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Drainable Deck Flashing

July 26, 2017

A 4-Dimensional look at deck flashing that works

Earlier, we posted a video by Glenn Mathewson about deck ledger code requirements. At the end of the video, Glenn took a closer look at a couple of good details, one from the EPA, the other from Building Science Corporation.

Glenn preferred the BSC detail, so we thought it made sense to put it into four dimensions for you.

The basic concept is that the flared drip edge protects the ledger and directs water away. Some details specify cutting the flashing at each joist, but that can cause wicking at the joint, and it takes extra work. Both are bad things.

Instead, keep the joists 1/4 inch (or so) off the ledger, but supported by the joist hanger. Make sure to choose a hanger with a base support of 1-1/2 inches (per code) plus the gap.

Here’s how to flash a deck ledger:

  • Apply peel and stick membrane over the base of the wall where the deck ledger will be. This will protect the sheathing from water and help to seal fastener holes, too.
  • Install the deck ledger—spaced off sheathing or not—with the appropriate fasteners (through-bolts are better than lag screws, nails and deck screws are unacceptable).
  • Install Z-style ledger flashing with a hemmed drip edge that extends over the face of the ledger and pushes water away.
  • Select joist hangers one size down from the joist size (such as 2x6 hangers for 2x8 joists) so that they fit below the ledger flashing. See Simpson’s joist hanger sizing chart for exact choices.
  • Install joists so as not to compress the flashing. Don't mash them against the ledger, let them breathe.
  • Housewrap over the flashing, siding over the housewrap.
  • Decking goes on top, avoid screwing through the flashing.

The distance from wall to avoid debris build-up is a personal one and open to discussion, but it is not specified here. Comment below please, if you have an opinion on it.


Building Science Corporation


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