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madmilitia
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# Posted: 29 Mar 2022 13:02
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hello,
i have a chunk of remote alaskain property, it's a 60 mile trip boat/snowmobile/plane. i have a lumber mill max length cut is 16.
i'm building a 24 x 24 cabin, the question i have is about rafters. my location gets alot of snow and i'm not always there, so i need to go 12/12 pitch.
12/12 requres rafters at 20 foot lengths. well i can;t mill 20 foot boards.
so what is the best way to sister/spilch/connect to 10 foot boards?
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Grizzlyman
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# Posted: 29 Mar 2022 13:07
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That’s a great question and I have no idea if there is a good simple method given the limitations.
However what if you were to double them like a beam with significant enough overlap?
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madmilitia
Member
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# Posted: 29 Mar 2022 13:14
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yeah so use like 2 10 footers and then run a 5 footer on each side of the joint to like sister/sandwich them togther?
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pabear89
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# Posted: 29 Mar 2022 13:27
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would think using 2 16'plus a 4' and flipping the ends to build your rafter beam.
having that 4' joint on both ends should give it enough support.
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Brettny
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# Posted: 29 Mar 2022 14:34
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Are you needing a 12/12 just to get snow to slide off?
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madmilitia
Member
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# Posted: 29 Mar 2022 14:57
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yeah, other wise i gotta build for an 8 foot snow load.
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Grizzlyman
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# Posted: 29 Mar 2022 15:08 - Edited by: Grizzlyman
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Quoting: pabear89 would think using 2 16'plus a 4' and flipping the ends to build your rafter beam. having that 4' joint on both ends should give it enough support.
This is what I was Thinking. Not sure of your spacing either with that much snow load but a beam like this should be more than fine for 24†spacing
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madmilitia
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# Posted: 29 Mar 2022 15:15
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what do you mean by flipping ends? so 16 foot plus a 4 on one side then th 4 plus 16 on the other side.
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Grizzlyman
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# Posted: 29 Mar 2022 16:13 - Edited by: Grizzlyman
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Yes. Just like if you were building a beam. Maximum overlap. Or 6 and 14 would be good too.
You’ll also have to cobble together your rafter ties too.
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Brettny
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# Posted: 29 Mar 2022 16:59
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8' snow load..I would be building an A frame. At least then your splice would be nearly vertical.
Can you put collar ties in at the 4' mark to take away some of the stress on the joint?
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madmilitia
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# Posted: 29 Mar 2022 17:02
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yeah collar ties are the plan. 12/12 with metal roof is enough to shed the snow that's what the others do.
A frame is the idea for my final cabin but this is the setup to stay in while i build the A
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NorthRick
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# Posted: 29 Mar 2022 18:13
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Make your cabin narrower and longer. 10/12 pitch sheds snow pretty effectively too. Even 8/12 on a metal roof with no dormers or valleys will slide before 8' accumulates on it.
I think I know at least a little about snow on cabins in Alaska.
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snobdds
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# Posted: 29 Mar 2022 19:05 - Edited by: snobdds
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I needed 24 foot rafters for my build, so I used two 16 footers and sistered them together. It sounds simple in theory, but it threw my layout off 1.5 inches on the bottom section. I had to really keep track of the layout and fasten accordingly. It also makes the screws on the roof a bit off, top to bottom. You only notice it if I saw something or you look really close.
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Brettny
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# Posted: 30 Mar 2022 05:04
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Quoting: NorthRick Make your cabin narrower and longer. 10/12 pitch sheds snow pretty effectively too. Even 8/12 on a metal roof with no dormers or valleys will slide before 8' accumulates on it
You would need to go prety darn skinny to use a 16' rafter and even get a 8/12 pitch. On the other hand you would need to go way to flat pitch to get a 16' rafter to fit
Have you considered getting a extension for your mill? In the end it may be far easier.
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jsahara24
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# Posted: 30 Mar 2022 09:51
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What's the interior layout going to be?
Wondering if you can't run beams parallel to the ridge in the midspan and support the rafters on a bedroom wall or similar? Then your splice will be supported?
We are in snow country in NY, and have a 9 pitch. If it doesn't go above freezing the snow doesn't slide, as soon as it goes above freezing on a sunny day it slides promptly...I'm not sure of the typical weather in AK, but I'm assuming you are staying below freezing most of the time...
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gcrank1
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# Posted: 30 Mar 2022 09:54
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I've seen mid-span beam rafter supports on traditional Nordic construction to do that, a variation on post & beam?
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Aklogcabin
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# Posted: 30 Mar 2022 10:27
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Hey welcome from another Alaskan. You could TJIs or manufactured I beams. I spanned 20' with 16" ones on a 3/12 pitch for our garage. They are rated for 60# dead load. With a 12/12 pitch 12" would work but you would probably need a ridge beam.
On our 16' wide cabin with a 12/12 roof I used 2×12s, 16' long so I have extra overhang. Putting the drip line near 4' out. I felt that having the drip line further from the foundation and having the cabin floor 4' high off the ground keeps the snow off the sides of the building. And making it so bears, well most bears hopefully can not reach the windows. Haven't had any trouble yet.
Any general location you building? Good luck building your cabin.
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snobdds
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# Posted: 30 Mar 2022 11:02
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Quoting: Aklogcabin Hey welcome from another Alaskan. You could TJIs or manufactured I beams. I spanned 20' with 16" ones on a 3/12 pitch for our garage. They are rated for 60# dead load. With a 12/12 pitch 12" would work but you would probably need a ridge beam. On our 16' wide cabin with a 12/12 roof I used 2×12s, 16' long so I have extra overhang. Putting the drip line near 4' out. I felt that having the drip line further from the foundation and having the cabin floor 4' high off the ground keeps the snow off the sides of the building. And making it so bears, well most bears hopefully can not reach the windows. Haven't had any trouble yet. Any general location you building? Good luck building your cabin.
Hauling TJI's in 60 miles would not be fun. He has to be able to cut it on his mill. He could do two 12 footers sistered together and he would be fine.
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madmilitia
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# Posted: 30 Mar 2022 12:29
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Quoting: Aklogcabin Any general location you building?
yeah up near skwentna.
the plan is 4 feet off the ground we got helical foundation screws. taht go down 50 inchs. prolly getting extentions to go down 72 inch. i'm fortunate that after 2 feet of dirt is sand.
i like the idea of building beams 16+4 foot on one side, then reverse 4+16 on the other and sister those togther. wonder if 2x6's would work or if i need to do 2x8's since essentally they will be a 4x6 or a 4x8.
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Grizzlyman
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# Posted: 30 Mar 2022 12:46 - Edited by: Grizzlyman
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My .02.
Id stuck with 2x8. After all you won’t be using southern yellow pine or maybe even Doug fir? likely what: spruce or white pine? And then probably not the best grade either
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NorthRick
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# Posted: 30 Mar 2022 13:05
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Quoting: Brettny You would need to go prety darn skinny to use a 16' rafter and even get a 8/12 pitch. On the other hand you would need to go way to flat pitch to get a 16' rafter to fit
No. I think you might be forgetting that there are two that meet in the middle. The cabin I am building is 20' wide, just 4' less than his plan. 8/12 pitch. From the ridge of the roof to the eave, with a 3' overhang, is 15' 10".
I would not be sistering rafters with a potential 8' snow load.
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madmilitia
Member
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# Posted: 30 Mar 2022 13:43
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yeah it's spruce, mostly beetle kill, so 2x8's it is
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ICC
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# Posted: 30 Mar 2022 14:03
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Quoting: NorthRick I would not be sistering rafters with a potential 8' snow load.
Right
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madmilitia
Member
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# Posted: 30 Mar 2022 14:13
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ok so what's the option with my restrictions.
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ICC
Member
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# Posted: 30 Mar 2022 18:21 - Edited by: ICC
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Let's start with the species of tree you would be using so I can enter data in the calculator?
And do you have a preferred OC spacing for the rafters?
8 feet of snow means nothing. The weight of snow is what is important. Snow is commonly from 20lbs per 12 inches of depth (per sq ft) on up to insanely heavy if wet/moist snow. Are your snowfalls generally powder or very wet? There is probably no data for PSF where you are, but if there was that info would be invaluable.
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NorthRick
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# Posted: 30 Mar 2022 18:46
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Quoting: ICC Let's start with the species of tree you would be using so I can enter data in the calculator? And do you have a preferred OC spacing for the rafters? 8 feet of snow means nothing. The weight of snow is what is important. Snow is commonly from 20lbs per 12 inches of depth (per sq ft) on up to insanely heavy if wet/moist snow. Are your snowfalls generally powder or very wet? There is probably no data for PSF where you are, but if there was that info would be invaluable.
I'm up valley from madmilitia and familiar with his area.
White Spruce. His comment about using beetle kill is concerning. On my property, the quality of the wood starts to significantly degrade after about a year after the tree dies (all the needles fall off).
I designed around 70 psf snow load and our "little" cabin has been through over 15 winters without a problem. That's the one shown in my photo in my post above. This year was one of the more heavier snows we've had in a while, including some mid-winter rain.
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ICC
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# Posted: 30 Mar 2022 20:49
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Quoting: madmilitia ok so what's the option with my restrictions.
My initial thought was to simply change the plan; build narrower and longer. After all it was mentioned this will not be the primary structure.
The AWC span calculator does not list white spruce in all their choices. So, I used the modulus of elasticity tables I have and it would seem engleman spruce is similar. I used that with 2x12, grade 2, L/180 deflection. 15 PSF DL for the longer rafter lengths that 12/12 pitch brings, LL of 160 PSF (I have seen that high a snow load in some areas of the rockies in CO and the PNW), with rafter OC spacing of 12". The AWC indicates a max horizontal span of 12 ft (1/2 of the planned 24 ft total width. That would be a "by the skin of your teeth" pass.
That would be a rafter length from ridge board to supporting wall of 16 ft and 10 inches according to my other calculator. So again, I think it would be best to rethink the plan and build a narrower and longer cabin.
Of course, if the actual potential snow load is much less or greater the span will vary.
Another thought I had and I am uncertain how practical, would be to build the roof structure with the 16 ft rafters as above, and sister up the rafters only at the tails. Not sure how long a piece to sister up for a 2 foot overhang, but probably at least 4 feet overlap. I dunno; I am not an engineer. Past experience tells me the engineers I know would do their damndest to talk me out of it.
I don't have much of an opinion on how structurally sound beetle-killed standing dead white spruce would be. But the beetle-killed pines around where I am don't stay upright for more than a year or two. I think a lot depends on whether or not other insects invade the wood and bore tunnels deeper into the wood after the bark beetles are done. The bark beetles I am familiar with stay just under the bark. I have seen deeper boreholes on some of the dead-standing trees we have cut here.
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spencerin
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# Posted: 30 Mar 2022 21:24 - Edited by: spencerin
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This is a question and not advice, but what about a Queen Post or Fink roof truss design? Put 2 10' boards end-to-end to obtain your 20' length, and the brace running perpendicular to the rafter would attach right at that joint/seam, thus supporting it. They make metal splice plates to fasten all that together.
Have no idea if it's code or not (doubt it), but it might still be structurally sound and get you what you need.
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NorthRick
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# Posted: 30 Mar 2022 21:47 - Edited by: NorthRick
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Quoting: ICC The AWC span calculator does not list white spruce in all their choices. So, I used the modulus of elasticity tables I have and it would seem engleman spruce is similar. I used that with 2x12, grade 2, L/180 deflection. 15 PSF DL for the longer rafter lengths that 12/12 pitch brings, LL of 160 PSF (I have seen that high a snow load in some areas of the rockies in CO and the PNW), with rafter OC spacing of 12". The AWC indicates a max horizontal span of 12 ft (1/2 of the planned 24 ft total width. That would be a "by the skin of your teeth" pass.
That's way too high for our area for a metal roof with 8/12 pitch or greater. If you want to be conservative go with 100 psf.
The quality of the wood, as I said, would be my biggest concern. For the most part, the spruce bark beetles burned them selves out of mature spruce trees 3 years ago in the Susitna Valley. My 5 acres doesn't have a single spruce tree still standing that is more than 10-15 years old.
I don't know what degrades the wood, but after about a year or so of being dead, a 30 mph wind will knock them over.
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ICC
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# Posted: 30 Mar 2022 21:57
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Changing to 90 PSF snow, 16" OC and 2x10 the AWC comes up with an 11 ft 7 in span max. Same issue on overall length and the tails tho.
Yeah, after a year or so the winds blow most over. It may be the shallow root system rotting away more than the trunk, but I don't know and hate taking chances on unknowns
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