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Old 11-11-2009, 08:53 AM   #1
 
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I have a roof with about a 5/12 pitch. The rafters are 17'6" from peak to plate and made of 2 x 6 pine #2. By installing a plywood beam 10' long 3' from the peak made of 2 layers of 3/4" plywood (glued and screwed) and bolted every 2' alternating top and bottom 2 1/2" from each edge. These were placed on every other rafter. How has this increased the load rateing of these rafters?
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Old 11-12-2009, 06:47 AM   #2
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Old 11-12-2009, 07:08 AM   #3
 
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Like the second example. Along side of the rafter.
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Old 11-12-2009, 07:19 AM   #4
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Dont want to be a smartxxx, but to what end? What exactly are you trying to accomplish?
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Old 11-12-2009, 07:44 AM   #5
 
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Trying to stiffen the rafters and meet the span requirements of a 2 x 6
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Old 11-12-2009, 08:14 AM   #6
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if you have that kind of access to the attic I would take span measurements, rafter size and pitch with ceiling joist size to a truss manufacturer and see if you can get a build in place drawing to stiffen the rafters
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Old 11-12-2009, 08:26 AM   #7
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Do you need the headroom? Maybe mid-span collar ties would work.
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Old 11-12-2009, 08:37 AM   #8
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Even a couple knee walls. Sounds easier and stronger than bolting all those scabs in.

But he's allready installed them and just wants to know how much it has helped.

I have no answer.
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Old 11-12-2009, 08:55 AM   #9
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Quote:
Originally Posted by dale.thomas View Post
Trying to stiffen the rafters and meet the span requirements of a 2 x 6
What happened to the rafters that they don't meet span requirements? Is this a new house, old house, addition? If it's an older house, were the rafters sagging?
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Old 11-12-2009, 09:04 AM   #10
 
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installing pv solar panels and need to keep the storage space open. Roff seemed a little spongy and the question arose about the 2 x 6 being strong enough.
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Old 11-12-2009, 09:11 AM   #11
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thats a long rafter for 2x6 soprised it is not already sway backed. would never pass muster today as single member, probably sagging under its own weight
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Old 11-12-2009, 09:15 AM   #12
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installing pv solar panels and need to keep the storage space open. Roff seemed a little spongy and the question arose about the 2 x 6 being strong enough.
If that's the case you should get an Architect or Engineer in there to determine what the loads will be adding the weight of the solar panels. How do you know what your doing will work? No one here can help you without physically looking at it.
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Old 11-13-2009, 12:44 PM   #13
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What has happened now, is the rafters have a very stiff middle section. There is still three feet at the top, and 4-1/2 feet at the bottom that has not received any treatment at all. So, in my opinion, the rafters load bearing capacity has been reduced, and here's why: Under normal circumstances, the top edge of a rafter is under compression, and the bottom edge of a rafter is under tension. These forces are evenly distributed along the entire length of the rafter. On an undersized rafter, over time the lumber will not be able to resist these forces and will sag and ultimately give way. This is because the cells in the wood have a finite amount of elasticity. The middle 10' of the scabbed rafters will not be sagging, because that section is no longer undersized. However, we still have to consider the upper 3 and lower 4-1/2', which is still under sized. The load follows gravity until it goes down to the foundation footings. At the point where the load travels into the 2x6, all of the tension that would have been spread throughout the middle 10' will manifest at the transition. And since there is a significant hole drilled in the last point of connection, a weak spot has been formed. So my prediction is the stretching of the cells within the 2x6 has been accelerated, and they will begin to crack at the bottom of the scabs. I try to think of it this way: If a load is introduced to the roof that will obviously destroy the structure, say a tractor trailer is dropped on the roof; what will break first? Hope this helps.
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Old 11-13-2009, 01:18 PM   #14
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Wouldn't a better solution be purlins at mid span with struts back to the center wall (assuming it's load bearing). Along with collar ties.

That's basically the construction of my house, which has rough cut 2x4 rafters, 24" OC. The rafter length is 20 some feet.

I think the struts function to cut the span in half. Whatever the "engineering," it seems to work. The house is about 60 years old now.
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Old 11-13-2009, 02:01 PM   #15
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Dale, only an engineer could tell you and he would need more info (for example what plywood and what dimensions, what load is your solar system adding etc) . If you don't want to get an engineer involved you might be able to use the IRC for some help if the numbers fit.

Pull up IRC Table R802.5.1 (3) which is the rafter sizes for 30psf snow load areas. Figure out the weight of your roof system and the additional weight of your solar system. It may (or may not depending on your calcs) be appropriate to use the 20 psf dead load section of this table. If your peak to plate measurement is 17'6" on a 5/12 pitch then your span is about 16'2". The table indicates that 2x10 rafters at 16" oc would span 16'6" at 20 psf dead load in a 30 psf snow load area if they are DFL #2s.

Oh and don't put a tractor trailer on the roof to test it. LOL
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