Cathedral Ceiling.

surfsupinhawaii

HI, I am building a log home and the roof system is a Cathedral ceiling with a 12/12 . I have a 3 1/2 x 16'' psl ridge with 2x12x24' long rafters. My question is what type of rafter grade you would use for a cathedral ceiling. I am worried about twist and cupping since I will be applying a wood ceiling to the bottom of the rafter. I priced out L.V.L lumber but it runs around $4.35 a foot. I think If I ordered #1 grade for the rafters I would get the same amount of cupping and twist out of my boards that I would get out of #2. Anyone have any comments or suggestions. Thanks

EasTnBldr

Given a 24' long rafter and a 12/12pitch ... your true "span" which is the distance on the flat must be around 15' to 16' depending on your overhang. Yellow Pine would be the best specie of lumber to use given the span ... it's strong but seemingly brittle if that makes any sense. As far as grades go ... guess what ... you are pretty much in the same boat with regards to cupping & twisting regardless of the grade. Yes, the smaller and tighter the knots ... the less crown and bow you'll likely have.

Cupping and twisting are driven more by the moisture content of the wood when it arrives on the job vs. the moisture content of the structure as you build it. Rain (or exposure to rain) and sun (or exposure to sun) play the biggest role what you are trying to avoid. I would worry more about the cupping and twisting if you were leaving the rafters exposed and putting a T&G roof deck down but if you are planning on covering the bottom or ceiling surface of the 2x12s ... the amount of cup and twist doesn't make much difference.

The only difficulty comes when you have to change measurements of your 1x6, 1x8, 1x12 or whatever size ceiling board you are using. I try to standardize two runs of my ceiling to maximize distance between joints and make for quick cut and application ... if you start to lose margin of your rafter on butt joints ... then you'll simply have to adjust your lengths.

supercrew

surfsup....

i've built a few luxury "cabins" with the 12/12 roof and cathederal ceiling. after pushing/pulling up 25' 2x12's, we decided to try something new on the next one. our solution, with the help of an engineer friend, we decided to use TGI's for our rafters. (the same engineered I-beam used in subfloors).

pros: lighter making install easier.
easier to layout for the roof decking, making the rafters straighter.
no bow or twist

cons: you CANNOT cut a birdsmouth, instead you need to rip a 45 degree
2x to place on the top plate for a solid set to nail to.

you have to cut the TGI square past the outside of the top plate
and add filler and 2x's to create the overhang

we had to block between rafters

now, i dont know if this will meet code where your at, nor do i know if it still meets code where we did it, but it did at the time and it makes for a very straight roofline AND a VERY smooth ceiling, probably cause each TGI is exactly the same height unlike dimensional lumber.

just a thought.

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i only work off drawings

Joe Carola

That's not true. You can cut a birdsmouth in I-joists. Alot of work involved but you can. Follow whatever brand joist your using installation guide. Scroll down to page 19 where it says birdsmouth cut.

http://www.ilevel.com/literature/TJ-4001.pdf

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Joe Carola

supercrew

thanks Joe on your diligence. i never seen any design plans for this application, we just sorta winged it and the enginner buddy just told us that cutting the birdmouth would put the tji integrity in jeopardy, so we took the easiest method without questions. and im glad, looks like that method would add many hours to the process before you could even think about sheeting.

thanks again Joe for the link, im gonna print that out to show some people. since i moved back to the city, everyone i have ever told this to looks at me like im behind the glass at a carnival freak show. ****, most of the framers around here, if you mention wall, floor or roof load, they think the lumber trucks on the way.

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i only work off drawings

surfsupinhawaii

I believe my rafter lengths are around 21' plate to ridge. I Have a lumbermill and have cut and dryed my own wood, understanding the causes and effects of cupping and twisting. I know how to fix this, but just wondered about what everyone else thought. It is too late for me to re-engineer my rafters to T.J.I. since I have a snowload of 78 and it is going to snow soon. I was also going to use LVL but it worked out to be too expensive. I think next time I will go the route of TJI. I used them in the floor and second floor, wanting to use them on the roof, but I have just run out of too much time. Thanks for all of the responses

supercrew

surfs up

i didnt have a clue that its ready to snow where your at. and its never to late to re-engineer. you would just be stuck with the lumber. or use for ceiling joist and make use of the loft/attic... its cool you have your own mill, cause i have a couple of cedar 8x8's and i would like to make into 1x's for some closets and trim work. this is in my own home so theres no deadline on the finishing.

so my question to you is...what is the most economical way for me to turn these timbers into usable 1x8 planks?

is that "chainsaw attatchment" any good?
am i better off finding a mill and paying?
your suggestion

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i only work off drawings