Author Topic: Yet another roof joist question...  (Read 6027 times)

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Offline NM_Shooter

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Yet another roof joist question...
« on: March 10, 2008, 01:54:42 PM »
Okay... sorry, but here comes another joist question.

Width of cabin is 20'.  i'm planning on a 12:12 pitch, and want to use 2X8 as the roof joists, on 24" centers, under a pro-panel roof. 

Is this adequate 2x material?

I've read that the maximum depth for the seating cut of the joist (onto the top plate) is 1/4 of the thickness.  Correct?

Thanks,

-f-
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Offline ScottA

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Re: Yet another roof joist question...
« Reply #1 on: March 10, 2008, 02:03:15 PM »
snow load?

Offline NM_Shooter

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Re: Yet another roof joist question...
« Reply #2 on: March 10, 2008, 02:31:31 PM »
Hmmm.. does "lots" count as a valid answer?  My land is 8 miles East of Chama, but 3000 feet higher, at 10,000.

http://www.wildlifephotogallery.net/discussionboards/02-04-07snow/chama.htm

This is in an area that gets as much snow as anywhere in the US right now, maybe with the exception of the Pacific NW or some parts of the Sierras.  I'm not sure how to find out what the snow load is... I'll call the county that this is in and ask to see if they have a number.

My hope is that with this pitch, it will not be an issue.

Don.. what's your snow load?

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Offline Willy

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Re: Yet another roof joist question...
« Reply #3 on: March 10, 2008, 02:53:23 PM »
Hmmm.. does "lots" count as a valid answer?  My land is 8 miles East of Chama, but 3000 feet higher, at 10,000.

http://www.wildlifephotogallery.net/discussionboards/02-04-07snow/chama.htm

This is in an area that gets as much snow as anywhere in the US right now, maybe with the exception of the Pacific NW or some parts of the Sierras.  I'm not sure how to find out what the snow load is... I'll call the county that this is in and ask to see if they have a number.

My hope is that with this pitch, it will not be an issue.

Don.. what's your snow load?


-f-
I sure don't see much snow load on a 12/12 pitch roof? I have 12/6 pitches and never had a foot on the roof at one time in 12 years. Normaly it slips off around 6 inches and I realy don't think any load of snow to speak of would stick to 12/12? I have metal all roofs also. Mark

Offline NM_Shooter

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Re: Yet another roof joist question...
« Reply #4 on: March 10, 2008, 03:08:54 PM »
I found a good whitepaper on this... I'm guessing a ground load of 150lbs.  My slope is 12:12, and I would classify it as cold metal, unobstructed.  I'm guessing the actual load to then be ~75lbs.

http://www.nysboc.com/forms/Snow%20Loads.pdf

I suppose the good news is that the ground snow might assist in keeping the walls together  ;D

I'm planning on a 2X12 ridge beam, and need to know what roof joists to use so i can start to draw out my gable end framing... sigh....

-f-
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Offline rakuz66

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Re: Yet another roof joist question...
« Reply #5 on: March 10, 2008, 03:18:59 PM »
I'm certainly no expert, but I am guessing a cathedral ceiling??  If so, one 2x12 seems a bit small to me, unless it's a regular ceiling with ceiling joists.  Why 24in ctr?  I would still go 16 in ctr even with a 12/12 pitch.  But, that's just me.

Offline MountainDon

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Re: Yet another roof joist question...
« Reply #6 on: March 10, 2008, 03:39:46 PM »
Re SNOW

I've heard that the Chama area has had 144" this winter. So far.  :)

As for my snow... I'm not sure what the area is rated at.  ??? We were just up there with the snowmobile.
http://countryplans.com/smf/index.php?topic=2335.msg50032#msg50032
The RV had about a foot, maybe a few inches more, on its flat roof. It looked like heavy snow, but the braced roof was fine. The Gazebo roof didn't have as much snow on it. The ground cover was 2 to 3 feet, more in places it had drifted. Pretty much typical.

My neighbor's cabin 12:12 roof had no snow on it at all. His addition with a 4:12 shed roof had less than the RV, with at least half of it bare metal.



« Last Edit: March 10, 2008, 04:55:46 PM by MountainDon »
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline ScottA

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Re: Yet another roof joist question...
« Reply #7 on: March 10, 2008, 05:51:13 PM »
Well a Doug-fir 2x8 is going to max out at about 45 pounds of snow load. Regardless of pitch I think you are suposed to design for the local snow load. This may fall into that overkill factor that seems so common nowadays. I'd think 2x8" would be fine but you might want to up it a size to be safe.

Offline davidj

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Re: Yet another roof joist question...
« Reply #8 on: March 10, 2008, 06:57:26 PM »
My cabin's 20' wide with a cathedral ceiling.  I've got #1 2x12s on 24" centers for a 120psf snow load.  This is without de-rating for the 12:12 pitch (which I think you can only do if the design is done by an engineer in CA).  The designer initially thought they'd have to be on 16" centers before doing the calcs, so I'm guessing this isn't over-generous.

Offline Willy

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Re: Yet another roof joist question...
« Reply #9 on: March 10, 2008, 07:47:54 PM »
Well a Doug-fir 2x8 is going to max out at about 45 pounds of snow load. Regardless of pitch I think you are suposed to design for the local snow load. This may fall into that overkill factor that seems so common nowadays. I'd think 2x8" would be fine but you might want to up it a size to be safe.
Nice part is he can derate the snow load a certain percentage because of the pitch being so steep. He needs to find out what the local inspectors require for the snow load and derating. California has rules all it's own I found out searching the net for a answer. Mark
This is the site I was reading on the snow loads it caught my eye cause I built a cabin in the town of Panorama Heights, CA right near this town;
://72.14.253.104/search?q=cache:CLzGnACUbtkJ:www.townofparadise.com/building/bldg_handouts/2005_res_const_guide_0905_rev.pdf+snow+loads+12/12+pitch+roof+calculation+paradise&hl=en&ct=clnk&cd=2&gl=us

Offline MountainDon

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Re: Yet another roof joist question...
« Reply #10 on: March 10, 2008, 08:02:53 PM »
FWIW, the IRC2003 has rafter span tables in chapter eight with snow loads up to 70 Lbs.

Link to IRC2003     http://countryplans.com/smf/index.php?topic=2590.0

The IRC does not have info specific to our areas as it varies too much locally.  I would think the Rio Arriba County officials would have suggestions for Frank's locale
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline NM_Shooter

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Re: Yet another roof joist question...
« Reply #11 on: March 11, 2008, 06:34:02 AM »
Well heck.

I called Rio Arriba County planning and zoning, and they told me that they don't have a snow load recommendation.  I guess I am not too surprised.  The guy I talked to said "I think you should at least plan for 55 load and 75 wind".  No kidding.  :-\ Sigh.

I received word from one of the ranchers ~4 miles from my property who said that they had over 18 feet drop this season, and that the snow pack is the heaviest he's seen. 

I'm guessing trout fishing is going to be a little postponed this spring.

Once again, I am second guessing (third / fourth guessing?) my plans to make a cabin large enough to accommodate 8 people.  I think I should make this smaller, stronger, and tighter.  If I need additional sleeping space beyond my 4 person family, perhaps I should create a second small sleep shelter cabin for guests.  Maybe make it dog run style, or just build it around a central deck area.

I am looking forward to starting so that I can quit fiddling with plans and just commit already  d* d* d*


-f-
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Offline Willy

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Re: Yet another roof joist question...
« Reply #12 on: March 11, 2008, 06:55:36 AM »
Well heck.

I called Rio Arriba County planning and zoning, and they told me that they don't have a snow load recommendation.  I guess I am not too surprised.  The guy I talked to said "I think you should at least plan for 55 load and 75 wind".  No kidding.  :-\ Sigh.

I received word from one of the ranchers ~4 miles from my property who said that they had over 18 feet drop this season, and that the snow pack is the heaviest he's seen. 

I'm guessing trout fishing is going to be a little postponed this spring.

Once again, I am second guessing (third / fourth guessing?) my plans to make a cabin large enough to accommodate 8 people.  I think I should make this smaller, stronger, and tighter.  If I need additional sleeping space beyond my 4 person family, perhaps I should create a second small sleep shelter cabin for guests.  Maybe make it dog run style, or just build it around a central deck area.

I am looking forward to starting so that I can quit fiddling with plans and just commit already  d* d* d*


-f-
OK so he said 18 ft but how much at one time and how much stays did some of it melt between snow falls? If it does not melt and all adds up your looking at allmost 2 stories of snow and I am sure he wasn't ranching in that. You only need to make sure the roof is built right. Why don't you go with 2X10s on 16" centers? Not that much extra cost. Big thing is if your not living there full time is windows being under the snow. Raise the cabin some off the ground if you can so the roof edge is higher. You don't want ground snow to build up to the eves if you can help it. You may want to think of shutters for over the windows for during the winter if your not there. 2X6 studs and 16" on center will make strong enought walls. Make your eaves stick out as far as you can to keep a lot of the snow away from the building. Other than that look at his home and out buildings to see how there built because you know they made it thru the winters. Sometimes ranchers like to mess with city folks moving out into the country we do it all the time up here! Mark

Offline John Raabe

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Re: Yet another roof joist question...
« Reply #13 on: March 11, 2008, 07:20:13 AM »
Here's a sample calculation using the American Wood Council span calculator with a link from the PlanHelp subscription site. First, input the rafter span (10'), species, spacing and dead and live loads. At a 12:12 pitch with a metal roof a snow load reduction of 50% is quite reasonable to most engineers (some will go higher).



I would use a 2x10 (or perhaps even a 2x12) for a cathedral ceiling where you will be insulating between the rafters rather than in the attic floor. If you are foaming the rafter without venting then the 2x10 will give you plenty of depth for around an R-40. I see no need to go to 16" spacing.
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Offline John Raabe

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Re: Yet another roof joist question...
« Reply #14 on: March 11, 2008, 07:34:30 AM »
Here's a bit more info from the Planhelp site on how to use this.

None of us are as smart as all of us.

Offline MountainDon

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Re: Yet another roof joist question...
« Reply #15 on: March 11, 2008, 11:29:06 AM »
I think I should make this smaller, stronger, and tighter.  If I need additional sleeping space beyond my 4 person family, perhaps I should create a second small sleep shelter cabin for guests. 
My wife and I agreed that we were really building this cabin of ours, for ourselves, with the possibility of having the occasional guest sleeping on the sofa bed. Or maybe camping out in the gazebo.  ;D  Or they could bring an RV up if they don't mind the rustic road. There's always the motel or the lodge 10-12 miles away as well.

Your original 16 ft wide might be a better bet. OMMV

As for the windows and snow... I want shutters for all windows. We're back up the slope and not visible from the public side of our gate. Shutters would offer some security against casual vandals, as well as the curious bear landlords and stormy weather.

Just because something has been done and has not failed, doesn’t mean it is good design.

Offline ScottA

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Re: Yet another roof joist question...
« Reply #16 on: March 11, 2008, 11:56:55 AM »
I went 16' wide to allow lumber sizes I could manage working alone. There is no way I could have put up 2x10 rafters by myself.

 

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