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lagarto
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# Posted: 8 Jun 2012 08:59
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Sorry if my terminolgy isn't exact, I'm building a shed-like cottage and I have the choice nearby to buy for my flat slanted roof PT lumber in 2x6x16, 2x8x16, 2x6x20 and 2x8x20--if I could use 20' boards and have 2' overhang, that would give me 16' wide of living space which would be wonderful. I will be using these boards 2-3' apart with purkins(?) nailed to them to hold the metal roofing down (no plywood on the roof) so weight will be minimum..so with a long slanted flat roof--can 2x8x x20'ers span 16' and hold the purlins and metal roof OK without any extra support other than the walls?
If I have to go to 16'ers, could I have 14' of living space with 1' overhangs, in this case would 2x6x16' work or 2x8x16 (nearly the same price)?
thanks!
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toyota_mdt_tech
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# Posted: 8 Jun 2012 09:16
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I think 2X6 is to small for a 20 foot span. Your concern would be from anchor to anchor point, ie wall to wall. The 2 foot overhang isnt an issue. I know a 2X8 is good for about 12 feet at 16" OC. You wanting to go 20, makes a big difference. Also the spacing. Are you going std roof spacing of 24" and do you know what your snow load is?
I think MtDon is the guy on this question.
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lagarto
Member
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# Posted: 8 Jun 2012 09:26
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Sorry, forgot to say I'm in C. Florida--no snow, lots of rain.
I can space however needed-I have an abundance of PT 6x6x14' posts that I'm using in the frame if that matters--24" roof spacing is not a problem-I've seen quite a few pole barns down here with 2x4x1/2" purlins/metal roofs that sure look like they span 16+ feet..I'll see if I can find MTDon and ask him, thanks!
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neb
Member
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# Posted: 8 Jun 2012 09:28
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I would go with 2x12 this is a very important part of this structure!!!!!!
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toyota_mdt_tech
Member
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# Posted: 8 Jun 2012 13:57
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With no snow load at all, just rain, I'd maybe go 2X8 at the smallest and then use 16" OC. Do you have your heart set on a "shed roof" I think a standard peak type would be easer to use smaller wood and have it much stronger. Add in a rafter and collar ties every 4 or use ceiling joist to tie them all in, then it would be cooler in those florida sun, as you could insulate the ceiling.
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fpw
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# Posted: 8 Jun 2012 15:37 - Edited by: fpw
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You can use a span calculator like the following:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
This will help you determine max span.
2" x 10" White pine with a live load of 20# will span 20' if it is select grade and spaced 12" on center.
On 16" center you only get 18 feet (note: Framing lumber is not normally select).
Free spans in the 16 to 20 foot range are pretty aggresive. If you install sometype of beam at the midpoint you get a much stronger roof.
Installing a kingpost with ridge is very straightforward and you can get much better strength as indicated by toyota_mdt_tech above. In that case to get 20' under roof, you are only spannign 10' so you can use much lighter weight material.
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MtnDon
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# Posted: 8 Jun 2012 19:09
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MtnDon on vacation
Why PT? odd for a roof. Insects?
PT is usually southern pine. Use the AWC calc referenced above. If using a different wood species adjust the calculator. 2x8 on 16" centers, grade 2 is minimum for 20 lbs LL and 10 lb DL. 2x10 = better. Remember these are maximum spans; never hurts to go up a size.
What about roof/ceiling insulation? That alone is a good reason to use 2x12 as a minimum.
Most cheap metal roofing panels are not rated for structural use which is what you are asking the panels to do if there is no roof sheathing. As well you will have condensation on the underside with purlins very likely. Myself I do not see any advantage to doing a metal roof on purlins with no sheathing in a building people live in. Barns, okay. The sheathing greatly stiffens the roof, gives the building racking resistance purlins do not. That's my opinion.
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Martian
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# Posted: 9 Jun 2012 13:04
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Don's right about the purlins/sheathing issue. Decking the roof gives you so many advantages over purlins that, unless you physically cannot lift the decking, it doesn't make sense to use purlins unless you are getting them free. Decking, with a layer of felt and closely spaced screws, is a lot quieter than purlins. Having lived in Florida, I know how hard it rains down there. The decking will also cut down on hail damage; a large hail stone WILL penetrate a 29ga metal panel on purlins in a shed roof configuration! I've repaired a few. Regardless if you deck or not, use at least 26ga metal. You'll be glad you did in 10 years.
Tom
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MtnDon
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# Posted: 9 Jun 2012 15:20
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Good point on the gauge of the metal, Martian. The stuff sitting on the shelves at many big box stores is 29 and it does make a difference. 26 costs more but so does almost everything when you select a better than minimal quality. Much of the metal used on commercial buildings is 24 gauge.
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Bevis
Member
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# Posted: 10 Jun 2012 23:22
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Quoting: lagarto Sorry, forgot to say I'm in C. Florida--no snow, lots of rain.
...and high winds at times. Down here use the 26 ga as Martian suggested. I have it on my barn/shop for a while now. Been through 3 hail storms, and 2 hurricanes with just a bent section of panel.
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lagarto
Member
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# Posted: 11 Jun 2012 08:54
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Thank you all..it is southern pine, the supplier originally told me they only had PT, but today someone else there told me they had non-PT but only in 2x8x16-no 2x10 or 12 near that length and I have to get it there due to transport problems. Using 26 guage is not a problem and I will do so. I am going to go with the a 14' wall to wall structure, just hard to decide on the shed roof or not, but now I know my limits and can get started--thanks all for your help!
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