Rebuilding a Cistern Roof - need ideas.

Discussion in 'Remodel Forum & Blog' started by drick, May 1, 2019.

  1. drick

    drick In the Trades

    May 16, 2008
    I have a 10ftx10ft cistern that is used for irrigation only. It has a flat roof constructed out of 2x8's, trex decking and an EPDM roofing membrane over that. There is no ventilation into the cistern which kept the light and bugs out and mostly kept it from freezing during the winter, but after 10 years 2x8's are rotted from constant exposure to evaporating moisture in the cistern (its like a rain forest on the inside). My initial plan was to just replace the 2x8's with plastic lumber but it was cost prohibitive.

    I need some way of keeping the moisture away from the 2x8's and the underside of the roof. Ideally I want the water in the cistern to stay a hair over freezing in the winter as well. My plan is to replace the rotted 2x8's with pressure treated 2x8's and screen the open ends for ventilation. For roofing I'll reuse the trex (so far its held up to the water) and a new 60mil EPDM membrane. Then below the joists I would use 2 inches polyiso to help keep the water in the cistern from freezing. My problem is I'm unsure of what to use for a moisture barrier below the polyiso (just over the water)? I'm thinking maybe 45mil EPDM but I'm worried about how I would go about sufficiently attaching it without defeating its waterproofing ability. I'm thinking something more rigid would be a better way to go but I'm not sure what to use. Thoughts?
  2. James Hughes

    James Hughes New Member

    May 6, 2019
    I am not a professional but I can tell you that it is risky, so I would recommend you to visit this site for roofing advice and contact them online. I had the roof damage problem because of the winter snow so I hired them for the work.
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  4. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    I was thinking 10 mil or greater polyethylene. Stapling to the middle of the bottom of the joists. For the top don't have a vapor barrier, but maybe house wrap or roofing material that lets a little vapor through -- more than the bottom layer. So the joist wood gets to dry out over time.

    I was thinking about material that might be used right under concrete when pouring a house slab. Reading up, I found references to poly being a vapor retarder vs a vapor barrier.

    That EPDM sheet sound good.

    How about FRP? I don't know its permeance. It looks pretty good. Commonly comes in 4x8 sheets.

    Teks roofing screws have a self-sealing neoprene washer.

    I don't propose to have the answer. Just discussing.
    Last edited: May 9, 2019
  5. drick

    drick In the Trades

    May 16, 2008
    Update: Reach4 your FRP idea got me thinking. I decided to use cement board (cheaper than FRP) under the joists and apply an acrylic primer and paint to make it more or less waterproof. I kept going back and forth between the poly or the house wrap over the cement board. In the end I went with 6mil poly thinking that any trapped moisture won't hurt the cement board. I'll admit I'm sort of regretting that now but its done so I'll see how it all holds up. Thanks for your help Reach4!
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