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Kenry
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# Posted: 16 Nov 2020 01:22pm
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Hello.
I have subscribed a little while ago and have been reading all the great information here!
I do happen to have a question concerning construction of a 12x16 shed.
I would like to do a 7/12 pitch roof, and I am thinking of doing a cathedral style so that inside it does not feel so claustrophobic and will give it a larger feeling inside.
I am wondering on what size joists will I need to be able to add insolation before covering it up with tongue and groove or plywood.
The plan i found online indicates 2x4, but i realize that is for a cabin that does not need insolation.
So i am hesitating between 2x6 or 2x8?
This will be a 4 season cabin but only used on the weekends, heated with a wood burning stove. Winter here in Québec, Canada can dip down to -40°C (-40°F).
2x6 or 2x8 joists? And what kind of isolation do you suggest Batted or Foam?
Cheers.
Kenry.
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snobdds
Member
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# Posted: 16 Nov 2020 04:51pm
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2x6 for 16 inch centers
2x8 for 24 inch centers
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gcrank1
Member
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# Posted: 16 Nov 2020 06:30pm
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My old rule of thumb to reach across has been the width size times 1.5
So a 2x6 can span 9' and a 2x8 span 12'
If you use a runner or piers beneath you can cantilever one or both ends a bit.
By doing that for your 12' you can still have the 9' span for a 2x6 and split the difference of the other 3' at a 1 1/2' cantilever for your ends. Doable for a small one story build imo, others may differ.
I agree on the cath ceiling a lot, and dont use truss rafters, they dont look good up there.
Make the pitch the steepest you can comfortably walk about on, it will shed water and especially snow so much better, but if you make it too steep you will hate it somewhere about half way through the build and everafter.
Im loving our new cabin's metal roof.
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Nobadays
Member
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# Posted: 16 Nov 2020 08:57pm
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Reading the OP... are you talking about the floor joist - first floor and or loft - OR, it reads like you are talking about the roofing system. Are you calling the RAFTERS joist?
Snow load as well as roof pitch (wich you provided) go into sizing rafters.
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Brettny
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# Posted: 17 Nov 2020 06:13am
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For a cathedral style ceiling you need to know how much insulation you want to fit in there. With as cold as your area gets I would tend to go bigger than smaller.
What R value do you want? What type of insulation are you useing?
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Kenry
Member
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# Posted: 17 Nov 2020 09:29am
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Nobadays: excuse me if things are not clear... Im translating words from French to English, as well... Im new to "Construction".
I did Google search prior to posting to make sure I had the lingo down and I still seemed to screw things up!
You are correct... Im talking about roof RAFTERS (not joists)
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Kenry
Member
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# Posted: 17 Nov 2020 09:42am
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Also, thank you to everyone who has replied. IM SO SORRY for mixing things up!
Lets start over, I will try to be more clear.
I want to build a 7/12 pitch cathedral roof.
Im not sure if I should use 2x6 for Rafters or 2x8 for Rafters.
Québec code is R41 for roofs with out Attics.
Source: URL
Will a R41 fit in 2x6 rafters? Including rafter vents for air flow?
Im starting to wonder... if this is just a weekend getaway, do I really need R40? Or if I can get away with less.........
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Atlincabin
Member
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# Posted: 17 Nov 2020 09:56am
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7/12 is pretty steep to walk on if you are using metal for the roof. Ours is 5/12 and it's about as steep as I would want to walk on safely.
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Kenry
Member
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# Posted: 17 Nov 2020 10:17am
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Roof will be asphalt shingles and not metal.
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gcrank1
Member
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# Posted: 17 Nov 2020 10:46am - Edited by: gcrank1
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In my 1983-84 cabin build (we just left it this past Sept 1, 2020) I did a cath ceiling at about 7:12 pitch, it was a bear to sheath, tarpaper and shingle (both times!). And was a bit of a trick to walk up there doing anything, like chimney work, etc.
We are in south central Wisconsin, USA with some long cold winters and some years pretty good snowfalls. My pitch was never a problem with snow load, even though I had one stretch of span in the rafters (the open cath ceiling side) that was longer than the actual measured 2x5 rafters were supposed to be good for. I did have a retrofit plan for add-in support if needed. I had also planned to use 1" rigid styrofoam insulation between the rafters, a 1" or so 'breathing' gap between that and the plywood roof sheathing, then t&g 'boxcar siding' but never got around to it in all those years.
Heat was only an old, but refurbed, fairly leaky upright 'barrel' wood stove with a straight up stovepipe (way preferable for good draft over 'out the wall').
So, about 400sq.ft., high cath ceiling, NO insulation, No problem heating, though I did regret having to look at all those long roofing nails penetrating the sheathing.....
In reflection I think after putting down the sheathing I could have put on stringers topside, the styro in-between, then roofing. And if I had used the boxcar siding as the the initial sheathing it would have been a lovely effect inside. It is faarrr easier to work from above than overhead!
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Alpine_Junky
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# Posted: 17 Nov 2020 05:01pm
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You can't get R41 in a 2x6 or a 2x8. You can extend the 2x4s with nailers provided they are rated for such. You may desire to blow in insulation vs using foam due to cost. If you are building scissor trusses, you might be well served to see what pre-built and delivered ones will run you; super easy to set too. Most of the time the quote is free and the folks using the CAD program will have some very good ideas to offer you.
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AKfisher
Member
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# Posted: 17 Nov 2020 06:00pm - Edited by: AKfisher
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They do make a batt insulation specifically for cathedral ceilings. Rockwool R30, fits in 2x8. I used this insulation in my 2x10 rafters as i needed an airspace to vent. Works well.
https://www.lowes.com/pd/ROCKWOOL-COMFORTBATT-R-30-Stone-Wool-Batt-Insulation-with-So und-Barrier-23-in-W-x-47-in-L/3693532?cm_mmc=shp-_-c-_-prd-_-bdm-_-google-_-lia-_-102 -_-insulation-_-3693532-_-0&placeholder=null&gclid=Cj0KCQiAhs79BRD0ARIsAC6XpaWu69NBIf _oUVODSdqVJ3mR60VzHdMzkC2lOTpde2Kx8nnX44CiwLMaAssKEALw_wcB&gclsrc=aw.ds
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