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flat roof

CaptainMayhem | Posted in Construction Techniques on March 13, 2009 06:20am

So, this summer I am finally going to build a home for my family. I think that I have my design figured(we’ll see how it changes with the prying eyes of an engineer)and I think that I would like to go with a flat roof with a concrete topping pad so as that will be my driveway/ parking. The lot drops about 3 or feet every 12 or so. So it would set right into the ground with the rear wall completely below grade and the front completely above grade. One floor. 32×32. I’m thinking that 16″ I joists spanned 16′(rested on some sort of center girter with posts) on 16″ centers would be more than enough, however the engineer may require a deeper joist to support roof slab. I am confused about my insulation air space in the roof though.. I plan on dropping 2″ of blue foam in the bottom of each joist bay and then r-38 fiberglass above that..Since it is a flat roof, do I vent it? My gut says hell yes, but I have read alot of articles saying not to..which doesn’t make sence to me at all, but I want to do this right…As I said before, I am going to take the plan to an engineer first and I’m sure he could tell me, but I want to have all my “facts” straight first..

All I ever wanted in life was an unfair advantage…

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  1. DanH | Mar 13, 2009 06:29pm | #1

    I think a bigger issue than ventilation per se is how you're going to handle moisture migrating downward from the slab. Ventilation may be required to dissipate that moisture, depending on the specific schemes employed.

    The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
    1. User avater
      CaptainMayhem | Mar 14, 2009 08:32pm | #11

      bituthane/grace ice and water shield will go the distance as far as stopping water infiltration. Obviosly bituthane on my back wall and what ever portion of the side walls that are below grade and the ice and water shield over the roof deck, under the concrete slab.

      As far as the roof vents go, it won't be a "built up roof"( which incorpoprates tar on foam on tar on foam etc. etc. It will be a standard fiberglass affair. So, still omit the vents?

       

      I know it would be a pain in the butt to shovel my roof every time it snows, but, I have to shovel my driveway every time it snows already, so, that's not a big concern.

      The rail system will be anchored to the walls and will be custom ornamental iron installed by the company who made them( a good friend) and will hold warranties and guarantees against any such failures/malfunctions..

      All my plumbing vents and heating stacks will be directed near the edge of the roof and honestly, I'm not that worried about the seal..Henrys has worked on every application I've seen it used on, and a tube or two goes a long way...

       So back to my question....Do I vent given that it's fiberglass and not foam?All I ever wanted in life was an unfair advantage...

  2. fingersandtoes | Mar 13, 2009 06:31pm | #2

    No need to vent the flat roof. The membrane stops all air movement so moisture can not infiltrate the insulation.

    The areas that should be thought through carefully are how you will handle plumbing vents, anchoring a railing through the roof, and the membrane at rear wall of the house that is buried - that is how are you going the detail the membrane where it terminates below grade.

  3. Piffin | Mar 13, 2009 08:26pm | #3

    this si sounding like an underground house that has a lot of interrelated issues, but you seem to be dealing with them one at a time.

    You present a special atypical case here but the whole house is part of the question and answer, leaving us like a blind man with one hand on the elephant.

     

     

    Welcome to the
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    1. DanH | Mar 13, 2009 09:57pm | #4

      TAKE .. YOUR .. HAND .. OFF .. OF .. ***THAT***!!
      The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

      1. User avater
        IMERC | Mar 13, 2009 11:25pm | #5

        HEY!!!! let the elephant have good time.... 

        Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming<!----><!----><!---->

        WOW!!! What a Ride!

        Forget the primal scream, just ROAR!!!

         

        "Some days it's just not worth chewing through the restraints"

        1. Piffin | Mar 14, 2009 01:51am | #6

          LOL 

           

          Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

    2. User avater
      CaptainMayhem | Mar 14, 2009 08:36pm | #12

      Like what? Help a brother out... I am still at the point where it's not too late to change plans, however I like the idea... I have built plenty of houses with full basements, either block, icf's or framed with a bituminous layer to the exterior..Could you list some more "issues" to concern myself with?All I ever wanted in life was an unfair advantage...

      1. Piffin | Mar 14, 2009 10:18pm | #13

        I'd have to see your plans and photo of site. Your description of this project is pretty ambiguous. My mind can picture lots of types of houses that fit your description. I'd rather speak to actuall than to what I imagine here and send you off on the wrong trail.But the basic answer to your Q is no venting needed.From your last post it sounds like you plan to rely entirely on I&W and Henry's to keep water out of a house though, so that tells me you will need a way to evacuate running water, not just airborne moisture. 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

  4. cliffy | Mar 14, 2009 03:15am | #7

    I think a flat roof on a house that will be bombarded with snow  for half the year is going to be a lifelong pain in the but.   That is just an opinion of course.

    Have a good day

    Cliffy

    1. Billy | Mar 14, 2009 04:34am | #8

      True, but he's going to have a heck of a time parking on a pitched roof... 

      I agree it may not be worth the time and money and trouble to build a roof he can park on.

      Billy

      1. DanH | Mar 14, 2009 04:36am | #9

        I think the thing to do is to build the roof and then build the house under it.
        The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith

        1. Billy | Mar 14, 2009 05:15am | #10

          Hey, that's the picnic pavillion approach!

          Maybe he should do what they do on the coast -- build the house on stilts and park underneath.

          Billy

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