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  1. #1
    frank1975's Avatar
    frank1975 Guest

    Question strange roof depression

    I wonder if anyone can shed some light on this. We just bought this 20 year old townhouse but observed some strange shingle depression at both sides of the roof (see pic below), does anyone know why it is like this ? I know the roof is at end of its useful life, but just want to make sure it is a normal wear out instead of some serious structure issue. My inspector is not quite sure of the cause , think might due to "OSB roof sheathing may be too short in this area".

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  2. #2
    Join Date
    Mar 2007
    Ormond Beach, Florida

    Default Re: strange roof depression

    Based on only those photos, which do not show much other than the depressions, my guess is that whoever installed the roof sheathing installed it with the face grain run vertically up the roof on each end overhang and that the roof sheathing is deflecting under its own weight because it was not installed properly and is therefore not being supported properly.

    The other reason may well be 'yea, the didn't install roof sheathing there at all, they just shingled over it, but an inspector could typically determine that while on the roof ... of they were on the roof.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( )

  3. #3
    Join Date
    Mar 2007
    Atlanta, Georgia

    Default Re: strange roof depression

    Townhomes require a 4 foot fire resistent barrier between each unit. Some builders use drywall for the first 4 feet to meet that requirement. Could be they put drywall on top or or instead of sheathing and it is sagging. SHould be visible from the attic area even if the roof was not walked.

    "The Code is not a peak to reach but a foundation to build from."

  4. #4
    Join Date
    Nov 2010

    Default Re: strange roof depression

    From the pictures I cant tell if there is an overhanging eve, also the left pic reveals something going
    on with the siding just under the sagging roof sheeting. Maybe during the building of the TH the sheeting
    got wet before it was protected, or maybe there has been or still has some moisture issues going on.
    I would have someone take a better look at whats going on because it is not normal for a roof to sag like that.

    If there is moisture found, hope there no mold/mould LMAO
    (Sorry just had to add that)

  5. #5
    Join Date
    Mar 2008
    Charlotte NC

    Default Re: strange roof depression

    Some times when installing a second roof the roofer will cut back the original shingles from the eaves and rakes to not show the doubled roof. Looking in the attic at the nail pattern that penetrates the sheathing can indicate whether or not there are two roofs installed.

    If the shingles do not dip before they turn up the rake board covering may have been too wide and the top edge is above the plane of the sheathing.

    The beatings will continue until morale has improved. mgt.

  6. #6
    Join Date
    Apr 2011
    Bozeman, Montana

    Default Re: strange roof depression

    I think Jeff is correct, that you have other issues as well. It does appear as though there may be problems with the siding in the left picture (the one that seems to show a valley terminating against a wall, hmmm). It also appears as though your roofing was stacked. When you decide to reroof (soon) Make sure you contact somebody qualified to look beyond the obvious.

  7. #7

    Default Re: strange roof depression

    In my experience there are a number of possible causes for this type of deck deflection. As Jerry indicated, it is often caused by aligning the grain of the decking vertically instead horizontally across the rafters. Deflection such as this can also occur when the roof sheathing is installed in a 'single span condition', where the decking is cut so that it extends from only one rafter to the next. Decking should always be installed in minimum two-span condition, spanning at least two rafters, so the tensile strength of the surface plies help support the sheet at either side of the rafter.

    Another possible cause is illustrated in the photos below. The framer used milled siding material at the overhangs to provide a more suitable underside appearance at open soffits. Not only is the siding material thinner than the plywood decking used over the rest of the roof, it is installed in a single-span condition - and aligned vertically. As we say in Texas, "no bueno"!

    Don Putnam
    Austin, Texas

    I am actually younger and better looking than I appear.

    Decking Sag2.jpgDecking Sag1.jpg

  8. #8
    Join Date
    Feb 2008
    Caledon, Ontario

    Default Re: strange roof depression

    Looking at the original post photos the photo on the left has something else going on at the gable end. The siding is damaged, or at least thats what the photo appears to show.

    The value of experience is not in seeing much, but in seeing wisely.

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