thanks a lot guys, this is what I needed so 2x8's on 24" o/c it will be. At least we don't have to worry about much snow here in SW WA like we did 40 years ago. Weather sure has changed.
Thanks again, Al
"Al" wrote
Stick this is your saddlebag for future reference:
OK not to get too technical about this, but this all matters where you live (snow loads or wind loads ?). According to the wood rafter span tables you would at least need 2x10's at 12" on center, and this is assuming a 20psf live load, which I doubt you will have, unless you have snow loads. If you are getting a building permit :=O, you will quickly find out what is needed. Now before everyone jumps my case about over designing these roof rafters, for the dead load, we all know that in the future, this clear fiberglass roofing *could* be replaced with a sheeting/shingle roof system and thus the size and spacing advised in this thread so far will not be anywhere near enough to carry the dead load. Like I said in the previous paragraph, if you are getting a building permit (structural construction), then you will quickly find out what is needed.
JAW
Al wrote:
|Thanks a lot guys, this is what I needed so 2x8's on 24" o/c it will be.
Hold on a minute. If this fiberglass is anything like what I used years ago, 24" centers is going to allow a bunch of sag between sheets.
Since you will never have any live load and the dead load should be zip too, I'd be more inclined to have a bigger population of smaller joists. Span tables are developed for a particular allowable deflection and have (almost) nothing to do with yield strength.
As before, the deflection I think you need to worry about is in the fiberglass, not the lumber.
O-K, I forgot to mention that I'm putting 1 x 4 @ 16" o/c at right angle to the joists (rafters) to screw the fiberglass to. Done this in the past and never a sag problem. Snow load here in Longview, WA? Haven't seen more then an inch or two in many years, then it only lasts for a day or two. Just enough to get messy.
Building permit? Never got one for anything I've done. Why make it easy for them to raise my property taxes?
Thanks for the chart, Swingman, tried to find one with Google with not much help. Al
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