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I'm trying to build a deck where I can use the few feet under it as storage space.

I'm thinking of using 8mil (or 10mil, if nessacary) plastic sheet or vinyl sheet between the joists and the decking. Edit: 20" wide Aluminum flashing is also an option

The plastic or aluminum sheets would have to be cut in a way that one side is wider than the other, so when secured to the parallel joists, it would form a gradual slope of excess material.

I would custom bend some aluminum flashing at 90 degrees that would drain directly onto the plastic from under the siding. Vertically, where this deck attaches to the house would be at the last two runs of the siding, and the bottom of the ledger would be lined up with the top of the concrete foundation. The concrete is about 4 feet above ground, I shouldn't need any further drainage below the deck

I would either use an oversized rim joist with a gutter at the end to catch the runoff, or place a beam in a place where I can put a gutter on it.

Question 1: Does this setup seem sane, safe, and effective?

Question 2: I would prefer to use cedar decking, but I'm not sure if the wood on plastic would trap water and cause rot issues. Can I use real wood here, or must I use composite?

Question 3: What would maintenance be like for this system? Would there be potential ice issues in the winter?

Question 4: Would I be better off sandwiching the plastic between the joists and a layer of PT 1x2 instead of between the decking and joist?

diagram

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My first concern when I saw your question was the plastic trapping water agains the underside of the deck and causing it to rot. Your solution alleviates my concerns somewhat since you included a method for drainage, but I'm still a little worried about the plastic sandwiched between the joist and the decking never drying out. What about building a separate "roof" suspended just underneath the deck joists? –  Doresoom Mar 20 at 15:37
    
The plastic has a much shorter lifespan than your deck, it's a temporary measure at best. The right thing to do is to slope and sheath the deck and use an appropriate waterproof deck sufacing. –  bcworkz Mar 20 at 17:27
    
@bcworkz It wouldn't cost much more to use 20" aluminum flashing instead of plastic, actually. What's your opinion on using that? The available waterproof decking is overpriced, ugly, and uncomfortable in my opinion. –  kavisiegel Mar 20 at 18:46
    
I really think using plastic and gutters is a very bad idea. i can see nothing but problems. I like Doresoom's idea of a separate roof under the deck. –  shirlock homes Mar 20 at 20:37
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Regardless of whether you do this or not, you should flash above your ledger and slope the deck away from the house a couple degrees. Here's a good picture of an unflashed ledger: inspectapedia.com/exterior/Deck_Collapse002_DGs.jpg Also - don't ever use aluminum in contact with pressure treated lumber, ACQ is highly corrosive to aluminum. –  Comintern Mar 20 at 22:57
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1 Answer 1

up vote 2 down vote accepted

Site-built deck drainage

Here is an article in Fine-homebuilding on site-built deck drainage.

Rubber roofing drainage

In this article they use EPDM rubber roofing material.

I wouldn't hesitate to use this or creative alternatives, but I wouldn't consider this as a "waterproof/roof" system. I would consider it rain-proof where it diverts rain and rain-runoff from underneath the deck.

Retail deck drainage

There are products available that are designed for this as well. Like Timbertech Dryspace for example.

Timbertech Dryspace

My opinion

Using either a site-built or retail product, I still wouldn't store anything under there that would be ruined from getting a little bit of moisture now and then.

If you want a real waterproof roof, then build it as a code-approved roof with code-approved materials installed how they were intended to be installed.

Issues

One potential issue I can think of right off the bat is that dirt and debris will collect in there, so do you have a plan on cleaning this out? You will need access from time to time to at least stick a hose at the highest part to rinse it out.

Another potential issue is that this would limit the airflow and potentially increase the humidity between the decking and drainage material. This could contribute to things like cupping or premature finish failure on wood decking.

Real Roof (info as requested in comments)

60mil vinyl can be considered an approved roofing surface. You would use something like this if you are not putting another surface over the deck.

enter image description here

Another roofing option (if you wanted to add wood planks) would be to do flat roof (i prefer a torch-down roof): then build panels out of your deck surface, or attach the decking to sleepers (see image below), or use wood tiles

Torch-down sleeper deck

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Thank you for the link to the fine homebuilding article! I knew this setup must have been used before. Rubber is a great idea. I'm going to be storing things like lawnmowers and chainsaws under there. I'd guess the climate of under a deck on par with a shed at least, would you say that's reasonable? I'll also be installing some deck skirting, to prevent sideways rain –  kavisiegel Mar 21 at 16:30
    
Regarding the REAL waterproof roof deck, do you have any links to anything on that? I guess I'm just intimidated by it. As far as the issues.. The article with the rubber suggested srewing in the last few planks so they're removable to allow cleaning. I'll likely do that if I do this. Airflow.. I can see that as an issue too. I suppose a solution for it would be a more dramatic slope of the rubber material? –  kavisiegel Mar 21 at 16:35
    
real roof info added to answer –  fungku Mar 21 at 16:54
    
there is not much you could do to help airflow other than maybe increase the gaps, or add some ugly venting. Just research the decking and finish materials to see if there are any clearance or airflow requirements. I wouldn't worry too much if using western red cedar, but the choice of stain/finish would be critical and need to read the technical data sheets. –  fungku Mar 21 at 17:00
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