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.
Thanks!
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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 (http://dekdrain.com/) 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.
-Ed
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 (http://dekdrain.com/) 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.
-Ed
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