Protect Deck Joists
A rubberized sealant provides inexpensive protection for the tops of decks joists.
I recently built a small deck and wanted to extend the life of the joists by protecting the tops of them with butyl tape, since even pressure-treated lumber rots out eventually. There is specialized flashing tape made specifically for this purpose, but it costs $0.50 per ft., and I had about 110 ft. of joists to cover.
In search of a less expensive option, I came across Flex Seal, a liquid rubberized sealant. It’s almost half the price of three 50-ft. rolls of tape, and the info on the can said I would get more than twice as much coverage as I needed; plus, I figured it would be faster. I simply lined up the joists on sawhorses and coated them with a chip brush and a small roller.
They were dry enough to handle in about four hours. The deck I built was low to the ground, but if you try this method on joists that can be seen from the bottom, you might want to stack them on their side to avoid seeing the inevitable drips that you’ll get when applying the sealant with them stacked vertically like I did.
I’m not sure this method will protect the screw penetrations as well as a tape-style flashing, but I know it will add years of protection to the top side of joists, where water can get trapped between the deck boards and take longer to dry off after a rain. I did not use the whole can, but I guess that a full can could have covered about 200 ft. of joists. That would put the cost at about $0.20 per linear foot, including the price of the brush and roller.
— Mark Petersen; Waco, Ky. Edited by Charles Miller.
From Fine Homebuilding #318
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View Comments
I’m not sure where you’re buying joist tape at $.50/foot, as even Home Depot carries a 50ft roll for $18. You might say this is easier, but what about the blocking? Joist tape allows me to cover everything onc in pace, including the spacing between blocking and josts as well as rim. I’ll stick with tape.
Your point about covering board seams with tape makes sense. Water could get trapped there.
A paintable sealer makes good sense everywhere else.