I just purchased a camp and come to find out that my rafters are a bit undersized for the live load. According to code, they are about 1 foot short of code standards. Is there anything I can do to help decrease the rafter span???
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Sure, add a wall.
edit: and the appropriate load path of course.
Spheramid Enterprises Architectural Woodworks
"Success is not spontaneous combustion, you have to set yourself on Fire"
Edited 2/11/2008 6:15 pm ET by Sphere
Sphere , are you talking about a knee wall 1 ft out from the outside wall???
Any where to reduce the point to point bearing, and thus the open span. Even a header only (beam) ( properly sized) and posted on each end would do.
edit: or sister a deeper rafter along side existing that WILL meet span calcs.
Spheramid Enterprises Architectural Woodworks
"Success is not spontaneous combustion, you have to set yourself on Fire"
Edited 2/11/2008 7:11 pm ET by Sphere
A knee wall will be of little help. The minimal load transfered to teh knee wall is then transfered to your floor joists. Are your floor joist bare minimum as while?
Don't know as it'll make you feel any better, but the place we sold in Denver had 15' 2x4 rafters on 24" centers. Every other one resting directly on top of a wood sliding window. Resting, 'cause the the minimal header was bent down to the window. Everything, bowed like hell.
This was an addition, undetermined age, to a 1911 building. Worked for the 7 years we lived there. Second owners since then are still alive. I beefed it up slightly when I cut a hole for a roof hatch and added stairs. Straightening was out of the question.
PAHS Designer/Builder- Bury it!
Assuming it has an open attic, add some collar ties. If there are already collar ties, add some more. It won't actually reduce the span, but it will help distribute the load.
http://www.tpub.com/content/engineering/14069/css/14069_213.htm
If it helps you feel better you might compare the looks of what you have with a higher grade of lumber than what you looked at on the span table. If you have crappy wood with large knots you might be over spanned by more than the table would indicate, while something like tight grained old growth might span more.
If you are that close it would be quite simple to add a few additional joists. Along other lines, (this method works but won't meet code unless spec'ed by an engineer), adding a layer of plywood under the joists will stiffen up the floor a great deal.
Many times being that close doesn't really hurt anything, but if you really want to there are dozens of ways to beef things up.
Good luck
Beer was created so carpenters wouldn't rule the world.
Actually, the new raters(2x8's) are sistered alongside old 2x4's, so *I think* that would help shorten the span, but I couldn't find any thing in the code books to determine whether it would or not. I did place 2x6 collar ties on each rafter pair in the upper ridge area about 16" down from the peak, they run 5 ft across. It's a small roof area 15 wide x 22 deep. Code states 2x8's for my live load will span 9'5" and my span is 10'6" Just wondering if being short a foot is something I need to worry about. Does anyone know if this sistering or use of collar ties helps my case??/ thanks.
Are you using a horizontal length for the rafters or the actual length from one end to the other? Just wondering since you mentioned the roof being 15' wide.
Beer was created so carpenters wouldn't rule the world.
If this is an existing structure and you're not doing any permit-required renovations, why does it matter if it doesn't meet code standards?
How old is this camp? How many years has the roof withstood the snow load? Has it collapsed yet? If not, then there' s not a problem. Enjoy your new camp.
Solar & Super-Insulated Healthy Homes
I was thinking floor joist when I first responded, if it seemed a little strange to recomend sheathing the underside of a roof! *chuckle*
Beer was created so carpenters wouldn't rule the world.
I'm using horizontal length at 10'6". I'm short 1 foot. I guess I could just let it go, but I am curious if using collar ties and the fact that the new rafters are sistered to old 2x4 rafters help.
15 ft is left to right, 22 ft is front to back. Rafters run from front to back spanning the 22 ft. I got 10'6" horizontal by dividing 22= 11, minus the walls and T-111 sheathing left me with 10'6" span , unless I did it wrong.
Edit: I'm going to retract that statement. I was wrong. The rafter span is 1/2 the width of the clear span of the building minus 1/2 the thickness of the ridge board.
Some Qs: are the sistered members in contact with the wall top plates and the ridge board? Are there ceiling joists that run between the bottom of the rafter pairs (parallel to the rafters) and sit right on top of the wall double top plates? What centers are the rafters on? (16"? 24" what? What centers are the ceiling joists on? do you have any idea of what kind of wood the rafters are made of? What do you think is your live load (snow load?) Can you post a pic?
Edited 2/12/2008 9:11 am ET by Matt
Just a follow up, I consulted with a PE and they said although the 2x8's are a bit undersized: the fact that there are 2x4 sisteresd alongside and collar ties every rafter pair make the rafters adequate for the camp.
I think someone already stated that collar ties will help distribute the load but are not a substitute for correct sized rafter. (though in practice they will help and possibly help to the point that it is ok)
The 2x8's sistered to 2x4's are stronger than 2x8's by themselves if nailed or screwed thoroughly and in a reasonable pattern. (like every 12" offset or something like that)
You will not find any info in any span table on sandwiching 2 different sized rafters together though, and the only way to determine if it will work is to hire an engineer to do a complete load analysis and verify the connection (nails, screws etc)
By the time you are done this, the 3' of snow on the roof will have melted and if it melted and ran off, leave well enough alone. If it melted on the floor cause the roof collapsed, then maybe you should have added some 2x10's
Good luck.