Slightly Warped Roof Decking


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Old 08-28-07, 07:33 PM
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Slightly Warped Roof Decking

Our house is 30+ years old. It is in need of a new roof. However, there is no leakage with the present roof...no rusty nails, no moisture in the attic.
However, there is a slight "bowing" to the decking that is only mildly perceptable from the street (existing shingles are the old-fashioned flat shingles).
We plan on replacing with dimensional shingles.
One builder recommended that we could "un-bow" the existing decking with some sort of supports, rather than replacing all of the decking.

My question: should this be done with 2x2 lumber or should something like steel angle iron be used to support the bowed sections by installing between the rafters in key locations ?
 
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Old 08-28-07, 08:24 PM
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If your rafters are on 24" centers, I would be willing to bet that you are seeing some dipping of the sheathing between rafters due to the weight of the layer(s) of shingles on the roof. If 1/2" sheathing was used, it's a bit thin for such a wide span.

If the roof is swayed- meaning a large section of roof is dipping, then it might be a different matter- the roof may need some added support- beams and struts that could actually jack the roof up or at least prevent it from sagging more, provided they run these struts to load bearing locations.

Or it could be that he was suggesting some rafters be added... or maybe he was thinking that some 2x4 blocking between rafters would help. Hard to say. It depends on what type of "slight bowing" you mean- the roof as a whole, or minor bowing between rafters.
 
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Old 08-30-07, 06:45 AM
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Thanks much...

But on further review, we've got 16" centers, and yes, the thinner 1/2" decking. However, again, no rusty nails showing to indicate any leakage or potential deck water damage.
I do see several wood 2x8 braces installed about 4 foot from the peak of the roof to keep about 6 of the rafters from sagging. However, I noticed that almost ALL of the rafters are slightly bowed. As usual, the former residents made a half-assed attempt at the proper repair. There were not enough of these braces and they were placed "too high" to be totally effective.
Since we plan on building-out a dormer on the non-street side of the house, the street side only needs to be fixed. I plan on notching-out the rafters about 3" and installing a 3"x4" steel I-Beam that will be support by verticle 2x4's every 48" or so. I will use shims to insure an even distribution of the pressure against each rafter. The I-beams will never show as a wall with sliding doors will be built for storage space when the dormer goes in.
Good idea ?
(Note: scrap steel is plentiful in my area...we are lucky to have a great yard that is actually like a supermarket for old steel and aluminum)
 
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Old 09-01-07, 02:02 PM
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When all the rafters are bowed, it is usually because older houses often have rafters that are too small for the spans they cover. Horizontal collar ties and permanent gusset supports that assist in transfering the weight to load bearing walls would have prevented that. Age and gravity eventually will cause an unsupported roof to sag- especially when it has had several layers of shingles on it much of the time. If you have 1/2" sheathing, it's likely those layers have been removed, and the house has recently been reroofed, but the sag remains. Jacking up a sagged 2x4 sometimes helps, but other times it will create two smaller sags on each side of the support.

It's hard to give advice sight unseen, but here's a few random thoughts (opinions) based on what you've suggested so far.

Notching your rafters in any fashion is probably a bad idea.

You suggest adding a beam and supporting it with vertical supports. That will work fine, provided the vertical support (either a kneewall or beam post on each end of the beam) sits directly over a load bearing wall that will transfer the weight directly to the foundation. If you are using the kneewall to support a load (jacked up rafters that are under tension) as well as a heavy steel beam, the kneewall cannot simply be built across ceiling joists, since the load you are supporting must properly be transferred to the foundation. Alternatively, the kneewall could be built over a beam that has been let into the ceiling joists, provided the ends of that beam transfer all weight to the foundation.

Additionally, if your plan is to reclaim unused attic space, it's likely that the ceiling joists (possibly 2x6's?) may be undersized for use as a floor.
 
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Old 09-06-07, 10:11 AM
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thanks once again...

obviously, you're pretty experienced and familiar with this sort of problem....and all of the thoughts are greatly appreciated.
Both rafters and floors are made from 2x8 lumber, so there is plenty of structural integrity there with 16" centers.
Alternate to notching, the "I" beam could be drilled and lagged into the rafters, but the load transfer would not be perfect and the verticle end supports (4x4 lumber" would have to be cut or routed to "fit" the odd angle of the I beam.
Myself, I don't see how a 2" notch would cause that much loss of strength in the rafters. Ideally, I think a 2" wide by 4" tall I beam would have enough rigidity to only require supports at the load bearing surfaces...which would require a length of beam of 20' or so.
Finally, let's keep in mind the bowing is visually pronounced now because of the cheap, flat shingles that are presently on there....and they are at least 15 years in age. Correct me if I'm wrong, but I think the dimensional shingles will help to "hide" some of these bowed areas.
 
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Old 09-06-07, 09:18 PM
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The reason not to notch is because of the potential for the bottom half of the 2x8 rafter to split along the grain as the beam is jacked. You'd be wise to get an engineer's approval before attempting that. Also, I'm not sure how much you would need to jack in order to straighten the rafters out... it took years to sag into it's current position and trying to remove all that sag in a short period of time would run the risk of pulling something apart.

Unless you've worked with steel before, I was also thinking that maybe a 20' steel beam is heavier than you think. You are right that laminated shingles will help hide some of the sags.

The only other thing you might want to eyeball is whether your ridge cap is straight or not. Usually if the rafters bow slightly, the ridge will also bow down with them. Jacking up the rafters on each side to take the bow out can sometimes cause problems- the rafters may want to pull away from each other at the ridge as you jack them up in the middle (depending on how much pressure it takes to lift them). Once in a similar thread I suggested that before jacking the rafters, opposing rafters should be bolted together with a collar tie that is as close to the ridge as possible. In theory, this would help keep the rafters together at the peak of the roof, allowing you to apply more pressure as you jack the middle of the span.
 
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Old 09-09-07, 06:45 PM
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thanks...great advice.

especially about the ridge caps.
Yes, these must be tied together as you said...on the top...as I had considered that I'd be using something like a portable jack to put pressure on the rafters before securing them....that could blow out the "peak" !
My brother-in-law came-up with a better approach that uses the steel as the bottom support rather than at the rafters....thus no notching will be necessary.
It will be a 2"x4" I-beam that will be supported at both ends on a loadbearing surface and will be set approx 1/2" above the floor joists in the attic. A knee wall will then be build with 2x6 lumber at each rafter, a little less than 1/2 way up the roof. A hydraulic jack will be used to pressure the rafter right before the verticle support is "toed-in" to the rafter and then secured to the I-Beam with a bolt and nut.
We will use an electric winch to pull the 200+ lbs of steel thru the attic vent opening in the side of the house.
That's the plan right now.
Comments greatly appreciated of course.
 
 

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