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I am using hidden deck fasteners which screw in to the joist and then hold the board via a notch in the side of the board, pretty standard I think. This fastener will sit on the ledger to hold the first board (closest to the house), but to do this it will need to screw through the flashing which sits on top of the ledger. Can I just put some silicon caulk over and around the screw to prevent water from getting in there, or am I doing something completely wrong?

 Home Depot item 525072 <!-- From http://www.homedepot.com/p/Veranda-ArmorGuard-Hidden-Deck-Fastener-Kit-BKT-BRD-VER-HF-C90/202671704 -->

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4 Answers 4

Aluminum flashing can not be in contact with pressure treated wood(it will eat away over time) I use ice and water shield or a window wrap in between flashing and PT ledger board this will seal around screw and not leak. To be honest though the amount of water that will make its way under the clip and through the hole and back towards the house is soo little,if any, it will not do any damage.

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Do place a little silicone caulk around the screw area before the clip is located and screw is set. You may want to even drill a pilot hole to help pinpoint everything This will sandwich the sealant between the clip, aluminum flashing and deck board. It is correct that there may be just a little passing by there. But a little in the wrong place can cause unnecessary problems. This method will insure the caulk stays in place. –  Jack Sep 19 at 23:10

I ended up not putting any screws through the flashing. I used the hidden fasteners on one side of the board and nothing else for that first board. No screws, or hidden fasteners that went through the flashing that hung over the ledger. It has been a year and I have had no problems with that board. I could most likely pull up that board if I tried (but it might actually be hard), but it hasn't moved on it's own. The other end is pushed as much against the house as possible so that helps. I like this method because if I do have issues I can easily do more, including what was mentioned in other posts, but I haven;t ruined the integrity of the flashing by trying it out.

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Silicone caulk is about guaranteed to leak in this type of application. It hardens more or less fully, then temperature changes come along, and it loosens and leaks.

Butyl rubber is more suitable, as it never fully hardens, so it moves with temperature changes and is less prone to peeling off and leaking.

Another option would be to use some sort of EDPM rubber washer that was thick enough to span from the fastener head to the flashing, but that's presumably "about half a deck board" so quite thick.

A third option would be to screw the single side of the single board next to the house wall through the board into the framing, beyond the flashing on the ledger, and save the "hidden fasteners" for the remainder of the deck. Counterbore and use wooden (or whatever your deck is) plugs if you really don't want to be able to see any fasteners.

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It may depend on the specific fastener you're using but you shouldn't use the fastener type that would penetrate the flashing at the ledgerboard.

The end closest the structure, like the terminal side, likely has a special fastener that can be attached only to the joist.

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