Dished Roof


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Old 06-06-06, 08:00 PM
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Dished Roof

Hi. I would like to do a tear-off and reroof on my home. The house is a 1950's colonial. Rafters look to be 2x6. There's a ridge beam that looks to be about 1x8 or so. I've got what I think is called "dishing," where the ridge seems fine but there's sagging about midway between the ridge and gutter pretty much across the whole roof. I think the weight of the shingles is too much for the roof. I would like to try to correct this before reroofing. Any suggestions?

Thanks.
 
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Old 06-06-06, 09:59 PM
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When wood is distorted, it rarely can be straightened perfectly. Take your rafters for instance. If you take a curved rafter and jack it up in the middle, you will likely have a rafter with two smaller curves. So don't expect perfection- if you just want to make it "straighter than it is now", that would be a reasonable goal.

You need to consider how important your attic space is, because there are probably 2 ways to go about rectifying the problem. 1). horizontal collar ties or 2). perpendicular bracing.

You'd want to start on a gable end and work in one direction, not start in the middle. You'd also want to do this after you tearoff. From inside the attic, place a stringline on the bottom edge of the rafter you will be working on, and run the line from the ridge to the top plate, or as close as you can get to the top plate. You will then see how much the rafter sags.

Lay a 2x12 across several of your ceiling joists. (preferably near a load bearing wall or, (*) if none is available, a partition wall) Set a bottle jack on the 2x12 and rig up a temporary jack stud that you can place on top of the bottle jack and under the rafter. Pin the jack stud to the rafter with a couple long screws. Jack the bottle jack until your stringline appears straight. Then give it a couple extra jacks (slightly bowing the rafter up 1/8" or so) if you think it's safe to do so. (the jack is also exerting downward pressure on your ceiling, which is why it's preferable to do this near a load bearing wall... you don't want to crack your ceiling!) Leave that bottle jack in place and do the same thing to the rafter on the opposite side with another bottle jack.

Now you need to decide which method to use- collar ties or perpendicular bracing.

1). Collar ties work by creating a triangle in the peak of your roof. Once a collar tie is nailed horizontally to the side of the rafters (in the middle of their spans, right above the bottle jacks), the rafters will not be able to spring back down when the jacks are released, because the collar tie will hold them apart. (additional blocking or bracing will be required to keep the collar ties straight, to prevent them from bending laterally.) The drawback to using collar ties is that they will block off the headroom in your attic and you will have to duck underneath them. If that's not an issue, collar ties might be the easiest way for 1 or 2 people with little experience to straighten the roof. You would just repeat this method all the way down the roofline. Collar ties also will not create any pressure on your existing ceiling or walls.

2). Perpendicular bracing (perpendicular to the angle of the rafters) could also be used, but the problem with using perpendicular bracing is that you need to have load bearing walls or a partition wall(*) below your bracing. Bracing to the ceiling joists is not acceptable, because you need to transfer the roof load to something substantial. Perpendicular bracing would also limit the space in your attic by creating a "wall" on each side of the attic, but the center of the roof would still maintain its current headroom. You could jack and support each rafter individually, but there is an easier way- Rather than jacking the rafters individually, you should be able to support the bottoms of several rafters all at once with a 2x10 or similar (size can be judged by the number of rafters, amount of bow and the pressure that will be exerted on it) positioned perpendicular to the direction the rafters run. The 2x10 would be braced on each end by 2x4's or 2x6's that have one side notched out around the 2x10, and are attached to it. The ends of the 2x10 would then be jacked up until the bow has been removed from all the rafters. The bracing on each end of the 2x10 would then be secured and more bracing in the middle of the span could be added- all of which would need to sit on the load bearing wall, or partition wall (*). (You can brace across the tops of the ceiling joists if they are within very close proximity to the supporting wall.) You would repeat this process across both sides of the length of the roof.

As I mentioned, the problem with perpendicular bracing is that it needs to be in the center of the rafter and you have to have something to brace the bottom end of the bracing to. Very seldom is there a wall where you want it to be, which often means that you have to build a "header" of sorts, and support that beam on each end, for example. So it's like you have to look at the roof, look at your interior walls, look at where you can brace, and then figure out how the heck you are going to build your bracing. And you can't just brace anywhere, because you might transfer that pressure somewhere you don't want it to be. (make a bow in your ceiling)

I hope this makes sense, it's pretty hard to describe. I hope my suggestions don't break some local codes that I don't know about. Perhaps there will be some other ideas out there too. Shimming the roof straight (under new sheathing) would be one idea, but that really doesn't fix the problem, only covers it up, and that could still sag again over time.

Good luck!
 
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Old 06-07-06, 03:25 PM
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XSleeper,

Thanks for the reply. I think the collar ties are the way to go. The attic space over the house really isn't that usable anyway. The access panel seems like it was an afterthought - just some drywall cut away between two joists in a closet ceiling; so you can't really fit any boxes up there. Even if you could, there's no floor. I think I can get the wood for the collar ties up there (if not, I guess I can remove sheathing), but a floor would be something else entirely.

A posting in another forum said that bracing is best left to an engineer, which I would agree with, considering what you described.

Thanks again.
 
 

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