# DIY + cost effective under deck waterproofing a for a dry patio space



## thompwa (Aug 26, 2019)

When we bought this house several years ago, I knew I would end up needing to replace a good portion of the deck. There was a lot of rot and poorly built additions/different levels. It just needed to go.

I recently undertook a complete demo/rebuild of this deck area and in the process, created a weatherproof "dry" deck/patio area.

Figured I would capture how I did that on the cheap in case anyone else was interested in doing something similar. If you don't care about the process and just want to see the finished result, scroll to the end.

A few pics of the deck from the MLS photos when we bought the house:







I have seen several folks use the Trex Rain Escape System  and DekDrain System for a number of years and it seemed like a good solution for what I wanted to accomplish. Main problem I had with these systems was the cost for the kits. For my deck, those kits would have run me about $2000. Everything related to the waterproofing I did in this post cost me right at $235 in materials(minus the gutter which I would have had to have either way).

So, like all good projects I started with some thorough demolition. I want to say in total, I hauled just over 12000lbs of debris away spread over about 5 loads in the dump trailer with all of this.







As a part of demo, I also had to remove a small tree that was in the way of where we wanted to pour the new concrete pad. In this case, I chose to dig up the old tree with a mini-ex and remove the complete root ball as to not leave a stump to rot in the ground and create a void.





Once all of the debris was cleaned up, I had some guys come in and pour a 45'x18' concrete patio. Since there wasn't great access to back the truck up to the pad and I wouldn't have wanted them tearing up my grass anyway, we pumped the concrete from the driveway.



*Now for the deck*. We were fairly limited with the size/shape of the upstairs part of the deck. The old cantilevered deck that was there when the house was built is accessible from two upstairs bedrooms so it had to be put back much the same way it was before. We almost never use the upstairs deck, but if nothing else, I figured that it created a good "roof" to keep me cool and dry underneath. I chose to cut off the old redwood cantilever joists and go back with a ledger in this case. The joists were in marginal shape, and probably could have been reused but since everything else was going back new, I figured I might as well go back new there too.

Rebuilding took about a week. I went with 2x8x8s for the depth, 2x12x12s for the double beam, and 6x6x12s for the posts. _More on why the 2x12 beam is important later._ The overall deck dimensions ended up being about 42'x8'.





Once the deck was framed, I got going on the waterproofing. For materials I went with a 50ftx10ft roll of 40mil EPDM roofing material, a few rolls of EPDM joist tape, and Cap fasteners.

Step by step:
I started by rolling out and cutting "fan shaped" pieces out of the EPDM roofing in order to create the sloped troughs between each joist bay. The joist bays are 16" on center and I chose to do 4 joist bays at a time so I cut sections 4x16.5" on center on the high side and 4x19.5" on center on the down hill side by the length of the joists - about 8'.




I just laid the fan shaped cutout over the joists and drooped the membrane over the joists. I would tack each one with a few cap fastners once I had it in the right spot and work my way across until each "trough" was the right slope at the top and bottom.



I repeated this process, overlapping the joint by a bit at each 4th joist bay until the entire deck top was covered. and then created a 1' wide continuous strip from some of the leftover material that I used as "flashing" adhering with glue and cap fasteners to the back band against the house.



Once the membrane was down on top, I wanted to create a "backsplash" that would channel the rainwater down into the gutter below the deck. For this I cut sections that were about 21" wide by 15" long and wrapped them around the inside of the beam.



The bottom part of this flap hangs down into the gutter that gets mounted to the back side of the beam.





Next came the gutter install. I hired a guy to hang a seamless gutter because I was scared the section gutters from a bigbox store would leak like crazy. We tucked the ~40' section of 6" gutter behind the flaps and fastened it to the back side of the double 2x12 beam. The 2x12 mixed with 2x8 joists allows a gutter to nest right below the joists with enough slope to do the job. I dont have a great picture of this, but you can see how it was installed for the most part in these pics.





one of the last couple of steps on the top side (before screwing down the composite deck boards) was to close up the seams. I went with 3" wide EPDM joist tape with a plastic backer paper for this. I ran lengths of this joist tape across every joist and any exposed wood or seam. This really made all of the difference. The joist tape has a self healing quality to it so when the screws penetrate the tape into the joist, it should eliminate most if not all of the water that could drive down into those holes.



The last step up top was more precautionary than anything, but I decided to go with it at the end. I decided on the wide section of the each trough to cut a "V" shaped groove in the end of the membrane to allow any small debris that was to get between the deck boards to wash through and into the gutter without any trouble. Unfortunately I didn't take a pic of this part but each "V" cut was about 2" wide and 1" deep. This seemed to be sufficient as pollen, leaves, and other small junk collected. It washed right out with each rain.

For the under side, I chose to tap into the exterior wall mounted light to add a few waterproof LED can lights under the deck. This certainly isn't necessary, but I figured I might as well while I had it opened up. This is the kind of thing that would be significantly more difficult if I waited until later.

As you can see in the pic below I chose to just run vinyl soffit material under my deck to hide the joists and waterproofing. I figured that was the safest bet in the event that it does develop a drip or some water splashes up from the gutter. Luckily we've had quite a few big rains lately and I've been able to test this solution at each step. So far I've been really impressed. I haven't noticed any issues so far (knock on wood). Even if I do get a leak at some point in the future, it should be fairly easy to isolate and fix (or live with). Assuming that would happen drove me to use the materials I ultimately went with anyway: pressure treated lumber, EPDM, waterproof LEDs, etc.

Here's the final pic from under the deck. It turned out pretty well and I'm happy with it. Now I just need to touch up paint, clean up the patio and get it all set up for summer



I'm still working on finishing up the hand rails, so I'll update once that's done but here is a pic from a few weeks back when I was starting to put the deck boards on.


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## gm560 (Feb 22, 2018)

Looks good. I am currently building a deck and am doing a DIY rain escape as well. I bought a pond liner on amazon that was 45 mil thick. It was a warehouse deal so it was only $60. Sweet deal. I will cut it into strips like you and cover seams with flashing tape. Estimate for the trex stuff was over 1500 and ill be in for like $120. DIY is a beautiful thing.

I will be using your post as a guide as I complete the job, thanks for posting!


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## dfw_pilot (Jan 28, 2017)

Wow. Awesome project and fantastic write up.


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## thompwa (Aug 26, 2019)

gm560 said:


> Looks good. I am currently building a deck and am doing a DIY rain escape as well. I bought a pond liner on amazon that was 45 mil thick. It was a warehouse deal so it was only $60. Sweet deal. I will cut it into strips like you and cover seams with flashing tape. Estimate for the trex stuff was over 1500 and ill be in for like $120. DIY is a beautiful thing.
> 
> I will be using your post as a guide as I complete the job, thanks for posting!


Nice! Good luck with the build! Had not thought of using pond liner. That will probably work just as well assuming that both sides of the material are the same. I was shocked at how fast that epdm went down on my deck. Working alone, I think I had it all capped down In an afternoon.

The worst part of mine by far was the demo lol


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## gm560 (Feb 22, 2018)

Digging footings is pretty brutal too.... oh and getting building permits during a pandemic...


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## thompwa (Aug 26, 2019)

gm560 said:


> Digging footings is pretty brutal too.... oh and getting building permits during a pandemic...


Lolol @ building permits...yeah. "Forgot" that step.


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