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  1. #1

    Flat roof, rooftop AC Ducting question

    looking at an older house in Las Vegas (built in the 70's). the house has a flat roof and 2 rooftop ac units. It appears that the house originally had only one unit, with the second unit added later. The issue is that when they installed the second unit they installed the ductwork in the ceiling of the kitchen. the house has no attic so they create a drop ceiling for the duct work. The end result is the kitchen now has a 7' ceiling. My question is would it be feasible (and advisable) to remove the ductwork from the drop ceiling and relocate it to the roof (adequately insulated if possible)? I know no prices here but wonder the ballpark cost of such a project?

  2. #2
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,170
    they could have furred down just portions of the ceiling rather than dropping
    the whole ceiling...but it depends on configuration of your house.

    I would NEVER put ductwork where it is exposed to direct
    sun. IMO there isn't enough insulation to compensate for the
    heat.
    again..not knowing the config of your house it is difficult
    to know.

    don't know exsimated costs as it varies from area to area.

    and would you really want to cut several holes into the flat roof?
    I'd run away from that..or have you hire a professional roofer
    and require a disclaimer releasing me from water leaks when
    they happen. and the will..just a matter of time.

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  3. #3
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    You are going to have a lot of heat transfer to and from the desert environment... You might have to upsize your unit to meet the losses in cooling and heating. It is do able but it won't be cheap, then again re-ducting the inside might cost around the same. I would recommend relocating your ducts into soffits and walls. Also see if the ducts at rectangular... you can reduce the height and increase the width of the ducts.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  4. #4
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,633
    Put a gable roof on it and move the units,wont be cheap but will increase the value of your home

  5. #5
    thanks for the replys, it seems like the best solution my be to put a gable roof over the existing roof and running the ductwork as needed. not a cheap option but probably the best solution. the weird thing is that the problem is only with one of the 2 rooftop units that created the drop ceiling in the kitchen, the other is run in the walls.

    if we were to do a gable roof would we keep the one part rooftop units we have and just leave them under the roof?

  6. #6
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,633
    You would have to move the unit to the outside of the new roof

  7. #7
    Join Date
    May 2012
    Location
    San Diego, CA
    Posts
    352
    Quote Originally Posted by jweddylv View Post
    thanks for the replys, it seems like the best solution my be to put a gable roof over the existing roof and running the ductwork as needed. not a cheap option but probably the best solution. the weird thing is that the problem is only with one of the 2 rooftop units that created the drop ceiling in the kitchen, the other is run in the walls.

    if we were to do a gable roof would we keep the one part rooftop units we have and just leave them under the roof?
    If the zone doesn't need more than 4-5 tons, maybe a ductless split would be cheaper than a new roof. I don't know what your house looks like but many architects are designing flat roofs to look more modern.
    You can call me Sam

    It should be a crime to be a mechanical engineer in San Diego
    Summer Design Temperature: 83 F Dry Bulb ~ 69 F Wet Bulb (California Climate Zone 7)

  8. #8
    Join Date
    Aug 2010
    Location
    West Monroe, LA
    Posts
    1,532
    Quote Originally Posted by jweddylv View Post
    looking at an older house in Las Vegas (built in the 70's). the house has a flat roof and 2 rooftop ac units. It appears that the house originally had only one unit, with the second unit added later. The issue is that when they installed the second unit they installed the ductwork in the ceiling of the kitchen. the house has no attic so they create a drop ceiling for the duct work. The end result is the kitchen now has a 7' ceiling. My question is would it be feasible (and advisable) to remove the ductwork from the drop ceiling and relocate it to the roof (adequately insulated if possible)? I know no prices here but wonder the ballpark cost of such a project?
    I would look at another route as all the pros are stating. Metal ductwork on your roof will not look good and will lead to problems. I have seen 1st hand and never looks or works well for long periods. Mini split might be the way to go due to the flat roof.

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