Need help with rafter spans


im building a cover over a slab of concrete out back. its a 15x25 piece. what size of wood do i need for the rafters?
im thinking 4 by 4's in the corners and one in the middle of the 25' sections. 4x14 around ancoring to the 4x4s and 2x6 spaning the 15 feet from 4x14 to 4x14. will this work or is it an overkill?
im using plain wavy sheet metal roofing.
thank you
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i mean 4x10's instead of 4x14 sorry
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Where do you live? Lots of snow or wind loading? Also, what roof pitch are you thinking?
Josh
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south texas. no snow, except once recently. wind might be a factor. i found the wind average to be 18mph. i was going to do just one slant. about 1 foot difference on the pitch. ex. left side 9 feet high, right side 8 feet high. just to run the water off.
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When I lived in Florida, I built a pole barn which would be somewhat similar to what you're doing. I think you might want to consider a similar design.
First, you can eliminate the heavy and expensive 4x10's. Don't worry about running beams along the 25 length to support the ends of the rafters. However, I'd up the poles to 4x6; 4x4 is too flimsy. Orient them so that the long side (the 6" side) is parallel to the short side of the slab (the 15' side). If I understand what you've described, you should have six poles total, three at 8' on one side and three at 9' on the other. Using 2x10's, make six rafters about 18' long (It's only a little over 15' between the poles, but this will give you some extra overhang to keep rain out). These rafters get attached in pairs to the poles with 1/2" threaded rod and nice big washers, so that they form a rafter-post-rafter sandwich. When you're done, you should have three separate free-standing structures consisting of a pair of posts 15' apart and a pair of rafters attached to either side of the posts. Add some 2x4 blocking in between the 2x10s every foot or so along the length of the rafters.
At this point, you connect the three separate sets of rafters with 2x6 purlins on edge at 24" on center, spanning about 12' from one set of rafters over to the next. I'd use 2x6x14' purlins so that you get some overhang on the ends of the roof. In any event, you'll want the total length to be an even multiple of 2' since the corregated or 5-V roofing is usually 2' wide. For this roof pitch, the purlins are basically joists. They are 2x6's on edge running parallel to the long side of your slab, perpendicular to the rafters. Connect them with H1 Hurricane ties like these: http://www.strongtie.com/products/connectors/H.html#gallery You should be able to find them right at the borg.
Speaking of hurricanes and high winds which you could certainly get in south Texas, I would recommend using screws for the metal roofing, not nails.
My barn made it through all the hurricanes last year (and this year too, though I no longer live there) with no problems at all. During Frances I had to go out to feed the horses in 70+ mph winds; the wind was howling through the barn but the roof didn't make so much as a creak. Meanwhile, back inside my house it sounded like the roof was going to blow off at any second.
I've put a few pictures online which show how I built the roof so you can see what I mean. My construction had a steeper roof pitch, so I have collar beams as well as rafters, but it should still work for you. The engineer I spoke to said I could get away with 2x8 rafters for my 13' span, but I'd recommend the 2x10's for yours.
The pics are at http://www.flickr.com/photos/jcaron2/search/tags:barn /
Josh
P.S. I drew up plans for the barn. I can probably track them down and send them to you if you want.
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Hey Josh,
Thank you for this excellent write up! It's really helped alot
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Sure. I found the plans. I can email them to you as PDF if you want them. Just let me know.
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Call your county extension agent, and ask if they have plans for a pole barn. Also, try googling "department of agriculture pole barn plans"
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Randy Thomas wrote:

Wow. So many plans, and me with no real estate.
er
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You need to get a copy of the UBC [Uniform Building Code]. It has extensive rafter tables for many wood species. Your local Building inspector can sell you one or give a local source. Building according to these standards should ensure that you don't find out you have to re-do the whole thing at inspection time. Bugs
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I have found these sites useful:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp http://www.awc.org/calculators/span/reversecalc/reversecalc.asp
I would use them to create rough plans then, as already suggested, get the go ahead from the inspector. He may give you an easier time if you do your homework first.
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