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Waterproofing the underside of a deck

Hi –

I live to the north of Boston and I’m looking to replace my wood rotted deck with some type of composite this year. The specifics of this deck is that the underneath portion houses a pool filter, electric sub-panel and a bunch of outlets along with lights and sound. So, it needs to stay completely dry under there. The dimensions of this area are roughly 20’x5’.

My questions are:
1) in order to keep it dry, do I want to use some type of rubber membrane under the decking boards or some sort of water diversion system (DrySnap, or others) underneath or anything else? I've been reading that in instances like mine, certain brands of composite can lift or sag if there is little or no ventilation between the boards and the membrane. That will be an issue with my deck and I’ll mention that it’s going to get total, blistering sun during the summer months.

2) If anyone is familiar with this ventilation issue, do I still want to go with composites and maybe install a bit differently. I’m only guessing, but instead of an 1/8" gap between boards, maybe 1/4”? or other installation ideas?

3) or is PT the way to go in my instance? I haven’t read any ventilation issues with PT, but might it bend or warp as well with the underside heat?

I’ve thought about it most of the fall and winter and was leaning towards Azek boards and CertainTeed railings, now I’m not sure with all that I’ve been reading.

Any help or advice will be greatly appreciated because I’m at the point where I cannot move forward with planning this project. You can imagine, but I don't want to put the time and money into it, only to have it look crummy in a couple of years. I really need your ideas and experiences.

And if you need to know more about more situation, please do not hesitate to ask.
Thanx in advance,

Re: Waterproofing the underside of a deck

Choose wood. If you have the money, AZEK. PT is good too. There are other woods that you may consider, but be careful before you write a check.

Composition fiber (that turns into dust very shortly) - no.

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