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Pancakes
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# Posted: 25 Nov 2022 06:20pm
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So I got these plans for an A-frame cabin online. And it calls for a special ridge board that is 16"x2"x20' that all the rafters get attached to and meet at the top.
The problem is any place I call for a board that size wants an insane price ($350+!)
My question is if I could get 2 smaller (And more affordable) pieces for the ridge board? Then tie them together with steel plates or something in the middle?
It doesn't really seem like it has to be a single piece to me? Since the structure of the A-frame is more off the rafters?
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Atlincabin
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# Posted: 25 Nov 2022 07:18pm
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When you say two smaller pieces, do you mean two shorter pieces (like2 - 2x16x10') or two narrower pieces (like 2 - 2x8x20')? If the former, I would be reluctant to tie these together to make a single longer piece. If the latter, one might be able to put the two pieces together.
I'm not an engineer and haven't seen the plans, but perhaps you could ask the person who drew up the plans why the 2x16 is required. Maybe a 2x12 would work as well.
Another alternative would be to make your own beam out of laminated plywood. I would use sheets of 1/2" plywood and stagger the joints, use good waterproof wood glue and mechanical fasteners (nails, staples, or screws). Again, I'm not an engineer so unable to comment on whether that would meet the structural needs of the project.
Just providing some ideas. Any my 2c is worth a lot less these days with inflation....
Cheers,
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spencerin
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# Posted: 25 Nov 2022 08:29pm
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You sure it calls for a 2" x *16"* x 20', and not a 2" x *6"* by 20'? A 16"-er isn't common, hence the price. I agree with Atlincabin, consult an engineer. But, think you'll find it doesn't truly require a 16"-er. Regardless of the dimension, do get it in one 20' section, no splicing.
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DaveBell
Moderator
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# Posted: 25 Nov 2022 08:41pm
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You should stop with the A-Frame idea and go with a box shape and shed roof. Everything would be so simpler.
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ICC
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# Posted: 26 Nov 2022 12:34am - Edited by: ICC
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A ridge board is NOT a structural member. A ridge beam would be structural, but an A-frame uses a ridge board. Any A-frame does.
What is the purpose of a ridge board if it is not structural, you may be wondering. A ridge board is used to: (1) make it easier to nail the upper ends of the rafter pairs together at the top and (2) to hold the rafters at the correct spacing from one rafter pair to the next. That is all.
The angled cut at the top end of the rafters is called a plumb cut.
The ridge board is usually 2x material. The ridge board must be at least as wide as the plumb cut; the angle-cut pointy end of the rafters. This is so the entire length of the plumb cut rafter end has a full flat surface providing support along the entire length of the plumb cut. The pair of rafters are nailed to the ridge board. Good plans should list the size and number of nails required. Or check Chapter 6 of the IRC, in the nailing schedule.
Failure to fully support a rafter end does not meet code and may lead to the rafter end splitting.
When the rafters are more typically moderately pitched, the cut end of a 2x6 rafter may only be slightly more than 6 inches, so in that case, a 2x8 or would provide enough width to fully support the cut rafter end.
With steeply pitched a-frame rafters the plumb cut gets much longer. A 2x8 a-frame rafter could easily have a 12 inch plumb cut. If the rafters are cut from wider stock the plumb cut could get close to 16".
So what to do if a single-piece ridge board is too large to handle or outrageously expensive? Remember this is not a structural member.
Piece it together. If it is to be 20 feet long in total, use two 10 footers, or whatever lengths you have left over or can buy and handle easily. The ridge board will be cut and nailed to butt together between two pairs of rafters and a piece of 1x, or 2x or plywood scabbed over the splice as a connector.
I hope that is clear. I cannot post images from where I am at present.
Two or more 2x may also be stacked one above the other to make a ridge board that is wide enough to provide the support surface. I would suggest staggering the splice joints along the length of such a pieced-together ridge board. If you really need about a 16" ridge board width I would try sticking three 2x6. If the ridge board width is not quite as wide as the length of the plumb cut it is much better to have that unsupported bit up at the upper point of the rafter plumb cut than at the heel of the cut.
I am not fond of a-frames myself. They can be very limiting in the square footage that has usable headroom without ducking. With the lack of side windows, I think most of them look like tunnels. Limited vertical wall space for hanging things. Etc. But some people like them and that is their choice. They do shed snow well so that is sometimes a reason to choose an a-frame.
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Aklogcabin
Member
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# Posted: 26 Nov 2022 10:19am
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The rim joists material for laminated joists is called versalam. Not sure about spelling. BCI joists come in 16" wide along with the 12". And come in and length you want. Not sure about price but probably not near 300 bucks. You may want to price them for an option.
For what it's worth, I think a frames look OK. If it fits your needs and you like the style, then I would suggest that. Nothing like telling yourself that you should have followed your dreams and ideas.
Good luck n stay positive
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Tim_Ohio
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# Posted: 26 Nov 2022 11:15am - Edited by: Tim_Ohio
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I might add, and it’s probably obvious to most, but structurally the rafters need to come together opposite each other for structural stability. Also, collar ties are necessary to avoid any walls the birds-mouth cut of bottom of the rafters rest, from splaying outward. There is outward force exerted on the walls, less so with a taller pitch.
By the way, in older construction it was not uncommon to use a 1†thick ridge board, since, as stated it is not structural.
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Brettny
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# Posted: 26 Nov 2022 12:41pm
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For an A frame I wouldnt even use a ridge board. No need for it as you can just make trusses and stand them up.
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MtnDon
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# Posted: 26 Nov 2022 02:30pm - Edited by: MtnDon
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Of all the components that come together to build a home, I believe collar ties are the most misunderstood in their reason for being. Many builders intermix the purposes of rafter ties and collar ties. Both may be found in the same roof assembly and both have different purposes. The attached pdf copy of a finehomebuilding.com article contains an excellent explanation.
I agree with Brettny's comment. Many A-frames are built using a series of triangular frames erected like a truss-framed roof. The commonly used truss framed roof has no ridge board at all. The trusses are temporarily held in position by bracing which may be left in place or removed after the roof is sheathed. The sheathing holds the trusses or A-frames in place.
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Tim_Ohio
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# Posted: 26 Nov 2022 07:52pm
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A true A-frame has no rafter ties. The walls/roof terminate at the floor or the tops of knee walls. In the case of knee walls, the floor and foundation are to be below grade. The grade serves as a counter to splay, as it is brought in on the outside of the structure. The collar ties should still be used. If the floor is at grade, the floor serve the same purpose as rafter ties, as far as I understand.
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