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There appears to be a school of thought in deck building that it is worthwhile to protect the tops of joists by covering them with a waterproof barrier. That way any moisture can't sit on the top of the joists, soak in, and gradually cause rot or premature failure in pressure treated wood.

(Here's Matt Risinger's example of this).

However I've also read the viewpoint that while this practice may protect the joists it can also increase drying times for the deck boards. Assuming the decking is wood this seems like a significant risk.

I would imagine that the value of this is highly dependent on climate, the wetness of the site, solar exposure, etc. But I'm assuming the site is wet or else this wouldn't seem to matter one way or the other.

Also, though a cynical point of view is that this idea is driven by product company marketing strategy; but that itself doesn't mean it has no merit.

Is there yet any industry consensus on practices in this area? For example: building codes, studies being done as decks have aged, etc.? I'm open to global perspectives although I'm in the US myself.

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    I can't answer the question as asked, but I can say, after demolishing, repairing, and restoring many aged decks, that the tops of the joists aren't exactly an Achilles heel. If the deck is old enough that the integrity of the fastening surface is degraded beyond utility, so are the joist ends, beam connections, posts, etc. At that point, since the decking is probably being replaced anyway, it's a fairly short step to a total rebuild. Whacking one proverbial mole doesn't yield much extended deck life. – isherwood Mar 3 at 19:05
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    I'm with isherwood on this one - the tops of the joists are one of the last things that I would expect to fail on a deck. Even the decks I've worked on where the decking was completely rotted off the fasteners, the tops of the joists were almost always sound. Also, if you're using a composite decking, you'll find that most? of them have cups or groves milled into the bottom of them and don't really have a lot of direct contact area with the tops of the joists that would prevent them from drying out. – Comintern Mar 3 at 23:13
  • I'm going to throw a counter argument, however I've only rebuilt six or so decks. In all instances, The tops of the joists were rotted with the top boards, and screws. The rest of the structure was sound. So in my experience it would be worthwhile if you get the correct protection (one that will not absorb moisture, that will compound the problem). That being said, your deck with moderate care will last ten years, you'll still have to re-surface it at that point, which is why there is no consensus, it's potentially a 20 year cycle before value is seen. – Chris Mar 4 at 4:18
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There’s always “Good”, “Better” and “Best” in any construction project. I’ve never seen the tops of pressure treated deck supports protected with “peel and stick” membrane, and I live in a very wet area of the U.S. (We get about 60 - 80 inches of rain each year.)

All of the ideas he recommended in the video you referenced are great, but membrane on the top of the joists seems extreme. In fact, there is a greater chance that rot will occur at the ends of each support, (because water “wicks” into a board far more in end grain than in side grain,) than on the tops of the supports...which he doesn’t mention.

BTW, there is nothing in the Code that requires a membrane on top of deck supports.

  • I agree that the protection may be extreme (assuming you're using a wood surfacing) but it's improbable for the membrane to be worse than a net neutral. The reason the tops of the joists rot is because of the separation between the deck boards and the structure cause a point for water to be captured and absorbed. Since the two points are now moist (and protected from wind and sun), the wood has an accelerated rate of degradation. The membrane will limit the damage to the joist, with no effect on deck boards (because osmosis, more water will be distributed to points with ventillation, or sun). – Chris Mar 4 at 4:29
  • @Chris You say, “The membrane will limit the damage to the joist, with no effect on deck boards...” Hmmm...no effect??? The membrane will trap the moisture on the backside of the deck boards and delay the drying of the deck boards...even the OP acknowledges that fact in their 2nd paragraph. However, the real problem is the absorption of moisture into the end grain of joists, decking, posts, etc. I’d spend my money and time in protecting the end grain of all those pressure treated members...especially the cut ends. – Lee Sam Mar 4 at 5:09
  • this isn't a situation like moisture behind a vapor retarding membrane. The point of contact between the deck board, and the joist structure is the only points of serious rot risk. The two points are like sponges, if the top gets wet, it will wick into the bottom, and that meeting point will retain water almost indefinitely. With the membrane to separate the two points, the deck boards will still rot at the same rate, but the structure will be re-usable. If (most regions) code is followed, cut ends, and posts are a secondary concern. – Chris Mar 6 at 2:45
  • @Chris I guess we’ll have to “agree to disagree”. I didn’t say it was “like moisture behind a vapor barrier”. I said the real problem is END GRAIN. However, you say, “cut ends and posts are a secondary concern”. I disagree. More moisture will enter through end grain and develop more rot at ends of boards (including deck boards) than at sides or edges of boards. Like I said, “I’d spend my time and money in protecting the end grain of all those pressure treated members...especially the cut ends.” – Lee Sam Mar 6 at 4:55
  • I agree fully about the end grain being a prime point of degradation. My point was that there are products (end grain sealant) available to minimize the issue. Most code takes care of the posts/end grain (in Ontario you're required to seal ends that were cut, and posts are to be a minimum distance from the ground, and seated in a saddle, not cast in concrete, for example). – Chris Mar 8 at 2:21

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