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EPDM-based under-deck drainage system

26390 Views 5 Replies 3 Participants Last post by  robertcdf
Hey all,
I've been researching under-deck drainage for some time. I have a simple 16' x 20' deck and am finishing the underneath to support a paver patio and a shed.

I've decided against systems like dry-snap and underdeck because they cost about $8/ft installed and $3-6/ft in parts. My wife nixes the idea of sloped corrugated panels as being unattractive.

I came across this solution, which seems like a nice one:

The deck is old and the boards are in bad shape, so pulling them up and laying new boards isn't an issue. The deck itself is flat, so something that puts the grading between joists is attractive, especially since we were hoping to not lose height under the deck (we have some taller things to put in the under-deck shed).

But I have a few questions and am hoping someone can help me out here:

1. All I can find around here (Ellicott city, MD) is EPDM liners. Is a 45mil EPDM liner as good a product as a polypropylene geomembrane?

2. The deck has two sets of steps with a ~ 4'x4' landing between them. I am considering just laying EPDM flat over the steps and landing w/o drainage. Is that a terrible idea? Not really sure where I would drop a gutter for steps.

3. Would likely wind up with an aluminum siding as the "ceiling" under the deck. Perhaps a fiber cement board. Any other good options?

Again, I've spent a few days searching this and other forums, but would welcome any additional advise here. Thanks in advance!

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Hey all,

1. All I can find around here (Ellicott city, MD) is EPDM liners. Is a 45mil EPDM liner as good a product as a polypropylene geomembrane?

I realize this is an old thread, but I have the same question. Did you ever get any information on the EPDM vs the polypropylene geomembrane? Also, the site links to a manufacturer but doesn't say specifically which product is being used.

This was from a while ago! Glad I left thread notifications on. :)

I finished the under-deck waterproofing and it has been working without issue since last August. That isn't a very long time, but I think we'll be good.

I'll summarize all of my lessons learned in case this helps anyone in the future:

1. The best thing to do is put a roof over your joists. Lay some sheathing at an angle for run-off and coat it in EPDM rubber or use a torch to melt product (torch-down roof). Cut runners at the opposite angle, and then lay your deck boards over the runners for a flat surface. I couldn't do this because my joists were almost exactly 2.5 inches below the sliding glass door, so there wasn't room for decking, sheathing, etc... and I didn't want to pull the ledger down and start over.

2. EPDM is fine for this application. 40 mil EPDM is pretty cheap, I purchased 20' x 40' sheet of the stuff, with 10 tubes of EPDM sealant for less then $400. They make heavier EPDM, but it is unnecessary and the additional weight pulls on the draping between the joists.

3. There is a company called DekDrain ( that uses an EPDM solution that they have patented. I talked to them on the phone and they use standard EPDM, but they pre-cut it for you to their specification. Since they have a patented design, you can't hire someone else to repeat their design more cheaply. I wound up not repeating their design based on some of the weirdness in my deck layout. They ship you pre-cut materials and you do the installation yourself. All of their instructions are available on-line. I believe for my deck their system would have cost me ~$3000, but I can't remember.

4. Almost all of my research after this original post led me to believe that systems that screw or otherwise adhere between joists leak over time. Joists expand and contract, materials lost plasticity with temperature extremes, etc... the EDPM rubber approaches all share something in common: lapping the rubber atop the joist, not trying to keep a waterproof seal against the side of the joist. In my very amateur opinion, that's the difference between leaking and not leaking.

5. It is harder than it should be to get a good slope. My joists were spaced weirdly: some 8" on center, some 22" on center, and no two the same. So measure twice, cut once.

6. Do each joist individually. Don't try and lay large sections over multiple joists and cut in-place. It is ridiculously hard to do. Besides, having 2 or even 3 layers (cap piece) over the top of each joist makes a bit of a gasket.

At the end of the day, the waterproofing cost about $500 in materials and maybe a full weekend of work. The whole refresh for me was maybe $2k and 3 weekends, but I needed to replace all my railings, all my deck boards, all of the rim joists (my rim joists were toe-nailed into my ledger and nothing else), and some joists were missing hardware.

Hope this helps.

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We use this same system on a number of decks each year, the most critical part is making sure that water coming down the house wall is transferred to the EPDM joist system. I use 40 mil EPDM for all of the projects we do too and it works great. We use contact cement to glue it to our steel frame system and then to itself, it also works great (a little time consuming).

Where is the best place to get the EPDM? I see a lot of pond liners, but that route appears far more expensive.
Where is the best place to get the EPDM? I see a lot of pond liners, but that route appears far more expensive.
A real roofing supply house, however they might not sell to someone who doesn't have a business.
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