Waterproofing options for a deck over walkout area below

What waterproofing options for a deck over a walkout area below are there?

I have a large upper deck over a walk out basement and would like to be able to utilize the lower area below the deck as an additional area but need it to be protected from water, rain etc. The upper deck will allow water to run through it thus making it unsuitable for use.

What options exist for 'waterproofing' the deck below? I thought about putting zip board over the main deck floor joists, then laying rubber before putting down the deck but have concerns that the water will still seep through the deck boards and be 'stuck' between the decking and the rubber.

Editor's Comments

The need to waterproof a traditional deck board style deck has been around for a long time. And there have been a number or interesting solutions that have come to be.

In the pacific northwest where rain is so prevalent for a good part of the year, homeowners have largely foregone the use of 2x6 decking and gone to plywood substrate decks with waterproof vinyl flooring membrane welded together. It's a proven solution in really wet areas created dry spaces below but also getting around rotting deck boards.

For those of use who still like the look of traditional decking the challenge exists where water will drip between the boards rendering the are below not usable during or after rain.

I would not do what you are considering. That will be a recipe for problems. The deck boards will remain wetter and subject to a higher risk of rotting faster.

Some of the solutions that have emerged are extruded aluminum interlocking boards, and joist bay water collection and drainage systems. Let's take a look at each.

Interlocking Aluminum Decking

These are extruded aluminum boards that interlock and click together created and tight and seamless look. They still have the appearance of deck boards and in fact some have coatings which very closely match wood grain.

The are quite effective at shedding rain to the end of the deck where water is collected by a common eaves trough and diverted to a down pipe and carried away.

They last forever but are more expensive and you may want to ask and see what it sounds like when rain hits the surface? Does is sound like rain hitting a tin roof? I do not know the answer and I doubt its a big deal but might be worth asking about. Also how hot does aluminum get? Again I don't know but I do know the dark composite colors of decking can get very hot.

Joist Drainage Systems

This is probably what you should most carefully consider. I am thinking of solutions like Dek Drain which are really well done and allow you to keep the traditional deck board design look. There are other copy cat systems emerging but Dek Drain seems to me to be the original and market leader.

Trex has a system called Rain Escape also. So there are some options for you to consider. Both of these look like good systems.

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