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Thread: Ponding on new flat roof

  1. #1
    DIY Junior Member yds's Avatar
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    Default Ponding on new flat roof

    We have a small 6X16 flat roof off our new third floor addition. Soon we will be building a deck over it. The roof was recently refinished (old roof taken up, new plywood layed, and covered with 2-ply modified bitumen roofing) when we added the third floor. Since it has been finished, each time we get a rain, I get ponding on the roof over an area about 2X6, against the support beam of the new addition. The pond has a depth of about 1/2 inch, and will dry up generally over a half week or so of no rain. My contractor is trying to tell me this is ok. I'm not so sure, especially since we will be building a deck on top of it, and therefore will have little access to roof in case of needed repairs. BTW, I live in Toronto so we get plenty of freezing temps over winter.

    Looking for advice on whether I should worry about this, or not.

  2. #2
    In the trades Dana's Avatar
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    I'd worry about it (it's hard not to!) The shading provided by the deck will protect the roofing somewhat, but no roofing is forever, and freeze/thaw on a wet roof takes it's toll over time.

    The ideal would have been to add a bit of tapered EPS foam insulation above the roof deck before the roofing went down to provide both drainage and a thermal break over the framing elements, reducing the mold & ice-dam hazard while increasing the energy efficiency. (Dead-flat roofs are a LOUSY idea for residential construction except in arid zones.) Torch-down roofs over EPS foam are common in commercial construction. Tapered EPS is pretty standard too (but can be custom ordered to almost any taper.) Nailer-decks can be applied over the EPS by through-screwing the nailer deck to the structural deck if it needs a bit more substance. Some vendors sell EPS or iso panel products with nailer-deck pre-lamninated onto the insulation.

  3. #3
    Moderator & Master Plumber hj's Avatar
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    You SHOULD NOT have a FLAT roof. Most codes would insist on at least a slight slope, along with cant strips, so you do not get ponding. Water standing on a roof is ALWAYS detrimental and will cause premature failure.

  4. #4
    In the trades Dana's Avatar
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    I'm thinking the solution here going forward is to build a sloped steel roof mounted on purlins under the deck when you build it, leaving sufficient vent gap to allow the bituminous roof to dry. It'll keep the bituminous roof mostly dry without having to rip it up and start over. Be sure to flash & seal properly at any penetrations of the steel roof before the deck goes over it. Even 3" of pitch over a 6' run will drain reasonably on steel roofing in anything but a gale wind, but 6" would be better.

  5. #5
    DIY Senior Member chefwong's Avatar
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    Ponding will occur on all flat roofs to a degree...there should be a slope regardless.
    If you put a ball on it, does the ball roll....with the slope ?


    Anything more than 2 days and not evaporating, AFAIK, will cause problems.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    A week is long enough to grow a new batch of mosquitoes...
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7
    DIY Junior Member modbit's Avatar
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    Ponding water is not ok. I don't know why a contractor would tell you this is ok. If you have ponding water on your roof 48 hours after a rain, then you have a problem. Hopefully, you will address the problem, before needing major flat roof repairs.

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