Sagging Roof on Shed Dormer


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Old 08-19-08, 07:42 PM
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Question Sagging Roof on Shed Dormer

Hi All,

Our house (Built ~1929) passed inspection with a bit of sagging over a upstairs Shed Dormer. The ceiling appeared to be sagging before and after we purchased the house. The sagging (inside and out) hasn't changed in the 18 months we have had the house, however one of the projects in our list was to pull down the plaster & lathe ceiling upstairs in the sagging area to see what is going on and hopefully be able to repair or at least stop it from getting worse for the long haul.

First, I was hoping that changing the plaster and lathe to Sheetrock may light the load pulling down on the Shed Dormer rafters. Will this help any?

Also, I am no expert, but I noticed several things that appear to be wrong, at least by todays standards (and possibly by 1929 standards!) It looks like the shed dormer rafters are 2x4, the collar ties are 2x4s, they aren't spaced evenly in the dormer area (two are ~22" OC and one spacing is ~30" OC), also there is no ridge beam.

Wow, that is a lot of stuff. My question is:
Can I jack up the current rafters and collars, then sister everything with 2x6s? I can angle cut the rafter sisters and I can go back all the way to where the collar ties end for both? Will this be enough?

Since pictures are far better at explaining everything, I will attach a link to a few: http://members.cox.net/jr.due/shed_dormer_pics.htm

Thanks for the help guys!
 
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Old 08-19-08, 08:14 PM
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Sad to say, straightening out a roof that has been bowed for 75 years is easier said than done. But straightening the inside is certainly doable.

You'd start by cutting out one collar tie (ceiling joist) at a time, and replace it with a 2x6 collar tie before moving on to the next one. You could put in additional 1x6 struts if you like, since they help stiffen the bounce of the roof somewhat. Jacking the rafters and then installing the struts would only serve to put stress on the new 2x6 collar ties, so I wouldn't recommend that.

30" is a little far for drywall (and roof decking) to span, so you should probably put in an additional pair of rafters and another collar tie in that one area.

It's probably difficult to get pictures up there, but a few wider angle shots inside the room might help put things into perspective. And maybe some pictures of the dormer itself so we can have a better idea of what we're looking at. Those are my initial thoughts.
 
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Old 08-19-08, 08:40 PM
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More shots coming!

Originally Posted by XSleeper
Sad to say, straightening out a roof that has been bowed for 75 years is easier said than done. But straightening the inside is certainly doable.

You'd start by cutting out one collar tie (ceiling joist) at a time, and replace it with a 2x6 collar tie before moving on to the next one. You could put in additional 1x6 struts if you like, since they help stiffen the bounce of the roof somewhat. Jacking the rafters and then installing the struts would only serve to put stress on the new 2x6 collar ties, so I wouldn't recommend that.

30" is a little far for drywall (and roof decking) to span, so you should probably put in an additional pair of rafters and another collar tie in that one area.

It's probably difficult to get pictures up there, but a few wider angle shots inside the room might help put things into perspective. And maybe some pictures of the dormer itself so we can have a better idea of what we're looking at. Those are my initial thoughts.

XSLEEPER, thank you for the advice. Sounds like a great plan. I will take some additional pictures and post them to the same link.

http://members.cox.net/jr.due/shed_dormer_pics.htm

I can get a few wider angle shots of the inside tonight (give me about 10 minutes). If we need some external shots then I can get them tomorrow.

Thanks a for the great response.
 
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Old 08-19-08, 09:23 PM
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The picture you took showing the windows is great. I wasn't visualizing that part.

The picture makes it look like the roof is sagging a LOT. If that's the case, then it may call for some heroic measures to correct it. Sistering 2x6 rafters (or LVL- $$$) onto the existing in that area might help raise that sagging roof, however it would reduce the width of the top window trim by at least 2", because they would have to run clear from the ridge to the exterior wall. (and, I'm guessing, there isn't a header above that window either!)

The other thing you could do is leave the existing rafters alone and put a header underneath them, but the problem you get into there is that the header has to bear on something load bearing that transfers the weight to the foundation... it can't all just rest on your sidewalls and floor. If you don't have any codes or inspectors to please, that's one thing. You're really only trying to straighten (stiffen) a small area. If you were to try it, it looks to me like a 6" LVL header would be plenty. If the sagging rafters over your windows were numbered 2, 3, 4... you'd want the LVL header to be located above the collar ties, and it would span from rafter 1 (left of picture) to rafter 5 (right of picture). You'd need to support the header with a pair of trimmers on each side, down into those sidewalls by the windows.

If you were to do this by the book, you'd probably really need to put a microlam in the floor below those sidewalls that would span from the exterior wall to your center load bearing wall (I'm assuming you have one), and your trimmers would rest on that, transferring the stress to your load bearing points. (my brother-in-law had to do this when his shed dormer was built on his house in Omaha.)

Keep in mind this is just a carpenter's opinion- the ones who tell us what to do are the architects and they get paid to run the numbers and tell us what is both safe and workable. If you'd like to spend $100+/hr for one of them to take a look, they'd probably be willing to give you a solution that is by the book.

If you wanted to go back to the solution that doesn't really solve the sagging roof, but makes the ceiling look straight, you'd shoot your new collar ties across, making sure they are straight (they wouldn't get nailed to the exact same spot)... and then once they are up you could sister some new pieces of rafter from the collar tie down to the exterior wall top plate, which would create a false ceiling below the bowed rafters. This obviously wouldn't be a perfect solution, but it would make the ceiling look straight from inside the room.
 

Last edited by XSleeper; 08-19-08 at 09:41 PM.
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Old 08-20-08, 05:25 PM
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Thanks for the help

Hi XSleeper,

Thanks for the advice. It sounds like I have two options, do it expensive and right, or perform a patch to stop it from getting worse and either way I can make it look good from the inside.

Our long term plan includes adding a bathroom and some additional space upstairs, so I think this may be an opportunity to "do it right".

For now, I think I will go with the patch it up plan.

When adding the new collar ties, the span is ~12' what 2x would you recommend (is 2x6 ok?). Also, do you think I can move the collars higher towards the ridge to give me a little more headroom? It seems like I have read that the closer to halfway on the rafters the more effective a collar is and not to go further than 2/3rds the rafter length, is this correct?

Thanks again for all of the help. It is not easy to find someone so helpful, knowledgeable and able to explain things clearly.
 
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Old 08-20-08, 07:23 PM
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IMO, 2x6 would be fine, and yes, that is a good rule of thumb for collar ties. When you go to set your collar ties, a laser level can be very helpful for helping you get them all in the same plane. If you don't have one or know someone to borrow one from, it would be worth renting one, methinks.

Thx for the kudos.
 
 

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