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Author Topic: Cathedral ceiling design  (Read 565 times)

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Offline Coastallogger

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Cathedral ceiling design
« on: December 29, 2022, 08:09:07 AM »
I am building a log home with this design rafters.


Half lapped, 12:12 pitch with birdís mouths.

As per traditional design in these parts at least, they go 4í OC with 2x sheathing running horizontally forming the inside ceiling.

On top of that I need 11 inches of insulation plus air gap, so I need dimensional lumber to create that void.

The problem is, they donít make 4 foot insulation batts, and the pitch is too steep to use blown in cellulose.

I could put the insulation rafters 24Ē OC, but then the problem would be the roof would be supported by the mid point between the rafters, which may create sagging on the ceiling sheathing. Although with a 12:12 pitch and metal roof, there is t much weight pushing directly down, it is hard to say.

The other thought was, I need to make overhang ladders anyways, so my thought was, why not carry those all the way gable to gable if they have to go in one rafter anyways for the cantilever effect? 

The only issue with that I can see is then I will have to fasten my metal roof directly to those insulation rafters. It saves on strapping, but then my question is why donít they do it that way all the time? All the build books I have show the insulation gap rafters going vertically, then strapping horizontally on top of that. What benefit do you get from that? Especially since some of them need to go horizontally anyways to form the overhang ladders.

Thanks for your thoughts!



Building 20X20 dovetail log cabin off grid.

Offline Don P

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Re: Cathedral ceiling design
« Reply #1 on: December 29, 2022, 08:56:43 AM »
I've done sort of similar. We made notches to create vent channels up the roof, ply deck, tarpaper, then sealed the metal to the deck.

I would have concerns with vented metal directly above fiberglass, condensation drips... but then again, I frequently drive by one I did 20 years ago that way, that is fine.

I would put 6" of foam on that, strapping over, sheath and metal



 

The timber rafters under there are on 4' the strapping above is on 2'. Long screws into the rafters and precisely measured screws through strapping, foam and into but not thru the 2x6 decking on the others.

I know 2x decking on 4' centers is good for 30 psf snow load on a roof. If you are higher than that it's probably worth looking closer.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Coastallogger

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Re: Cathedral ceiling design
« Reply #2 on: December 29, 2022, 09:22:20 PM »
Thanks Don,

When you say

ďI would put 6" of foam on that, strapping over, sheath and metalĒ

do you mean put the foam on the 2x that goes directly onto the structural rafters? Why then sheath it? You can strap and roof right over the foam I would imagine if you taped the seams.

Just another layer of protection if the tape fails?

The foam option is about the same cost, because you save on lumber, but it is simpler and faster. I like that.

Also, unvented roof assemblies I am told are always risky and should be avoided if possible. Not sure if there is truth to that.

For the overhang ladders, would you just stuff them with the foam?

When it comes to snow load, I find tables for my area, and it is close, but it mentions nothing about pitch and roofing material. A 12:12 pitch of metal is going to have a totally different snow load than a less steel asphalt roof. Does code specs not consider that? My code specs are about 40 psf where I live. But I have a 12:12 pitch metal roof so I feel I might be fine.

My building is code exempt so I can build what I want but I want to make sure it is safe and sound for my own sake, not the inspectors.

Brgds















Building 20X20 dovetail log cabin off grid.

Offline Don P

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Re: Cathedral ceiling design
« Reply #3 on: December 29, 2022, 10:22:53 PM »
Yup you can go through all the gyrations to extrapolate roof snow load from ground snow load. Just remember all it takes is one time in the life of that building to eff it up. Freezing rain, snow, snow, snow, rain, freeze, etc. I framed that roof with 2x12's on 12" centers and when they called about the ice dam leak, that metal clad 12/12 pitch had 3' of consolidated, heavy stuff up top. I was impressed, I sure couldn't stick to that roof  :D.


Here's the code roof sheathing table


 

The strapping and plywood sheathing above created a vented roof, those are vent channels between the 2x4 strapping. It did it without having a cold metal condensing surface exposed to the venting channel,

Are these intended to be vent channels carrying house moisture away or are we inviting a rush of air in order to wring out atmospheric moisture on cold steel?

Put the (5/8" minimum) ply over the strapping and you are set up for standing seam, which specifies a solid deck but it will also create a better vent channel under the ag rib style metal as well. Use the end caps top and bottom on the metal, seal it to the deck and vent under the ply. If the night air cannot touch the underside of the metal, it will not sweat on the underside. Now we've removed that moisture from the daylight venting load and its properly vented.

Around the perimeter of the wall line and then the overhang edge stand up a 2x the thickness of the foam + strapping. In other words make a foam stop around the perimeter of the building line and a ply support around the perimeter of the roof.... hmm if this is of interest I'll try to find more pics, that was a word salad  :D. OSB is flatter and easier to handle, ply has better screw withdrawal strength. Metal strapping over the ridge from sleeper to sleeper, in the end it is draped over the ridge.

If the foam option is twice the price it is cheaper than fiberglass in a roof. That isn't insulation it is simply a filter cleaning the sawdust out of all your warm air as it heads outside. Somewhere in the background I hear "We hold these truths to be self evident"
The future is a foreign country, they will do things differently there - Simon Winchester

Offline Coastallogger

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Re: Cathedral ceiling design
« Reply #4 on: December 30, 2022, 07:01:08 AM »
Yes I see your vent channels. I was just kind of musing about putting my corrugated metal roof directly on strapping which would go on top of the foam, but that would have to go horizontally, and I donít think the corrugations in the metal would be adequate venting perhaps. But it would save me a step and a lot of money. I am not doing standing seam so I donít need a solid deck. What would you think of the venting situation there?

I hear you that more ventilation could simply mean more condensation on the steel to drip onto the strapping and foam.


I would be interested in seeing the pics of you wouldnít mind. I am having trouble visualizing that!

I hear what you say about ďself evidentĒ I have had that feeling myself. My local pro desk guy who is a huge efficiency nerd and has been following my build closely has said it isnít all about R value.

And no the foam is only slightly more expensive because I save on all the lumber to create the space to put the fiberglass in. So it seems like a no brainer to do it your way now.

Brgds

Building 20X20 dovetail log cabin off grid.

Offline ljohnsaw

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Re: Cathedral ceiling design
« Reply #5 on: December 30, 2022, 12:50:43 PM »
Put the (5/8" minimum) ply over the strapping and you are set up for standing seam, which specifies a solid deck but it will also create a better vent channel under the ag rib style metal as well.
Well, this is a new wrinkle in my plans.  I like the look of standing seam but I don't want to spend that kind of money to sheath the entire roof in plywood.  I was going to do 2x T&G roof deck, 6" rigid foam, 1x4 vertical strapping and 1x4 horizontal over that with the metal attached.  So, no go on standing seam?  But regular corrugated is good?  If I were to shim the vertical strapping to flush it with the horizontal, spaced for the joints, would that work?  Or does the entire sheet of roofing need support from a solid surface?
John Sawicky

Just North-East of Sacramento...

SkyTrak 9038, Ford 545D FEL, Davis Little Monster backhoe, Case 16+4 Trencher, Home Built 42" capacity/32" cut Bandmill up to 54' long - using it all to build a timber frame cabin.

Offline Jim_Rogers

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Re: Cathedral ceiling design
« Reply #6 on: December 30, 2022, 02:17:59 PM »
  Or does the entire sheet of roofing need support from a solid surface?
That question may need to be asked to your roofing material supplier. They will have recommended installation requirements.
Jim Rogers
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Offline Don P

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Re: Cathedral ceiling design
« Reply #7 on: December 30, 2022, 05:11:24 PM »
It was tongue in cheek but if you see dirt in and through the fiberglass you've located where the warm air is leaving. Continuous foam eliminates so many small leaks.

I was in a short course at the nearby U. The professor was talking about how the wind usually gets under the lower corners and edges of the roof and begins working it up from there. In the forensics after the storms it is often poorly nailed there. It was a quiet classroom full of theory, engineers, architects, inspectors, and one carpenter. What the heck, "I've got one hand and one foot hooked on the top edge of a sheet of ply, the other foot is out on the slope under me, the gun is in my other hand. I'm leaning out, looking down and nailing that corner. We're trying to work with you all but the main thing I want right then is to get down! :D". They laughed and the porch lights came on. I guess "self evident" often depends on the view from where one is perched.

This was in my gallery, omit the purlins, turn the sheathing 90 and the same idea. The dams are over the wall lines.


 

 

I've solid decked the top with planed 4/4 boards if that is cheaper than ply in the math. We've also run tarpaper up and down the roof, over skip sheathing and then plopped the metal on it, the tarpaper was our "hygric buffer" (sponge  :D) It grabs or deflects the liquid water till it warms up and can vent out without dropping onto the insulation. There is also dewstop metal.

John, you are off the charts I'm sure, call their tech line but I'm certain you will have to have a solid deck under anything at 300psf snow. Not the 3/8 ply  :D
The future is a foreign country, they will do things differently there - Simon Winchester


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