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Low spot on a flat roof

jpf | Posted in Construction Techniques on September 27, 2009 07:22am

Hi,

I have a flat roof (rolled composit roofing) with a low spot that retains water. No leaks – YET but ….

Any thoughts on light weight material I can use to fill in/level out this low spot and how to check that rain will run off and not find an new low spot?

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  1. Piffin | Sep 27, 2009 07:32pm | #1

    flat is not a good idea for rolled roofing. It is not normally a log lived roof to begin with.

    So you may end up re-framing this soon.
    A common situatiuon when framed to be too flat, what happens is that the weight of the puddled water and snow lets the rafters deflect, they learn the new deflection, and more weight applied makes it worse and worse over successive seasons. Especially when the rafters are undersized to begin with.

    I'd have to know more about the framing and sizes etc to say more. A picture or three would help

     

     

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  2. florida | Sep 28, 2009 03:36am | #2

    There is nothing. If you move the water from the spot it's in it will just run to a different low spot. Even low slope roofs will develop puddles. One of the roofers will be along and tell you more but there are foam sheets available with pitch built in that might help but it would require new roofing. Piffin is right, new framing might be cheaper if possible.

  3. ponytl | Sep 28, 2009 05:53am | #3

    i hate flat roof that hold even a little water... now all mine are BUR but if the ares isn't too low... i build it up with layers of torch down...

    I know it's not the proper fix but on 160 yo buildings you do what you have to sometimes...

    you didn't say "how low" your low spot was... but for a fix "for now" you could buildit up with several layers of the same type roofing material thats on it now... first piece down being the smallest next a little larger... next a little larger... about 4 is max ... if we are only talking a 5x5 or so area...

    sometimes you have to do what you can do until you can do it the right way
    p

    1. Piffin | Sep 28, 2009 02:06pm | #4

      since roll roofing is what's on it now, all he would be doing is making a water sandwich trap and greater liklihood of leaks 

       

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  4. Clewless1 | Sep 28, 2009 03:55pm | #5

    Flat flat is not a good condition for a 'flat' roof. Normal minimum slope on a flat roof is 1/4" in 12. Do what the pros do ... use canted or tapered rigid insulation to get the slope you need and reroof. How big a roof are we talking about? When water drains, how does it get to the ground? e.g. scuppers w/ a parapet? eave and drip edge? Run down the wall?

    Check for flatness one side to the other to verify it is truly [framed] flat.

    You have rolled roofing which is the worst roofing for 'flat' roofs. Any fix attempts using what you already have is asking for trouble and headaches, IMO. If what is under it matters at all, bite the bullet and do it right. Water leaks in flat can be a royal PITA and the damage can far outweigh the cost of doing it right.

  5. oops | Sep 28, 2009 05:47pm | #6

    Sounds like a good location for your roof drain.

    1. McMark | Sep 28, 2009 05:57pm | #7

      Everyone is jumping on this "flat roof " thing, talking about slope.  Truth is many commercial roofs are sloped about 2 to 4 %, and are referred to as flat.  Rolled torch-down is also a common roofing material.

      What is often done on a truly flat roof, is that insulating material is used to give the roof the slight slope that is needed.  In your case you will have to remove the old roof in the bellied area.  Work with a laser or optical level will tell you the degree of tear-off that you will have to do.  Reframing may be needed.

      1. florida | Sep 28, 2009 06:32pm | #9

        "many commercial roofs are sloped about 2 to 4 %"And they still get huge ponds. The pigeons use them as birdbaths.

    2. roger g | Sep 28, 2009 06:02pm | #8

      Very good "out of the box" thinking. I mean it.
      Not saying it would work but you're thinking.roger

      1. oops | Sep 28, 2009 07:36pm | #10

        Yeah, I was going for a little tongue in cheek humor. I figured that Piffin would explain the error of my logic and set me straight.

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