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  1. #1
    Zane Remenda's Avatar
    Zane Remenda Guest

    Default Negative Slope on flat roof

    Hi all,

    I frequently inspect flat roofs. Very often there is a negative slope, thus creating stagnant water which reduces the lifespan of the membrane.

    For a tar and gravel (bitumen) roof, is anyone aware of anyway that the slope can be repaired? What about a negative slope on a "roll roofing"?

    Thanks

    Zane

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  2. #2
    Join Date
    Aug 2007
    Location
    Memphis TN.
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    4,311

    Default Re: Negative Slope on flat roof

    Quote Originally Posted by Zane Remenda View Post
    Hi all,

    I frequently inspect flat roofs. Very often there is a negative slope, thus creating stagnant water which reduces the lifespan of the membrane.

    For a tar and gravel (bitumen) roof, is anyone aware of anyway that the slope can be repaired? What about a negative slope on a "roll roofing"?

    Thanks

    Zane
    .
    One Way When it Rains it Pours – Roof Drains Explained

    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.

  3. #3
    Join Date
    Mar 2007
    Location
    Ormond Beach, Florida
    Posts
    26,248

    Default Re: Negative Slope on flat roof

    Reverse slopes can be corrected, but it is a bit expensive as they would need to cut the roofing back, install tapered roof insulation, then re-roof.

    That area looks like a perfect spot for a roof drain.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  4. #4
    Join Date
    Oct 2011
    Location
    Minneapolis, MN
    Posts
    780

    Default Re: Negative Slope on flat roof

    That's a very saggy roof! How old is the building? I'd be concerned about unseen leaks that have compromised the roof structure. When full of water there would be enormous weight on the center. Is that a roof drain I see in the foreground, the rusty pipe sticking up a little? That would definitely be the way to go. Otherwise if you raise the middle you're looking at reroofing the whole thing, since it's tar and gravel.

    Roll roofing shouldn't ever have a negative slope, it's not designed for flat roofs. But to increase the slope the repair Jerry suggested is suitable.

    Last edited by Kristi Silber; 12-14-2011 at 10:13 PM.
    Do not think of knocking out another person's brains because he differs in opinion from you. It would be as rational to knock yourself on the head because you differ from yourself ten years ago.
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  5. #5
    Join Date
    Feb 2008
    Location
    Caledon, Ontario
    Posts
    5,005

    Default Re: Negative Slope on flat roof

    Its not all that uncommon, difficult to always get a positive slope, and could be difficult to correct. Likely easier to put another drain in.


  6. #6
    Join Date
    Feb 2009
    Location
    Southern Vancouver Island
    Posts
    4,546

    Default Re: Negative Slope on flat roof

    I am surprised we don't see more pond pumps on roofs like that. The pump drains the puddle thru a drain hose and then shuts off. They are apparently common in jolly old England, where flat roofs are a curse.

    John Kogel, RHI, BC HI Lic #47455
    www.allsafehome.ca

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