# Vapor barrier under deck?



## Jmayspaint (Apr 20, 2014)

I'm a painting contractor and I've done several decks over the years. It seems like aside from UVs, that moisture from underneath is perhaps the biggest factor in the lifespan of coatings applied to the deck. Also seems like big factor in accelerating rot, and mildew growth that often occurs on the underside of decks close to the ground. 

I've wondered several times if installing a vapor barrier on the ground under a deck would help mitigate these problems. We're doing one now that seems like a good candidate for this idea. 

It's a new cedar deck around a pool. The boards are installed tight together, so any moisture vapor from underneath couldn't escape through the cracks. 









It's also fairly close to the ground and the slope goes away from the deck









I'm wondering if anyone has done this, or what you think of this idea. I'm thinking some heavy plastic installed tightly on a dry day would direct water out from under the deck and prevent the exposed earth from retaining so much moisture. 

What are the pitfalls of this idea? I suppose making sure it was installed tightly so as not trap moisture under it would be crucial. What other problems could arise? Do you think this is a good idea? 

TIA.


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## heavy_d (Dec 4, 2012)

I think its a horrible idea.


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## Jmayspaint (Apr 20, 2014)

*Vapor Barrier Under Deck?*



heavy_d said:


> I think its a horrible idea.



Thanks, care to share why? Think it would trap moisture under it?

Edit: just to be clear, I'm talking about heavy plastic on the ground.


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## Jmayspaint (Apr 20, 2014)

Here's what I'm thinking in a nut shell. 

Rather than have a mound of exposed earth under the deck that would take in, and disperse through evaporation, a lot of moisture. Have a plastic liner on the ground to shed any water that got under there. Perhaps add a fan to increase air flow. Basically just keep the air space under the deck drier than it normally would be. 

Best case scenario with decks of this type is to coat all sides of the boards before install. That helps keep moisture from entering and potentially causing problems with the coating. In extreme cases, I've seen moisture from below literally push semi trans stains out of the deck boards. 

This deck is already built. Coating the bottom is possible, but I'm wondering if taking steps to reduce moisture in this air space would work in lieu of trying to prevent its intrusion into the wood by coating it. 

The only negative I can imagine would be if mildew growth was promoted under the plastic. Though in theory, if the barrier was tight enough, seems like it would be inhibited by lack of air flow. 

Here's a pic of the deck I'm talking about. 








I'm thinking the plastic could be attached to the base of the pool. The ground slopes away from it, so any water would run out of the air space under rather than soak into the dirt. 

Thoughts on this? 

Sorry, I guess it's a big nut


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## Jmayspaint (Apr 20, 2014)

Another pic of the underside


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## Tom Struble (Mar 2, 2007)

good deck guys ive known usually clean up, level and put down a layer of stone under there


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## MarkJames (Nov 25, 2012)

I don't know how you would get the plastic tight, but if you did that, you'd still end up with puddles in the plastic to make it worse. How about pea gravel or crushed stone instead? Of course, not an easy job at this point.

And fill in all the low spots, including that back post in your pic. Proper grading is key. No pooling of water allowed. On the other hand, if it's sandy soil or something else with good drainage, don't worry about it.


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## Jmayspaint (Apr 20, 2014)

Ok, that makes sense. Pooling would be another drawback to the plastic idea. I'm not knowledgable in the construction end of decks, just trying to figure out the best ways to ensure my coatings succeed. 

Coming in at the end of this project, I'm worried about the stain's chances of succeeding here. 

The tight installation of the boards concerns me. Guess they could be allowing for shrinkage, but idk. 

The deck is in full sun most of the day, already a challenging environment. Add the pool, and the dirt/mud underneath and it's an even worse prospect.


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## parkers5150 (Dec 5, 2008)

if you where doing it prior to build i would say absolutely. Not plastic though, just a matter of time before that's it's own problem. I start one on monday and it will get and EDPM liner on grade below the deck.I both recommended it and charged for it..


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## Jmayspaint (Apr 20, 2014)

MarkJames said:


> On the other hand, if it's sandy soil or something else with good drainage, don't worry about it.



It's not sandy unfortunately, clay and mud as is common around here.


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## parkers5150 (Dec 5, 2008)

if your responsible for the finish... you are signing up for a losing battle. the pool chemicals alone are gonna have your phone ringing within 6 mos.


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## Morning Wood (Jan 12, 2008)

Your idea is flawed. The deck is outside. A vapor barrier is useless. Putting down stone is best bet


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## Mark122 (Sep 27, 2014)

parkers5150 said:


> if you where doing it prior to build i would say absolutely. Not plastic though, just a matter of time before that's it's own problem. I start one on monday and it will get and EDPM liner on grade below the deck.I both recommended it and charged for it..


curious as to what problems you are avoiding using epdm vs plastic? unless the deck is completely enclosed (impossible unless its inside a building) im not seeing the benefit to any kind of liner under a deck.


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