That's my opinion.ĭon's right about the purlins/sheathing issue. The sheathing greatly stiffens the roof, gives the building racking resistance purlins do not. Myself I do not see any advantage to doing a metal roof on purlins with no sheathing in a building people live in. As well you will have condensation on the underside with purlins very likely. 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. What about roof/ceiling insulation? That alone is a good reason to use 2x12 as a minimum. Remember these are maximum spans never hurts to go up a size. 2x8 on 16" centers, grade 2 is minimum for 20 lbs LL and 10 lb DL. If using a different wood species adjust the calculator. In that case to get 20' under roof, you are only spannign 10' so you can use much lighter weight material. Installing a kingpost with ridge is very straightforward and you can get much better strength as indicated by toyota_mdt_tech above. If you install sometype of beam at the midpoint you get a much stronger roof. On 16" center you only get 18 feet (note: Framing lumber is not normally select).įree spans in the 16 to 20 foot range are pretty aggresive. You can use a span calculator like the following:Ģ" x 10" White pine with a live load of 20# will span 20' if it is select grade and spaced 12" on center. 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. 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. With no snow load at all, just rain, I'd maybe go 2X8 at the smallest and then use 16" OC. I would go with 2x12 this is a very important part of this structure!!!!!! I can space however needed-I have an abundance of PT 6圆x14' 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! I think MtDon is the guy on this question. Are you going std roof spacing of 24" and do you know what your snow load is? You wanting to go 20, makes a big difference. I know a 2X8 is good for about 12 feet at 16" OC. Your concern would be from anchor to anchor point, ie wall to wall. I think 2X6 is to small for a 20 foot span. If I have to go to 16'ers, could I have 14' of living space with 1' overhangs, in this case would 2圆x16' work or 2x8x16 (nearly the same price)? 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? 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 2圆x16, 2x8x16, 2圆x20 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. Small Cabin Forum / Cabin Construction / How long can non-truss single roof beams be? Forums - Register/Sign Up - Reply - Search - Statistics.
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