I have been researching composite/PVC deck boards and the issue of how many sides are capped seems to not get enough attention, at least in the technical sense.

What should I think about with regard to the choice between 3 and 4-sided capping? All I have found is talk of looking up at it from below. In my situation, while I am planning a 2nd story deck, it will have a between-the-joists drainage system, so you will not see the underside of the boards. Does the fact of the drainage system mean I should favor 4-sided caps because of the moisture? Or are there other factors to consider?

  • Moisture isn't really relevant with synthetic decking. It's entirely about appearance.
    – isherwood
    Commented Mar 15, 2021 at 13:44
  • Can you explain or provide pics for what 3-sided & 4-sided capping is? That's a phrase I'm not familiar with. Are you referring to a "breadboard edge" where a board is run along the edge perpendicular to the decking as a finish detail?
    – FreeMan
    Commented Mar 15, 2021 at 13:49
  • 1
    @FreeMan seiffertlumber.com/…
    – MonkeyZeus
    Commented Mar 15, 2021 at 14:11
  • Thanks @MonkeyZeus. Interesting. I've not looked into plastic decking, but I'd have assumed it would last pretty much forever. Also, all the stuff I've noticed at my local big-box has always had a "wood grain" texture, but I never paid enough attention to notice if it was 3 or 4 sides. If the core material isn't good enough to stand up to weather, sun, etc. on its own, what the heck do you do about the "end grain"?
    – FreeMan
    Commented Mar 15, 2021 at 14:27
  • 1
    @FreeMan The texture is merely vanity so ignore that and concentrate on material. Unless you switch to 100% polymer, the typical composite decking is 20% wood fiber so the less of it easily reached by the elements, the better. Wood fibers will gladly decay inside of composite decking much like rebar rusts inside of concrete. It will take a few decades but over time enough of the fibers will decay leaving you with flimsy feeling composite deck boards.
    – MonkeyZeus
    Commented Mar 15, 2021 at 14:37

2 Answers 2


What factors go into choosing between 3-sided and 4-sided capping for composite deck boards?


Cost and how long do you want it to last.

I'm not a decking expert but composite decks with 20% wood fiber content are still prone to rot, albeit at a slowed pace. What you end up with is something like this: https://www.youtube.com/watch?v=ahiI6WTumB8

By having all sides capped you reduce the surface area for environmental variables to affect your boards.

If you switch to 100% polymer then you simply get to enjoy the longevity of the plastic and not worry about the wood filler. These too will degrade over time as nothing is safe from the sun. Maybe a stainless steel deck will provide the greatest longevity but you'd have to consider the cost.

One of the most important things to consider with any deck is joist spacing. If your joists are closer together then your deck will feel less spongey as the years go on. Also, if the span calls for a 2x8 joist then consider using a 2x10 or 2x12 if you really want it to feel sturdy.

Keep your deck swept, washed, relatively dry, and don't let microbes deteriorate your deck over time.

  • 2
    Stainless steel deck fails on "comfort" big-time. Might do for frying eggs in the sunshine, though, if that's something you want to do on your deck surface.
    – Ecnerwal
    Commented Mar 15, 2021 at 16:35
  • 1
    That video doesn't make sense. Wood would've fallen apart long before the synthetic decking did. What was his point?
    – isherwood
    Commented Mar 15, 2021 at 16:48
  • @isherwood My point in showing the video is to show that nothing lasts forever. Maybe I should find a less biased video as I do not endorse that person's message. I can place a disclaimer in my answer to watch the video without sound and closed captions ¯\_(ツ)_/¯
    – MonkeyZeus
    Commented Mar 15, 2021 at 16:51
  • @Ecnerwal I'm not opposed to cooking eggs on my brand new stainless steel deck :)
    – MonkeyZeus
    Commented Mar 15, 2021 at 19:43

Trex coats three sides of their product. TimberTech coats all four sides. Trex believes that leaving the bottom side open allows for moisture to escape. TimberTech believes their 'four' side coating prevents moisture from getting in. Who knows! It is not visible as to which one is used. It has to do with handling 'moisture'.

  • This is about pvc decking Commented Aug 2, 2023 at 2:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.