Drick
In the Trades
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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?
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?