To seal the seam between a doubled joist or beam, what type of caulking is best? The Home Depot Decks 123 book recommends Alex Plus (acrylic latex plus silicone) while a guy at Home Depot suggested Mono Ultra Exterior (latex). How about Blueskin?

I initially used Alex Plus, it rained before the caulk cured, the caulk became a shriveled and cracked mess, I scraped it and replaced it with Mono Ultra Exterior.

1 Answer 1


Maybe I'm just sloppy, but I've never bothered to caulk the seam on a doubled up joist with treated wood.

There was one time I felt I needed to worry about that, but that was with a cantilevered deck using untreated wood. In that case I just used aluminum flashing over the top of the joists.

I suppose if I were going to caulk it though, I would use a silicone caulk over a latex caulk since they generally adhere and hold up to water better (cleanup is messier, but it's a deck, so it probably doesn't matter).

  • +1 For the aluminum flashing. I didn't mention it, but I actually ended up using Blueskin on one of the doubled up joists, essentially covering the seam instead of caulking it. I'll wait a bit more to see if there any other acrylic vs. silicone comments before accepting yours as the answer...
    – Ates Goral
    Aug 3, 2010 at 15:41

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