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jeroen
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# Posted: 1 Oct 2023 05:41pm
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Does anyone know of any 8x12 roof framing plans that have been verified by an engineer? Or a good tool that I can use to calculate the required sizes/values?
I don't like trusses, so I'm looking to frame my roof using rafters. I'm having a hard time finding a good design. I'd like a vaulted ceiling, and I don't mind exposed beams/joists, but preferably not every 16".
Say I use 2x6 rafters, a 2x8 ridge beam, would I still need collar ties? If so, do I need them on every rafter? Do I need rafter ties? If so, do I need them on every rafter or can I do every other one? What size rafter tie? 16 or 24 oc?
I'm in New England so there will be snow. I think a 6/12 pitch would be nice.
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gcrank1
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# Posted: 2 Oct 2023 06:50pm
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Im no engineer....and agree that a 6/12 pitch would be nice to work on, but in a snowy area I think you may need more?
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Brettny
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# Posted: 2 Oct 2023 07:03pm
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Why do you need a engineered plan?
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razmichael
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# Posted: 2 Oct 2023 09:02pm
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Clarification - 2x8 is not a ridge beam. That is a ridge board that has no structural use just makes it easier to put up the rafters and you still need collar ties etc. a ridge beam needs to be sized based on loads, distances etc as well as be designed to bear the weight down to the foundation. Rafters can then be hung off the beams which substantially reduces the size (ignoring need for insulation space) and generally removes the need for collar ties. Disadvantage is can be harder to get up, impacts design and generally more cost. Not to mention the need to be very attentive to the span tables etc.
I used LVL build up beam for my 16x24 allowing for an open space and lots of loft headroom. All depends on the goal.
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Brettny
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# Posted: 3 Oct 2023 09:47am
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With a 8x12 your really not going to have much of a loft because the building is so skinny.
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razmichael
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# Posted: 3 Oct 2023 10:05am
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Agree. My main point was to clarify board vs beam. I then got carried away into my build!
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jeroen
Member
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# Posted: 3 Oct 2023 11:39am
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"a ridge beam needs to be sized based on loads, distances etc as well as be designed to bear the weight down to the foundation. "
That's my point. How did you all figure this out? Are there tools out there to calculate this?
I guess my question is: what size for what span, and pitch? I understand the physics behind it, but I cannot find what size ridge beam I would need for a span of X, with a roof pitch of Y, and Z inches on center.
There are span tables, but that doesn't tell the whole story.
If I go 24 on center with 2x6x6 rafters, then I have about 14x8lbs of rafters. Depending on the pitch, more or less than half of that weight would rest on the ridge beam. If I go 16 oc then I have 20x8lbs.
The pitch also determines how much of all that roofing material and sheathing weight is transferred to the walls, and how much to the ridge beam.
The easiest I guess is to not use a ridge beam, but is ridge board instead and use collar and/or rafter ties. But still, 16 or 24 oc? 2x4 or 2x6 rafter ties? How far from the top do I place the collar ties? Are there tools or tables for this, or does everybody just wing it?
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razmichael
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# Posted: 3 Oct 2023 01:27pm
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There are lots of calculators out there but I'll let someone more up-to-date on U.S. code recommend one. I just wanted to make sure you understood the difference between a beam and board as your original post described a 2x8 beam and I assumed you meant a Board. Sorry if you knew this and just made a typo.
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jeroen
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# Posted: 3 Oct 2023 02:52pm
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I've done some more research and am planning on doing the following:
-2x6 rafters 24 o.c. is (more than) sufficient per the rafter table at https://www.mycarpentry.com/rafter-span-tables.html. I could have gotten away with a 2x4, but I want the room to insulate.
-a 2x8 ridge board is required: "Ridge boards shall be at least 1 inch nominal thickness and not less in depth than the cut end of the rafter (IRC section R802.3)."
-I will use collar ties on every rafter, but could get away with skipping every other one: "If collar ties or ridge straps are installed to connect opposing rafters, they must be located in the upper third of the attic space. They must be at least 1 inch by 4 inches (25 mm by 102 mm). They must be spaced no more than 4 feet (1219 mm) apart."
-I will use 2x4 rafter ties on rafter 1,3,5,7: "When rafters are installed on 24-inch centers, rafter ties are typically installed at every other rafter. It’s not unusual to see rafter ties of either 2 x 4-inch or 2 x 6-inch."
I will then install Durovent 23-1/2 in. x 46 in. Rafter Vents against the underside of the roof sheathing, followed by insulation. Next I will hang the ceiling on underside of the rafters and the collar ties, leaving a mostly vaulted ceiling with exposed rafter ties.
Sources:
-https://www.mycarpentry.com/rafter-span-tables.html
-https://www.nachi.org/collar-rafter-ties.htm
-https://www.techsupport.weyerhaeuser.com/hc/en-us/articles/207291947-Ridge-Beam-vs-R idge-Board
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gcrank1
Member
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# Posted: 3 Oct 2023 03:03pm
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All this for an 8'x12' bunkie?
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Brettny
Member
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# Posted: 3 Oct 2023 03:40pm
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You cant calculate anything until you know your roof load for the area.
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jeroen
Member
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# Posted: 3 Oct 2023 03:57pm
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Quoting: Brettny You cant calculate anything until you know your roof load for the area.
30 psf snow load, 20 live load. I went for the table with 50 psf snow load, 20 live load.
Quoting: gcrank1 All this for an 8'x12' bunkie?
Yep, I'm a software engineer by trade. I like numbers and calculations. I am also learning for when I build my bigger cabin in the next few years.
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