Q: Earlier this year there was a discussion on JLC’s website regarding the proper width for flashing tape that’s used to seal around windows. Can you settle what is recommended and what is required?

A: Mike Guertin, a builder and remodeler in East Greenwich, R.I., and a presenter at JLC Live, responds: The 2015 IRC does not list a specific minimum width for flashing tape, but it does give us some prescriptive guidance. Section R703.4 subsection 1, entitled “Exterior Window and Door Openings,” says: “Flashing at exterior window and door openings shall extend to the surface of the exterior wall finish or to the water-resistive barrier complying with Section 703.2 for subsequent drainage.”

This means simply that the flashing tape needs to extend from the window and onto the surface of the siding or onto the housewrap.

The same subsection goes on to note: “Flashing at exterior window and door openings shall be installed in accordance with one or more of the following: The fenestration manufacturer’s installation and flashing instructions, or for applications not addressed in the fenestration manufacturer’s instructions, in accordance with the flashing manufacturer’s instructions.”

In other words, follow the instructions that come with the window or with the flashing tape.

The subsection then states: “Where flashing instructions or details are not provided, pan flashing shall be installed at the sill of exterior window and door openings. Pan flashings shall be sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or to the water-resistive barrier for subsequent drainage. Openings using pan flashing shall incorporate flashing or protection at the head and sides.”

This requires the bare minimum of sloped pan flashing with additional flashing properly installed at the sides and head of the opening.

The problem—and frustration—arises when you look for guidance from product manufacturers. I checked the instructions from several window manufacturers as well as instructions provided by housewrap makers and the makers of flashing tape to see if any specific dimensions were provided for a minimum flashing-tape width—and found none. The only common dimension I saw was for the upturned jamb legs of a sill pan (most manufacturers called for 6 inches).

But the biggest quandary for me was that many of the manufacturers’ instructions tell installers to follow the local building code or the flashing-tape instructions. Then the flashing-tape instructions often refer back to the window manufacturer’s instructions and the local building code, with the code leading you back to the window or flashing manufacturers in an endless circle of passing the buck.

My recommendation is to check all three sources: your local building code (in the event it is more stringent than the 2015 IRC); the window manufacturer’s instructions; and the flashing-tape maker’s instructions. Then follow the one that calls for the widest width flashing tape (if any dimensions are available at all).

The narrowest flashing-tape width that I’m aware of is 4 inches. Given that most window flanges are less than 2 inches wide, 4-inch tape is wide enough to meet the code requirement that the flashing extend to the surface of the housewrap.

For what it’s worth, I use 4-inch flashing tape for the side flanges and 6-inch tape for head flanges when the housewrap is installed before the windows are installed (so-called Eastern install). When the windows are installed before the housewrap (Western install), I use 6-inch tape on all sides unless the instructions direct me otherwise.