Roof structure needs to be replaced

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Old 12-09-10, 12:24 PM
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Roof structure needs to be replaced

I'm considering buying a distressed house that has major problems with the roof. Previous owners have added three additions to the house and the roof over two of the additions was poorly designed. The roof extensions have little to no pitch and parts of the roof have buckled. I assume the sections buckled while trying to carry the seasonal load of snow without any drainage. At any rate the roof structure needs to be completely replaced. I want to do it economically but still have a sound structure when complete. I'm thinking of having a contractor remove the roof trusses but leave the truss' bottom chords. I'm guessing electrical wire is running over the chords and I don't want to remove the drywall from the ceiling inside. Can I leave the previous bottom truss chords in place and then have trusses engineered to go over the top of the chords? Does anyone have any tips on how to economically replace a roof structure?
 
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Old 12-09-10, 01:40 PM
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NO. The bottom cord of a roof truss is not designed to carry any load by itself. The bottom chord may even bow under it's own weight. Many trusses only have 2x4 or 2x6 for the bottom cord and are totally inadequate for long spans without the rest of the truss structure. If you want to stay with engineered trusses I would have the truss company rep take a look at your application. I'm guessing the best way might be to remove the roof sheeting and install new trusses next to the old, leaving the old ones in place. It may be less expensive to stick build the new roof so I'd price it both ways.
 
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Old 12-09-10, 05:15 PM
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If the structure failed, I would doubt that you have "trusses". If you have ceiling joists and rafters, then yes, once you remove the rafters you can leave the ceiling joists and place your new trusses along side them, but the trusses must sit on the top plates, not on top of existing joists.

All electrical running perpendicular to the joists in the ceiling will need to be disconnected in order to do this.
 
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Old 12-09-10, 08:15 PM
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Joists

Yes after another look I realize that it is has ceiling joists. Why can't I build up the top plate and place the trusses on top of that? I will ask the framers to see if they can rebuild the rafters and add joists where needed to make the ceiling structurally sound. If the framers rebuild the rafters do I need an engineer to look at it?
 
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Old 12-09-10, 09:07 PM
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One viable option might include cutting your ceiling joists back 1 1/2", and install a rim joist all along the ends of them. Then top that with a sole plate that the new trusses could sit on. But the problem with that is that you would need to install exterior wall sheathing over this new sole plate and rim joist that would need to extend down onto the wall framing below in order to tie into the wall below. If you can remove enough siding and existing sheathing to do that then that might be an option.

Instead of using trusses, there's probably no reason that the roof couldn't be reframed with more pitch and larger rafters to eliminate your existing problem with snow load. Since you do have snow load, the rafters would need to be sized accordingly.
 

Last edited by XSleeper; 12-09-10 at 09:23 PM.
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Old 12-09-10, 11:44 PM
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Yes, that is generally what I was thinking. Building a new sole plate and the new trusses would sit on top of it. Then adding to the exterior sheathing to avoid messing with any electrical work. But it seems that having the framers build new rafters is going to be cheaper and simpler.

I appreciate all the great advice.
 
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