How to Install a Vapor Barrier Over a Nailable Deck

Roll the vapor barrier roll over the nailable deck surface until you have a strip that reaches across the entire length of the deck. Slide the loose end of the vapor barrier over so that approximately 1/2-inch hangs off the side of the deck and cut the other end of the barrier to hang 1/2-inch off the other side of the deck.

Line the vapor barrier strip up at the bottom, or lower edge, of the nailable deck, so that approximately 1 inch of the vapor barrier extends over the edge of the deck. Use a staple to hold the top edge of the vapor barrier in place at each end of the deck and snap a chalk line over the deck along the top of the strip.

Remove the staple in the vapor barrier at one end of the deck with pliers. Fold the now loose end of the vapor barrier back approximately 2 feet. Pull the backing that covers the adhesive on the vapor barrier in that section of the strip away from the strip. Line the strip back up along the chalk line and press the strip into place at that end.

Return to the other end of the deck and remove the staple holding the end in place. Fold the strip back by approximately 2 feet at that end and then fold the strip back again. Continue folding the strip back along the deck until you get back to the other end of the roof.

Peel the backing from the vapor barrier 2 feet at a time. Press the exposed section down onto the deck so that the top lines up with the chalk line.

Install the vapor barrier in this manner, moving up the deck toward the higher end. Lay each strip of the vapor barrier so that it overlaps the strip below by 4 inches. If you run out of the vapor barrier on a role halfway across the roof, cut a second piece to finish off that row, and overlap these pieces by 6 inches.