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    DIY Junior Member bjferri555's Avatar
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    Question Flat Roof Options / Advice...

    I need flat roof advice / options. Iím purchasing this home and there are minor issues with the flat roof. I never wanted a home with a flat roof but in any caseÖthere is living area beneath this roof.

    I donít want to be the homeowner that has flat roof leaks that either goes unnoticed or that require constant maintenance. Iíd love to convert it into a deck Ė or am I looking for trouble? I donít know whatís out there that is tried and true, any ideas? Am I better off having it pitched and using regular shingles or just keeping it as is? I believe it is painted on tar substance Ė itís not torched.
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    Retired Defense Industry Engineer jadnashua's Avatar
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    The best flat roof protection is probably a continuous membrane. If you decide you want to tile it, there are several methods that also would waterproof it. The roof needs some slope so stuff doesn't pool and if it can't slope to the outside, needs a drain of sufficient size, with the area sloped to the drain (similar to a shower). www.schluter.com makes a system that is designed to tile an outside roof deck, but if you don't want to tile it, and the area is walked on, you need something else. Most membranes are not designed to be walked on, but many can support an applied, floating wear surface on top of it. Flashing is critical on any roof, but even more so on a flat one, along with proper drainage.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Architect Spaceman Spiff's Avatar
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    I'd go with a continous membrane, adhered to at least 2" of foam insulation. That way if there is a cut in the membrane it still won't leak. Also, there are special pads for flat roofs so you can sit pavers on top of the membrane safely. They will protect the membrane and allow water to flow underneath, and have shims so that the paver surface is flat. I do this in commercial architecture on a regular basis.
    Spaceman Spiff aka Mike

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    DoD Army bjferri's Avatar
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    Can you be more specific about the membrane to use and 2" of foam insulation? Something I can suggest to a flat roof specialist. Thanks.
    Brian

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Sorry, but a flat roof specialist that can't offer these suggestions, is not a real specialist! Flat roofs are more of a commercial thing, and maybe search in that area verses residential purveyors.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the trades Dana's Avatar
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    Quote Originally Posted by bjferri View Post
    Can you be more specific about the membrane to use and 2" of foam insulation? Something I can suggest to a flat roof specialist. Thanks.


    Membrane roofs are exterior vapor barriers, so the foam helps protect the roof deck & rafters from rot by keeping it above the dew point of the inteior space air in winter, avoiding condensation at the roof deck, making it resilient to air leaks from the conditioned space. But the amount of foam required is climate-dependent, as well as the R value of the insulation in the rafter/joist bays.

    The dew point of 70F 35% relative humidity air is about 40F, so if you keep average temp at the structural roof deck in January stays above 40F with a 70F interior it's enough prevent moisture accumulation at the roof deck. In say, Gaithersburg the outdoor mean temp for January is ~30F so if you have R30 batts or cellulose in the rafters/joists (2x10s, nominally), you need at least R10 in foam above the roof deck to prevent condensation. That would take 2.5" of EPS (bead-board), or 2" of XPS (extruded polystyrene), or 1.5" of polyisocyanurate. In Baltimore the mean January temp is ~35F, so with R30 in the rafter bays you'd only need a minimum of ~ R5 above the roof deck, so 1.5" of EPS or 1" of XPS or 1" of iso would be enough, but you'd have more margin with 2".

    If you had R38 in the rafter bays (2x12s), that's about 25% more R, so you have to scale up the foam by ~25% (3" of EPS or 2.5" of XPS/iso for Gaithersburg, or 2" EPS or 1.25" of XPS/iso in Baltimore.)

    If the rafter bays are empty or don't have full depth batts/blown fiber, measure the full cavity depth and multiply by 3.5" to come up with a number to approximate a full fill of cellulose for calculating your foam-R, then retrofit it with cellulose. That way convection loops impede any interior air from finding it's way to the roof deck in the first place, and during the hours where it's colder than 30F outdoors and potentially condensing at the roof deck, any condensation that forms is wicked up & safely redistributed by the cellulose to dry seasonally toward the interior. If you don't want to mess up the interior with blowing holes it can be drilled & filled from above prior to laying down the rigid foam.

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    DoD Army bjferri's Avatar
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    Ahhh - commercial is perhaps the way to go. Providing I can find a commercial company that does residential. Thanks for the above technical information.
    Brian

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