Deck joists - waterproofing

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playden
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Deck joists - waterproofing

Post by playden » Wed Sep 11, 2019 11:00 am

Hi Shannon,

Watched your "Building a deck series" on Youtube and noticed that you didn't add any waterproofing for your joists. I will be building my own deck in a couple of weeks and was wondering if adding joist flashing tape is overkill? The entire deck will be PT wood.

Thanks
Paul

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A. Spruce
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Re: Deck joists - waterproofing

Post by A. Spruce » Wed Sep 11, 2019 11:15 am

Personally, I think it's a good idea that isn't used enough. You can buy the tape or you can buy 15# or 30# roof felt and cut that into strips for the same effect.

Even with PT wood, it will help keep water from pooling between the joist and deck board, saving the framing from damage. Today's PT isn't like it used to be, the chemicals are not as aggressive anymore (great for the environment, bad for wood preservation), but worse is that the chemical treatment does not permeate the material like it used to, resulting in about an 1/8" protective "skin", rather than three quarters of the board being saturated. For this reason, I would recommend treating all your cuts with coppernapthenate or a boron solution. Coppernap is getting harder and harder to find, whereas you can make your own boron solution from a $5 box of 20 Mule Team Borax from the laundry aisle of the grocery store.

Side note, a borax solution is also great at controlling ants and termites without being toxic to you, your kids, or your pets and animals.
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playden
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Re: Deck joists - waterproofing

Post by playden » Wed Sep 11, 2019 11:36 am

Awesome. Thanks for your quick feedback! Much appreciated!

playden
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Re: Deck joists - waterproofing

Post by playden » Wed Sep 11, 2019 12:05 pm

One more question, will the boron solution be ok for ground contact (for my deck posts) ??

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A. Spruce
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Re: Deck joists - waterproofing

Post by A. Spruce » Wed Sep 11, 2019 12:32 pm

As far as toxicity, it will be fine, borax is non-toxic to most things. It is an excellent mold/mildew and wood destroying pest inhibitor.

As far as a preservative for the wood against the ground, I don't think so, but even PT doesn't hold up all that well against the ground, at least not in my area.

Personally, I'm not a fan of buried posts for anything but fences because they will rot off at ground level where they never dry out. So, unless you need a buried post for structural stability, I would recommend a cement pier that is 6" above ground with a wet set post/beam saddle to attach your framing. With your framing isolated from the ground, you won't have ground related rot issues and less inclined for pest intrusion issues. Most of the time you can easily add structural bracing into the framing, if necessary, to make up for not having buried posts.
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Shannon
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Re: Deck joists - waterproofing

Post by Shannon » Wed Sep 11, 2019 2:20 pm

I did not use anything for joist flashing/protection as it was not common item at that time but things have changed and I do think it is a good idea for sure.
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