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 1×2 instead of between the decking and joist?
Best Answer
Site-built deck drainage
Here is an article in Fine-homebuilding on site-built deck 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.
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.
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