Sagging roof repair

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  #1  
Old 06-18-07, 11:16 PM
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Question Sagging roof repair

I'm looking for recommendations on how to straighten and reinforce a sagging roof.

This is an older, one story, stick built house with a 6/12 roof. The 2x4 rafters span 12' on 24" centers. There is no ridge board, but there are 1x6 collar ties a foot down. There is a patch work of purlins and rafter braces to the center wall under the peak. A quarter of the rafters have no support. It's obvious bracing was added as rafter sagging occurred.

The ridge itself has a sag starting the first rafter from each end. The gable sheathing stopped the end rafters from sagging. So the house is sway backed. Based on measurements and a level, the supporting walls are not pushed out. I'm scratching my head on that one.

I stretched a string along the length of the first rafter. It was bowed 2 inches at the center. I jacked the rafter straight and added a 45 degree brace to the center wall. I planned to do that for every rafter. However, then I noticed the opposing rafter had separated by about 3/8" at the ridge during jacking. It's not a "V" shaped separation, but a parallel one. So now I'm scratching my head again.

I plan on replacing the collar ties with plywood gussets. That would hold the rafter ends together. However, the rafter ends need to move with respect to each other as they're straightened. If install the gussets before jacking, the relative angles are locked.

So, how do I prevent the rafters from seperating at the peak while allowing their angles to change. Do I just need to jack each opposing rafter at the same time (i.e. as pairs)? How is this problem usually corrected?
 
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  #2  
Old 06-19-07, 09:04 PM
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One possibility you may not have considered is that the wall the rafters bear on could also be bowed down a little- it may be hard for you to tell since you can't get your eye down there, but maybe you could check that by looking down the fascia or rafter tails on the exterior of the house. If so, it would be "partly" to blame for the dip at the ridge, and you would then realize that it will be impossible to get the bow out of the roof. Obviously the bowed rafters which you mentioned are also a major part of the problem, but if the exterior wall has sagged, you will never get it perfect without jacking up that wall too.

In my experience, there is little point in trying to straighten old lumber. If a rafter has 2" of bow in it, and you jack it up in the middle, it usually results in a rafter that now has two 1" bows on each side of the jack. (I may be exaggerating, but this is an important point- the point is that the rafter has been bowed for so long that often if you jack the bow in the middle, it will just push the rafters away from one another at the peak... your 3/8" gap. This is because the board often wants to retain its shape- jack the middle of the rafter up, and you'd actually have to pull the ridge back down to bring them together again because they are often permanently warped.) But you can check the bottom edge of each rafter as you jack them with a stringline, from top to bottom and see if you are really getting them straight or if you are just making 2 smaller bows.

IMO, this is the kind of deal where you just do the best you can with stringlines and straightedges, realizing that it is never going to be 100% perfect- it may only be 50% better after all the work you go to. So ask yourself if it's worth it.

One thing you could try is to build a header of sorts, and use it as a straightedge to jack the midspan of all the rafters at once- it will keep the bottoms of the rafters lined up as you jack each end. (each end- jacking straight up off of a pair of beams where one end of each beam is on the exterior wall and the other end on another load bearing wall.)

A short collar tie near the very top of the rafters could be bolted with one large bolt through each rafter. This would keep the peak together while still hopefully allowing each side to pivot as the rafter slightly changes pitch during jacking. As you slowly jack in stages, you might also want to install some collar ties a little lower too, to keep the opposing sides apart (at first) but also to keep them together to prevent them from spreading too far apart as you jack the middle of the span even farther to take out the bow. The dillemma is that you need to jack and collar tie both sides at the same time to create a perfect triangle, which will require a lot of effort, and maybe some math if you want to get it as close to perfect as you can. Working on pairs of rafters opposite from one another is the best method. If you can hold the ridge together, you should be able to jack the midspan on both sides up 1/2 way... put in a collar tie 1/4 of the way down (length and location figured mathmatically), then jack the midspan on both sides the rest of the way, and put a collar tie there as well (length and location figured mathmatically), with temporary vertical supports only used as needed. Its highly likely that you will need perpendicular blocking between your collar ties if there is a lot of stress on them. Since the rafters will want to bow back down, the collar ties will want to curve horizontally as the rafters try to spring back toward one another. You may even need collar ties in the bottom 1/3 of the attic if you want it perfectly straight.

The problem people don't think about is that simply bracing a roof to interior non-load bearing walls can transfer loads and pressures to other parts of the house that were not made to support that sort of load. You are creating tension by jacking the rafters up, and when you transfer that tension to other parts of the house, the load doesn't just magically disappear. So hopefully that center wall you are supporting everything on is well supported down in the basement / to the footing. You're actually better off attempting to straighten the roof with horizontal collar ties- returning the roof to its original triangular shape- and use less permanent vertical bracing to accomplish this.

Realistically the only way to get that roof perfect is to totally tear it off and rebuild it, or sister new wood onto the old. You can hardly do that properly without removing the roof sheathing. So you'll have to decide whether it is worth all the effort to make it "a little better", or whether you're going to bite the bullet and tear the old roof off.

With all the collar ties and blocking it would take to really straighten that roof out, you could probably reframe the whole roof.

Hopefully this will give you some ideas that will help you along the way.
 
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Old 06-19-07, 11:22 PM
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Thanks xsleeper. That does give me a few ideas.

The center wall (now load bearing) is very well supported clear to the ground. The top plates of the exterior walls are level and at the same height as the other walls. The drywall is off. So it's fairly easy to check. Who knows, maybe all the walls sunk evenly.

I understand what you said about the double bow, that's what I expected too. But, once the brace was in place, the rafter actually stayed reasonably straight, with less than 1/4" bow. Off course, that's with the 3/8" separation on top. I'm hoping that won't happen if I tie the opposing rafters together at the ridge and jack both sides at once. Your idea of initially using bolts on each side as pivots, then adding collar ties as I go, may be the key.

The problem with collar ties is the connection area required. You need tons of nails to do them right. With purlins and braces, the load is tansfered as compression along the length of the brace. So far fewer nails are needed. I'll stick with purlins and braces as much as I can, but light collar ties might be the ticket as I'm jacking.

Thanks for the suggestions,
Barry
 
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Old 06-22-07, 05:06 AM
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rafters gaps

Have you sighted down the length of the ridge to see if it has bowed,it may be where you boosted up the rafter that the bow occurs ,tap the ridge back towards the gap side slowly and ease the brace back down then sight the ridge again. That may have to be done in a couple spots.
 
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Old 06-22-07, 09:41 AM
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I bolted 1' tall, 3/4" plywood collar ties about 18 inches down from the ridge to hold the rafters together but let them pivot. Then I jacked and braced the first 5 rafters straight. The rafter junctions didn't appear to move much. They never did have much of a "V" shaped gap to begin with. So I added about 40 nails to each tie. I also renailed the old collar ties that were loose. The jacking helped the dips in the roof some, but the ridge is still sway backed. In any case, the collar ties and braces should stop further drooping.

Since that section of the ridge is firmly tied now, I'm could put beam(s) near the ridge and jack the center up a bit, but that would probably pull the walls in. Also, there's the new braces to consider. They would need to rotate a little and that might not want to happen.

So maybe I'll settle for a sway backed non-dippy roof. It will need redecked at some point anyway. That might be a better time to fix it... or cover it up.
 
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