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jsrore
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# Posted: 18 Sep 2023 11:42
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Hi, we just started building our off grid cabin in Idaho, foundation done and outer walls are up.
here is my issue/plan... going to do a ridge board for roof 2x8... and 2x6 for rafters, ceiling joist to tie the walls together my plan to span the 30' was (2) two 20' 2x8 nailed and glued put on 24" center.
will this be too much and get a sag in the middle? it will be supported by the interior walls, the longest span for a few joist would be 23'
the other kicker - my wife wants a loft i dont think its doable with out a beam/support down the middle of the cabin, then i assume i would need to support the floor were the beams would rest.
any advice is appreciated
jeff
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Grizzlyman
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# Posted: 18 Sep 2023 13:57
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Not sure about a 23’ span but I can tell you that my22’ rafter ties are 2x8 and haven’t sagged at all in 2+ years. Could be the tension of the triangle that holds it taught- not sure. The joint might be troublesome on a longer spans with two nailed together??
Also- just a question but why 23’? I was able to find 22’ 2x8s when I did mine. I’m Just thinking that if you moved the wall for the 23’ span and made it a 22’ span (or even 21’) then you could join that rafter tie and a shorter one on top of an interior wall and the joint wouldn’t matter then.
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Grizzlyman
Member
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# Posted: 18 Sep 2023 13:59 - Edited by: Grizzlyman
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Also, I think if you’re spanning 15’ horizontal a 2x6 may not be enough for a rafter. I don’t have the charts in front of me but you may want to consider beefing that up to a 2x8- especially in Idaho with the snowload
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ICC
Member
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# Posted: 18 Sep 2023 14:29
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Quoting: Grizzlyman spanning 15’ horizontal a 2x6 may not be enough for a rafter.
It is very important to know the average snow load. The minimum figure used by anyone anywhere is 20 PSF. There are areas in ID where the snow load is much greater. Who knows when a big storm may dump a truly big load?
I would check that with the state of a reputable web source. Then you can either use tables or try the AWC span calculator.
https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/
like the calculator a lot. There is the webpage version and there are also Android and iOS versions. Some people have had issues with the app but it usually works fine for me and the webpage link always works as far as I know.
You also need to know the lumber species and grades that are available to you, the pacing and so on.
For rafters with a pitch greater than 3:12 use the deflection limit of L/240 unless there is no finished ceiling attached to the rafters. Then L/180 can be used.
The spans given are the maximums and the lumber sizes the minimums. If close to a max or min it never hurts to be more cautious and round upwards for more strength.
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ICC
Member
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# Posted: 18 Sep 2023 14:44
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Ceiling joists can be spliced but the splices should only be above a wall. The number of nails or structural screws must be the same as whatever number and size are recommended for the rafter heel to ceiling joist joint. That may be found in the IRC. Table R802.5.2(1) in the IRC 2021
https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction
Quoting: jsrore my wife wants a loft
A 30 ft wide loft is going to need much more support than just laying a floor across ceiling joists. A load-bearing wall somewhere close to center plus proper supports from the floor down to foundation.
You could do 30 feet with floor trusses as long as the side walls and floor can support and transfer the load to the foundation.
My opinion on lofts is that they are almost always too hot if the main floor is comfortable.
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ICC
Member
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# Posted: 18 Sep 2023 14:51
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Quoting: jsrore ceiling joist to tie the walls together my plan to span the 30' was (2) two 20' 2x8 nailed and glued put on 24" center.
A 2x8, #2 Doug-Fir ceiling joist, 24" OC, has a maximum span of 14'5" with a 20 PSF snow load. That would need a midpoint support on each rafter. That is solely when the ceiling joist is only supporting a ceiling. For a loft floor, the joists would need to be bigger, closer together or span a shorter distance.
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gcrank1
Member
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# Posted: 18 Sep 2023 23:01
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Ime with a prev 12x24 build c/w a 12x12 stand up in the center peak loft was that in the summer it was often too hot for comfy sleeping and in the winter too hot because the stove heat rises. We also had to kick that risen warm air back down where we were of an evening.
It also became difficult to get up into and out of as we became injured and aged.
It finally became pretty much storage. Id not build one again except for just storage so it doesnt need to be as big as a 'room' or high enough to stand in.
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Aklogcabin
Member
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# Posted: 19 Sep 2023 09:19
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We really like to have the extra room that a second floor comes with. And with a small deck and a door. Window on the other end it stays very comfortable. And in cabins there is always a lack of space.
To me the additional cost to go with 10' tall side walls and have an extra floor is minimal compared to the gains of doubling the amount of usable floor space. And I like my space. With a bunch of folks in a cabin it can get small fast. Even 4 adults can get cramped.
As suggested I would use larger roof joists. I used 2x12 so we could get better insulation along with structural design. Bigger is better.
Sounds like you have a plan. So good start. Hopefully you can start building your dreams n cabin soon
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jsrore
Member
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# Posted: 22 Sep 2023 11:12
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thanks for all of the great advice, after ALOT more research and using the span calculator i think i have come up with a plan..
12/12 pitch
snow load in our area 30-40psf
going to use 2x12 for ceiling joist
and i think 2x8 for rafters on 16" center
also going to run a 6x6 beam down the center cut them at 10' then brace i will also support the floor were i brace..i have a saw mill so i will make my own beams
we also are going to shrink the loft 1/4 of the cabin, so a small loft.
again thanks for the advice and if you think i missed something please let me know
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Brettny
Member
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# Posted: 22 Sep 2023 12:11
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Dont use a 6x6 horizontally. They look big but just cant span that much space.
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jsrore
Member
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# Posted: 22 Sep 2023 14:12
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so you dont think a 6x6 on 10' span will work?
what about 8x8 on 10 span?
i plan on bracing to the floor every 10' with another 6x6 beam and then reinforce the floor.
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Brettny
Member
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# Posted: 23 Sep 2023 07:38
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Doubled up 2x10 should work for that span but use the span calculator and theres nothing wrong with making them the next size bigger. Since you have a sawmill just mill out a 4x10 to the length you need.
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jsrore
Member
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# Posted: 23 Sep 2023 10:25
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yes great point, we are also looking at redesign of a few walls to get the ceiling joist span with in the span limits of the 2x12
then no beam would be needed.
lots of work to build your own cabin
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Brettny
Member
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# Posted: 23 Sep 2023 14:43
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You have your own mill. Use 14 or 16in tall lumber if needed. I cut all my wall studs for a 20x32' cabin I'm currently building on a mill I built. I would have cut all the floor joists but found a great deal on new 2x10x20' lumber. I have been cutting and squirling away nice hardwood like oak, ash and cherry for things like stairs and other interior more finish grade aspects of the cabin.
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