Decks/Sealing a deck

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Question
Hi,

A few things about the deck-to-be: It is more of a utilitarian work surface and platform for sheds. Also, I live on the Windward (rainy!) side of O'ahu, Hawaii. Deck framing will be conventional.

My initial idea was to finish the deck with traditional planks, but then I started thinking about using plywood instead and sealing it with a product like Liquid Rubber, Flex Seal or similar. I like the idea of the deck being textured. I also like the idea of completely sealing the surface. But my concern is, where does the water go? With planks, the water drains easily between the cracks (but then perhaps gets trapped and puddles and creates a problem for the piers?). It seems to me that sealing a plywood deck with Liquid Rubber will create a monolithic surface with nowhere for the rain water to drain.

I suppose I could take the best of both ideas and seal planks with Liquid Rubber, but brushing it onto plywood seems easier (and cheaper?).  

Any insight is greatly appreciated :)

Answer
Hi Shaun,

I must say I am skeptical of using plywood for the flooring of an un-roofed deck and even more so of sealing it with the products you specified.  There are specific and legitimate reasons why decks are constructed and finished using standard methods and products.

Treated wood posts and select framing are rated for ground contact and will resist decay even when exposed to persistent moisture.  All other treated materials are rated for above grade use.  If these materials don't dry quickly each time they are wet they will decay just like untreated wood, only at a slower pace.

Treated plywood is somewhat weather resistant but it is still venerable to decay if exposed to persistent moisture. Even if sealed, plywood will still absorb enough moisture (at the joints and from underneath) to cause it to expand and contract with changing weather conditions.  This movement will cause even the best coatings to crack and loosen at the joints between the plywood panels.

Additionally framing lumber is never consistently straight. Even carefully chosen top grade lumber will be slightly bowed, warped or twisted.  This is not a problem in a traditionally floored deck but a solid plywood floor will almost certainly have low spots in which water will pond.

Wood is porous and easily absorbs moisture, resulting in almost constant expansion and contraction of the wood as the weather changes.  This movement of the wood along with it's changes in moisture content tends to cause moisture to collect under the applied coating.

Most deck coatings are actually formulated to allow moisture to evaporate through the coating.  Products such as liquid rubber and flex seal are not formulated this way and will actually trap pockets of moisture.  This will result, over time, in the coating loosing it's bond to the wood as well as increase the likelihood that decay will occur in these same areas.

If you are adamant about a plywood floor it should be a minimum of 3/4 inch thick which will be sturdy enough when completed but I am unsure how long it will hold up before it starts to have structural problems.  I would also recommend a more traditional coating formulated for wood decks.  

For best results I recommend using traditional treated framing and treated or composite deck boards.

I hope I have been of some help in this matter. Good luck with your decking project.

Tony  

Decks

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Tony Wood

Expertise

I can answer questions pertaining to deck design and construction and offer insight into the pros and cons of various designs and materials options.

Experience

I am a life time resident of North Carolina with over 30 years experience in multiple facets of the industrial, commercial and residential construction industry. For the past 23 years I have owned and operated Wood's Home Maintenance Service, providing services primarily in Johnston, Wake and Sampson Counties of North Carolina.

Organizations
NFIB

Publications
Several Home improvement articles published on Angie's List and other online forums.

Education/Credentials
High School graduate ~ proficient in CAD (computer aided design), Open Office program suite.

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