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Thread: Sagging roof

  1. #1
    Jon mackay's Avatar
    Jon mackay Guest

    Default Sagging roof

    Does anyone know what might cause a sag at the roof surface?

    This roof had a sag at the front left and it was noticeable in the attic at the rafters but I couldn't pinpoint an actual cause. Could this be just the way the rafters were installed? Maybe the crown was installed different?

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  2. #2
    Join Date
    Mar 2007
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    Default Re: Sagging roof

    - weight of the shingles
    - rafters being installed crown side down
    - rafters overspanned

    I see this condition very regularly Jon. Often times, somebody will construct a kneewall or install vertical struts beneath the rafters in the attic to stabilize them and prevent further sagging. But in more extreme cases, I've sometimes seen these repair methods result in sagging ansd damage of the living area ceilings below.


  3. #3

    Default Re: Sagging roof

    The roof is sagging from the weight of the shingles and other construction materials. If purlins, bracing and collar ties were present, the sagging would be minimized.


  4. #4
    chris mcintyre's Avatar
    chris mcintyre Guest

    Default Re: Sagging roof

    Quote Originally Posted by Jon mackay View Post
    Does anyone know what might cause a sag at the roof surface?
    Quote Originally Posted by Darrell Covington View Post
    If purlins, bracing and collar ties were present, the sagging would be minimized.
    The bracing can (and should) be added at this point and possibly bring the roof back to its original plane. As long as the bracing is installed over the interior walls and the walls have the proper sills and piers, we do this often with very good results.


  5. #5
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    Default Re: Sagging roof

    JOHN

    what kind of ventilation does this roof have.i see a lot of white mold like substance on gable wall. is it sucking itself inward ???

    cvf


  6. #6

    Default Re: Sagging roof

    All of the above. Plus, I've seen where part of an attic had ventilation/ moisture issues, which caused more sag than other areas with the same span.


  7. #7
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    Default Re: Sagging roof

    You will see this quite often in older construction. In some cases additional rafter support or other repairs are needed. However many times in older homes like this it's just an observation and there are no repairs or recommendations to be made. From your picture it looks like the shingles are little uneven, which can make the roof surface appear to be uneven when looking across shingles.


  8. #8
    Jon mackay's Avatar
    Jon mackay Guest

    Default Re: Sagging roof

    Quote Originally Posted by CHARLIE VAN FLEET View Post
    JOHN

    what kind of ventilation does this roof have.i see a lot of white mold like substance on gable wall. is it sucking itself inward ???

    cvf
    That's not mold, its just the metallic gable vent.


  9. #9
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    Default Re: Sagging roof

    john
    that's a big out of form gable vent.how do you know that, just like i don't know the mold thing.that white -whatever it is is a bit strangely shaped to be a vent

    cvf


  10. #10
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    Default Re: Sagging roof

    Quote Originally Posted by CHARLIE VAN FLEET View Post
    .
    John
    that's a big out of form gable vent.how do you know that, just like i don't know the mold thing.that white -whatever it is is a bit strangely shaped to be a vent

    cvf
    .
    Charlie,

    He was There.

    The man said it was a gable vent.

    That's Good Enough for me.
    .

    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  11. #11
    David McGuire's Avatar
    David McGuire Guest

    Default Re: Sagging roof

    Off subject of the post here, but....Was there any clearance of the insulation to allow for ventilation in this attic? Kind of hard to tell, but it lloks like it butts right up to the roof deck. Second question, is that air duct completely flattened or what?


  12. #12
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    Default Re: Sagging roof

    With a build design like that, Id be looking really close at how the rafters are attached at the eaves. Loads pushing down on the roof will be forcing the rafters outward at their ends. Without any purlin or collar ties, those outer rafter attachments will be critical.


  13. #13
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    Default Re: Sagging roof

    John,

    I see this all the time, it's due to age, weight and the use of 2x4s as rafters. What I do is find out from the buyers what their intention is - remove the lathe and plaster that is usually associated with this type of roof system and materials - then fine - install a knee wall from one gable to the other. If it is already drywalled, then I let them know the adverse effects that can be caused by trying to even up the rafters to take the low spot out of the roof plane. Then it's their call.


  14. #14
    chris mcintyre's Avatar
    chris mcintyre Guest

    Default Re: Sagging roof

    Quote Originally Posted by Jim Hintz View Post
    I see this all the time, it's due to age, weight and the use of 2x4s as rafters.
    Probably 2x6's, maybe 2x8's, defiantly not 2x4's.

    Quote Originally Posted by Jim Hintz View Post
    ...remove the lathe and plaster that is usually associated with this type of roof system and materials...
    What exactly do you mean by this.


  15. #15
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    Default Re: Sagging roof

    Quote Originally Posted by chris mcintyre View Post
    Probably 2x6's, maybe 2x8's, defiantly not 2x4's.



    What exactly do you mean by this.
    Chris, if the ceiling is of lathe and plaster (which is usually shattered/cracked) and the buyers are planning on removing the lathe and plaster from the ceilings in a 1940's or so house as the one in the photos appears to be based on the unsupported 2x4 rafters and the 1x6 roof decking, then I advise them to hire a contractor to install a full length knee wall or purlin bracing on a runner that bridges all ceiling joists for even distribution of weight and once they get the roof rafters aligned where they are satisfied with it, then hang the ceiling drywall - that's all.


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