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SonnyB
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# Posted: 26 Mar 2012 09:41pm
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Im building a 20x32 cabin with 12' 2x6 walls. It will have a 10x20 loft on both sides with 12' opening opening in the middle. I am wanting a 10 or 12 pitch roof. What is the proper size rafter I need? I have been told a single 2x6 16oc is fine?
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MtnDon
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# Posted: 26 Mar 2012 11:22pm - Edited by: MtnDon
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Depends on snow load. 2x6, #2 in any commonly used construction species will work on 16" centers for the 20 foot width. Add much snow and the answer is no. 2x6 don't leave enough space for any serious insulation in the roof, especially if you use batts and need to provide ventilation. Many builders will use 2x10 or 2x12 so they can have sufficient insulation in their roof.
Another potential issue are those 12 foot high side walls with loft floors at 8 feet (more or less). The rafters on a 20 ft wide can generate some large horizontal outward forces on those side wall tops. With a setup like that you have basically no rafter ties. (Rafter ties are located in the lower third of the rafter triangle, not below... image below). I know this is disregarded by many and most cabins like that don't fall apart, break or cave in. Failures usually happen slowly. A sagging roof ridge is usually the first clue. The rafters push out on the wall studs. This causes a bending moment at the point the loft floor joists attach to the wall studs. This is especially more pronounced if the loft floor joists are resting on a 2x that is notched into the inside edge of the wall studs.
The open space in between the lofts can be even worse, unless you place rafter ties across the opening at the wall top. There the entire wall can tilt outwards, causing ridge droop.
rafters, collar ties & rafter ties
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SonnyB
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# Posted: 27 Mar 2012 07:44am
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The cabin will be built in N. GA so snow is only a few inches and only every few years. Do I have to have the collar ties at every rafter or every so many feet?
Thanks for the help!
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MtnDon
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# Posted: 27 Mar 2012 11:16am
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every one
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toyota_mdt_tech
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# Posted: 31 Mar 2012 02:26pm
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Don, when I submitted my plans, I was only required to have a collar tie every 4 feet. At 16" OC, that was every 3rd rafter. But that may be also because I have rafter ties on every rafter (ceiling joist) and they are tied right into the rafter.
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MtnDon
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# Posted: 31 Mar 2012 09:59pm - Edited by: MtnDon
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OOOPs, my bad.. Code is for every 4 feet. But here in the mountains the winds are so variable and at times with extreme gusts the local officials pretty much want collar ties every rafter set. I confused myself and was in a hurry. Sorry. A good substitute if one does not like the look of them in a cathedral ceiling is to use metal straps over the ridge, one side to the other, just after the sheathing is applied. Metal strapping should be 1 1/4" x 20 gauge.
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sh10134
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# Posted: 6 Apr 2012 12:30am
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MtnDon:
I've seen that image posted by you a few times and have a clarifying question. If balloon framing with high walls and a dropped loft (effecting a short knee wall in the loft), does it help any to extend the loft boards beyond the walls so they meet and connect with the rafters? This was suggested for my design by soundandfurycabin at http://www.small-cabin.com/forum/2_1891_0.html#msg27040. I've drawn it up for two of the rafter / loft boards in my design, as shown in the attached images.
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sh10134
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# Posted: 6 Apr 2012 12:35am
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The images there fail to show the intended boards attached to the long outside walls perpendicular to the loft boards, upon which said boards would rest. The first (cabin3.png) does show, barely, a possible center beam that would also support the loft boards and bear down through the shorter walls directly to the ground.
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MtnDon
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# Posted: 6 Apr 2012 12:49am
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That idea is much better then placing the rafters on the top of the kneewalls, with the loft joists dropped down the wall as is commonly done. As long as lumber is available to make a continuous span from left to right eve and rafter tail it would be much better. It's a variation on what a friend did. It was 20 ft wide with eves and floor joists protruding 2 ft on each side.
The floor joists still sat on the normal height wall tops, but the extra width created more headroom in the upper area.
Another thought, that is also not found in a code book, is to cut the notches for the ledger board into the exterior edge of the wall studs. That way any outward horizontal forces will be trying to compress the notch rather than open and cause splits in the stud at the notch.
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MtnDon
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# Posted: 6 Apr 2012 01:15am
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I want to add another thought, something to ponder. Many roofs I see designed and built on cabins with lofts and with cathedral ceilings seem to be deficient in space for good insulation. Strengthwise a 2x6 rafter would be good in many locations. But there is not space for any serious insulation. Using a 2x12 for the rafter will also provide more room for a good nail connection for the rafter to joist joint. Use of a true 16D nail, face nailed through the rafter and joist and then clinched over will increase the strength of that connection immensely. The larger lumber will make it possible to get sufficient numbers of nails in there without getting too close to the edges of the lumber. 2x10 could span 16 ft (#2 SYP) on 16 inch centers using a lighter loft LL of 30 psf, 15 DL, although 2x12 would also be stiffer.
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