Flashing over deck joists

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Old 06-21-16, 08:32 AM
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Flashing over deck joists

I have just started removing the deck boards on my deck to replace them, as they're in pretty rough shape. Even after 3 days of no rain, the top of the joists are still wet. They're in pretty good condition, but I'm wondering about placing some type of waterproofing over the joists to make them last longer (about 20 years old now) before I put the new decking on. I'm thinking about using a self-adhesive window/door flashing.Even though the joists are treated for above ground use, I get a little concerned about how long they stay wet.

What do you think? Good idea or overkill? Can't hurt, might help?
 
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Old 06-21-16, 08:39 AM
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I once had a Porch constructed where the Carpenter insisted on laying an 8" strip of Tar Paper or Roofing over the top of the joists before laying the decking.

I only owned the house for another 12 years or so, and don't know if this technique added to the life of the joists; but it sounded good !
 
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Old 06-21-16, 08:54 AM
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I've seen people put something over the top of posts, but not joists.

Why are the joists so wet in the first place?
 
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Old 06-21-16, 09:08 AM
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I do exactly as you describe, put a strip of flexible window flashing tape over each joist. Whenever I see rot on PT joists (fairly common to see at least some on re-decking projects) it's almost always on top of the joists. "Stuff" gets trapped on top of the joist in the space between boards and it holds moisture against the joist. I haven't been doing it long enough to know if it really helps, but It's not that expensive, doesn't take very long to do, and I figure it can't hurt.

I use a J roller to make sure it's well adhered to the tops and the part of the sides where it laps over.
 
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Old 06-21-16, 09:18 AM
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I am thinking the joists are close to their life expectancy. Of course I live in Washington and if the decking is in rough shape the joists will be soft and rotted. They will hold moisture for quite awhile.
 
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Old 06-21-16, 10:31 AM
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Thanks all! I'm heading off to the lumber yard this afternoon. Today I learned something, so it was a good day! While I worked for almost 40 years as an architect, it was strictly in commercial/industrial buildings, so I don't have much experience in residential.

The joists are still in good shape. If I poke the top with a knife, I get very little penetration. In fact, the decking is not in too bad a shape either, replacing it is more aesthetic than anything.
 
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Old 06-21-16, 02:36 PM
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Originally Posted by Carbide Tipped
". . . I use a J roller to make sure it's well adhered to the tops and the part of the sides where it laps over . . ."
Now that's a bit different from what my Carpenter did in the application I mentioned earlier.

He attached (with roofing nails ?) the 8" strip of Tar Paper lengthwise along the top edge of each joist, and left about 3" along each side suspended in mid-air away from the decking AND away from the sides of the joist too. That way, any dripage through the deck was directed well away from the sides of the joists, somewhat like an elongated umbrella.
 
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Old 06-21-16, 03:02 PM
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I bought a product called DeckSeal Self-Adhesive Flashing. It came in a 4" wide x 75' roll and is a peel & stick. It looks very much like an Ice & Water membrane used in roofing. The instructions just say to peel off the backer, center it on the joists and adhere it to the top and sides, working out wrinkles and bubbles. I don't get too excited about the sides of the joists getting wet because they'll dry out right away.
 
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Old 06-23-16, 04:51 AM
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Update:
I got all the joists covered yesterday with the flashing. It was very easy to do and sticks very well as long as the top of the joists are reasonably clean. The only thing I would say is that it makes the top of the joists VERY slippery because it doesn't have a grit like ice & water does. Gotta be really careful walking the joists. In fact, if I do this again, I'm going to seriously consider buying a roll of ice & water with the grit and cutting it into strips.
 
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