How to Install Vinyl Z Flashing With J Molding

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Things You'll Need

  • Chalk line

  • Tin snips

  • Hammer

  • Galvanized nails

  • Undersill flashing

  • Lag bolts

  • Socket set

  • Caulk

Z flashing and J molding are important to sealing out moisture when installing a deck ledger board.

Z flashing is commonly used in combination with J molding or J channel to prevent water from leaking behind siding when attaching exterior elements such as a deck ledger board to a vinyl-sided house. The Z flashing sits atop the ledger board while the J molding is used on the sides and can also be used along the bottom of the opening created in the siding to attach the ledger board. Following the proper procedures ensures that moisture is sealed out and damage to the existing siding is minimized.


Step 1

Snap a chalk line around the outline of where the ledger board will be located. Include space for the deck boards above the ledger board and any trim boards that will be attached to the sides. Cut away the siding with a pair of tin snips.

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Step 2

Nail J molding over the bottom of the opening. Pull the siding back gently in order to fit the molding beneath the siding. If necessary, remove one or two courses of siding and replace them after attaching the J molding.

Step 3

Attach Z flashing at the top of the opening. Attach J molding to the sides of the opening. Again, slide the pieces under the siding and remove pieces of siding as necessary and reattach them after you've nailed the flashing and molding in place.

Step 4

Tack the ledger board into place. Install undersill flashing on the cut edges of the vinyl. Attach the ledger board using lag bolts and apply silicone caulk around the bolt heads to seal out moisture.


If you need to use more than one piece of Z flashing to span the opening, overlap the pieces by at least four inches and seal the joint with silicone caulk.


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