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SubArcticGuy
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# Posted: 18 Jan 2013 02:31pm - Edited by: SubArcticGuy
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I recently had a post about ridge beam sizing. Now I am rethinking the roof design. I am building a 16'x16' cabin with 8' walls (2x4 16"OC). I want to have a small loft (6.5' x 16') at the back of the cabin for sleeping. I am looking at having about at least 4' of headroom at the peak of the loft.
I was originally planning a ridge beam and two side beams to be able to support the roof 6.5 off the front of the cabin to cover a small deck. Now I am wondering if I can just get away with 2 small beams off the front of the cabin to support the roof structure. Then I would put log support posts up on the deck to support the beam.
Does anybody have any suggestions for a roof design. I am wondering if just 2x6 rafters with a 2x6 ridge board would suffice. Could I use the floor joists for the loft and then collar ties near the ceiling for the rest of the joists to prevent the walls from bowing out?
It will be a metal roof with a significant pitch so snow loading shouldn't be too much of a factor.
Thoughts are appreciated.
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MtnDon
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# Posted: 18 Jan 2013 03:26pm
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1. Collar ties do nothing to restrain the wall tops. Collar ties are too far up in the gable triangle to have any effect. They are designed to keep the roof peak from peeling open under extreme wind conditions.
2. If the loft floor joists are located on the wall tops as rafter ties, then those will count as rafter ties and will keep the walls in place. That is dependent on proper nailing of the rafters to the floor joists / rafter ties.
3. By the time the steep pitch rafters are installed, the roof sheathed and roofed, then insulation inserted and finally some sort of interior finished the roof DL (dead load) could be 15, even 20 PSF. Just using that, with rafters on 16" centers each rafter would have a thrust of 106 pounds. That is, 106 pounds pushing horizontally outwards. If the load on the roof is greater (winds, snow) the "push" increases. 40 PSF total load = 213 lbs, 50 PSF total = load 266 lbs. If the pitch is reduced to say 10/12 the loads increase; 128, 256 and 320 respectively.
4. Your building probably will not fall down. Over time though the wall tops may move outwards and the ridge develop a sag. That may cause other issues to develop.
5. You were on the best track with the ridge beam.
6. As a 'make-do', or a 'better-than-nothing' crutch a couple of doubled up 2x4 across the width (2 evenly spaced locations in the open ceiling space; sandwiching the rafter and with spacers between them.) would help a lot to restrain the forces on the walls.
7. "proper nailing" of rafter ties to rafter... A 10D Common (not box) nail has a shear of 100 lbs in most woods. A 16D is 120 lbs. Box nails and most gun nails would be 79 lbs for a 10D and 88 lbs for a 16D. Divide the rafter thrust by the nail shear and that is the number of nails. There is also a table in the IRC; R802.5.1(9) With high loads and many nails it is sometimes best to use lumber that is larger than what the design loads call for to give better nailing with a larger member.
8. 2x6 rafters are fine; the ridge should be bigger. The idea is for the ridge board to offer full surface support to the angle cut rafter peak. The steeper the pitch the longer that cut surface. If you err place the ridge lower so the lower edge of the angle cut rests against the ridge board. If the tip is supported but the bottom is not a rafter can develop a split.
A 12/12 rafter tip will need more than a 2x8 to cover the entire angled end. A filler strip could be used at the top with a 2x8 mounted low enough to provide proper support.
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MtnDon
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# Posted: 18 Jan 2013 03:31pm
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Quoting: SubArcticGuy if I can just get away with 2 small beams off the front of the cabin to support the roof structure. Then I would put log support posts up on the deck to support the beam.
The log posts would be best NOT placed directly on the deck boards. They should either go right down to the footings or if there is a main beam under the deck joists, they could be supported on the beam. The deck boards should be scribed / cut to go around the posts.
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toofewweekends
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# Posted: 18 Jan 2013 04:25pm
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Give a thought to higher walls (9-10 feet) and use the extra foot or two in the loft. It will give you more head and storage room for practically no cost and without much more work on the structural integrity calculations. I've got a loft where the 12-12 roof comes down to the floor and I'm left with a lot of goofy 45-degree triangular spaces that go mostly to waste.
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SubArcticGuy
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# Posted: 18 Jan 2013 04:32pm
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The log posts would go right to footings and then also be used as an attachment point for the small deck on the front.
I am thinking I could put the 2x6 rafters on 24" centers (it would then be strapped on top with 1x4's and have tin installed. I could then put 2x6 rafter ties on all but the very center rafter. The one missing rafter tie would allow some space for the ladder to the loft (which would be tied into the end of a bunk bed). It would also provide more of an open space to help prevent the kids from trying to climb from the loft over to the other rafter ties  . The open rafter ties would also make useful locations for drying clothes with the addition of a couple of drying racks.
The one advantage I see with avoiding the ridge beam is that it will be much simpler to frame a window into the loft. Otherwise I will probably have to go with two windows on either side of the support post.
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MtnDon
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# Posted: 18 Jan 2013 07:00pm - Edited by: MtnDon
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Quoting: SubArcticGuy put the 2x6 rafters on 24" centers
Yes, that would be probably be ok... ... but not ideal.
Quoting: SubArcticGuy strapped on top with 1x4's and have tin installed.
You do realize that most metal roofing panels sold are not rated as structural? Without osb or plywood roof sheathing nailed to the rafters your roof is missing most of its structural rigidity. The 1x4's don't come close to providing the stiffness wood sheets do. Roofs w/o sheathing are also much more prone to moisture condensing on the underside. , IMO it is not a good method for a roof on a habitable building.
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Martian
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# Posted: 18 Jan 2013 07:07pm
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The question is, how long do you want this structure to stand? A friend has a saying, "Even a poorly built house will stand a long time. You may spend a lot of time doing repairs, however."
If you want a structure that your kids will be enjoying when they are old, then build it to last. The time and money you spend now, will save you a bunch down the road.
I do metal roofs quite often. If you use 1X4 purlings on 24" centers, there will be dips in your roof at some point. Which takes us back to the original question, what is your goal?
Tom
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SubArcticGuy
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# Posted: 21 Jan 2013 06:03pm
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Good point on the metal sheeting. I guess I will be sheeting with OSB first (which works out well since I don't have access to the tin until the snow melts and it has to be ski-doo'd out there. It will be a tarp roof for a season. I have also bumped the roof up to a 12/12 pitch to give additional headroom in the loft.
Does anybody have any thoughts on heating...the only issue I see with a loft is cooking the people up top with the woodstove going. Perhaps a heavy curtain across the front of the loft would help control the heat if it becomes an issue.
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