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I need a sealer Caulk to fill a approx. 3/8" gap between my Composite Decking boards to waterproof and prevent water from dripping between the boards and going on to a deck below. I want the rain water to flow over the composite decking boards to the edge of the deck and run off onto a roof that will take the water away.

What is the best product for this application? What product will adhere to and seal to Composite Decking?

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    I would not recommend sealing gaps between deck boards, decks are carefully designed and constructed and those gaps serve an important purpose. The best way to divert water from a deck and prevent it from falling through would be to enclose the upper deck or at least add a roof above it. Commented Mar 23, 2021 at 15:26
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    You're asking the wrong question. Caulk is not a solution for this--it will look terrible and it will fail in short order. To provide solutions we need more information about your deck. Photos would be great.
    – isherwood
    Commented Mar 23, 2021 at 16:13
  • You have a deck below a deck? Why not just waterproof both decks, which I imagine is mostly done for you if you're using composite decking materials?
    – TylerH
    Commented Mar 23, 2021 at 19:20

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Typically you need those gaps for expansion of the boards in hot weather- if they're tight to each other they can get damaged as they expand from heat.

They make under-deck panels that catch the water and direct it out a side that work very well. That would be a better solution than caulking, which would most likely need to be re-done every year or two anyway.

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