Rot caused by bituthene, Grace Vycor, storm shield, etc above deck joists?

CDragon

Can anyone point me to a long-term study or any kind of personal experience that sealing the tops of outside decking joists with a self-sealing membrane extends the longevity of the joists? In another thread, someone said that using a membrane is "said to prolong the life of the joists from 20 years to up to 40 years", but where did that figure come from?

I've been searching off and on for a number of days and while a lot of membrane-selling sites and contractors say they sell their customers on adding various "self sealing membranes" above the joists, I can find absolutely no proof that it helps. Even my neighbour who used to be in construction used a membrane over his new deck joists, so a lot of people are convinced it works, yet nobody seems to ever go back and check what actually happens to the deck in 5-10+ years. In fact, the only shred of long-term evidence I've been able to find is one person on this forum who said "I've replaced decking on older deck done both way, not sure why but the ones with tar paper or storm and ice shield were more rotted then the ones left exposed."

Now, of course, tar paper isn't necessarily self-sealing, but storm and ice shield should be. More importantly, I take what he said as evidence that if water ever gets under the "water proof" membrane, it will not dry out and rot will be accelerated. So even if the self-sealing membrane works for 5 or even 10 years to keep it dry beneath, if it ever fails then rot is accelerated and the beam will probably not last any longer than if no membrane had been present - it might even fail faster than without a membrane. And it's very hard for me to believe that a membrane is going to completely stop water for 40 years. Those screw holes are going to start letting water pass at some point due to UV exposure, freeze/thaw, shifting boards due to weather/humidity changes/wind/earthquake/whatever, or even if someone removes a screw and drives it back in without hitting the same hole in the membrane (will a membrane hole missing a screw completely self-seal? I doubt it). It seems almost inevitable to me that at some point the membrane will accelerate rot instead of stop it. Even water getting into one screw hole could spread over a large area due to capillary action between the membrane and the wood and not dry out for a long time.

I also saw one forum member say "There is always going to be moisture in wood that is used in an exterior application. Even if they don't get rained on, humidity alone will drive the moisture content of the wood above 20%... Caulking, painting, staining, or capping the joists will only prohibit the moisture from getting out of the affected area." I take that to mean that water will penetrate the wood from the sides when it's humid or when rain hits the sides, and keeping the top sealed will slow evaporation, especially since the top is the side that gets the most light/heat.

Thoughts?

ddawg16

Thoughts?

You sound like a lawyer or homeowner who is collecting information to use in a lawsuit.

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CDragon

No, I'm just a guy wondering if he should put water proofing membranes under his deck boards. I'm also frustrated I keep finding people recommending the stuff with no evidence that it doesn't actually make the deck fail earlier.

ddawg16

Ok....fair question......sorry for the comment....but between 'fishing' posts and "how much does it cost" posts, we start to get a bit 'testy' at times....

Hopefully one of the more experienced experts will chime in.....but my first thought....keeping water away from wood is the first order of things...

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CDragon

That always seems to happen with old message boards... People get tired of answering the same questions or just start seeing most questions as "stupid" and get testy easily.

I can tell you about my deck progress so far, and if you need to strip paint or are considering capping screw holes, read on. We bought this house a few months ago. I believe our deck was built in 1983 at the same time as the house. My goal is to remove the deck's old, flaking paint and replace with stain. I bought a 1900 psi pressure washer first since everyone recommends pressure washing before staining and before refreshing stain. I figured I'd try to remove the paint with it as well, but it was stuck on too tightly to remove without gouging the wood - even the flaking areas.

So I tried a random orbit sander but its disc clogged with melted paint and became very slow at removing paint after a few square feet. I tried using a rubberized disc cleaner but it didn't get any of the paint off the disc. So I did a lot of research and ended up building my own infrared heater (instructions here) but it was very slow, caused lots of sweating, and produced fumes that triggered my wife's asthma. Based on more research I think the IR heater can work quite well on some types of paint (100 year old lead based paint is often mentioned as being easy to remove with it) but other types don't "melt" much and hardly release from the wood with heat. The label on the spare paint used on my deck says it's meant to be used in marine applications so it's probably extra tough and the primer seems strong as well.

I was about ready to try something like the Paint Eater despite how much mess it makes and the discs being expensive when I finally found someone mention the "Paint Shaver Pro". I researched that and everyone who had used it loved it. It's expensive, but can be sold for a good price when you're done with it. I paid $460 on ebay. It works fast, makes no fumes, and sucks up most of the debris into a shop vac. It uses blades rated to last about 10,000 square feet.

The one downside to paint shaver is you must countersink all nails/screws or they'll either wear at the shaver's blades or actually have their heads ripped off by it. Of course you have to do the same if you're sanding your deck to stain it anyway. But with sunk screw heads you leave a place for water to collect, and I found "experts" who say that's bad, so I was trying to decide if capping the screws would make sense. I found one person recommend using pieces of wooden dowel to cap screws. But then I thought, wouldn't water just flow through the capillaries in the dowel into the screw hole anyway? So I did a test with food coloring which I can describe in detail if anyone is interested but basically a 3/8" long dowel glued with Tightbond III, DAP plastic wood, and PC Woody epoxy wood, even with fresh stain on top could not stop the red water from getting through the screw hole within a couple minutes.

So I was thinking a bituthene membrane might be the answer, and here we are.