Flashing New Window Installations
If installing both a new window and flashing, refer to window manufacturer’s instructions and ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights for the proper flashing installation method for the window type and wall configuration on the project.
Flashing Previously Installed Nail Fin Windows
If a nail fin (in new construction) window has been previously installed without flashing, the following instructions should be followed:
- Apply a continuous bead of sealant to the nailing flange of the sill in a manner that covers the nails and nail slots. Apply a minimum of 9” (229mm) wide horizontal sill flashing level with the bottom edge of the existing window by pressing the flashing into the sealant bead at its top edge. Cut the sill flashing long enough to extend a minimum of 9” (229mm) beyond each jamb. Fasten the sill flashing at the bottom and side edges (Fig. 31).
- Apply a continuous bead of sealant to the nailing flange of the jambs in a manner that covers the nails and nail slots. Continue the bead of sealant at the jambs vertically a minimum of 8 1/2” (216mm) above the head of the window to allow for bedding the top portion of the jamb flashing into sealant in the next step. Install the jamb flashing by pressing the flashing into the sealant beads at the window jambs. Extend the bottom edge of the jamb flashing approximately 1/2” (12.7mm) short of the sill flashing edge, and extend the top edge approximately 8 1/2” (216mm) beyond the head of the window, where the head flashing will be placed next. Fasten the jamb flashing along the edges further most from the window (Fig. 32).
- Apply a continuous bead of sealant to the nailing flange of the head in a manner that covers the nails and nail slots. Add an additional bead of sealant horizontally, in line with the top of the head flashing. Install the head flashing by pressing the bottom edge of the flashing into the sealant bead previously applied across the mounting flange. Extend the ends of the head flashing approximately 1” (25.4mm) beyond the jamb flashing at each end. Fasten the head flashing into place along the top edge (Fig. 33).
Flashing Previously Installed Window with Exterior Casing (Brick Mold)
If a window with exterior casing (e.g., brick mold) has been previously installed without flashing, the following instructions should be followed:
- Ensure that exterior casing is sealed to the exterior sheathing or water-resistive barrier with a good quality sealant.
- Cover the exterior casing with aluminum or vinyl trim sheet. This can be accomplished by using a portable field brake and bending instructions from the brake manufacturer. The trim sheet should be installed in weatherboard fashion. The bottom piece should be installed first, and each piece should overlap the piece below wherever they join.
- Install rigid head flashing (i.e., drip cap) on top of the top piece of exterior casing, covering trim sheet installed in the previous step. The ends of the rigid head flashing must extend to the outer edges of the exposed legs of the side J-channels. Cut a notch on the ends of the rigid head flashing and bend them down over the sides of the exterior casing. The rigid head flashing must be sealed to the exterior sheathing and to the top of the exterior casing (Fig. 34).
J-channel is used around windows and doors to receive the siding. Follow the steps below when applying trim.
- Cut and bend the tab of the top piece of J-channel down to provide flashing over the side J-channel.
- Fold the bottom end of the side piece of J-channel inward at the bottom of the window, to fit over the existing J-channel to prevent water from entering under the sill.
- Cut the side J-channel members longer than the height of the window or door, and notch the channel at the top.
- Miter cut the free flange at a 45° angle and bend the tab down to provide flashing over the side members (Fig. 34). A similar miter and tab may be provided at the bottom of the window, depending on the sill’s condition. The J-channel should fit snug to the window.
J-Channel Over Roof Lines
Install the flashing before the J-channel to prevent water infiltration along the intersection of a roof and wall.
- Keep the J-channel a minimum of 1/2” (12.7mm) from the roofing material. Chalk a straight line up the roof flashing to guide J-channel installation. Tip: You can use another J-channel laid over the shingles as a spacer to create the straight line desired.
- Overlap the J-channel (lapping the upper piece over the lower piece) if it is necessary to use more than one piece. See Fig. 54 in the Vertical Siding section of this guide.
- Extend the J-channel past the edge of the roof in order to ensure proper runoff. A diverter can be used; see Fig. 44 and 45 in the Horizontal Siding section of this guide.
- With dark shingles, or a south or west exposure, it is recommended to either use a metal J-channel or to install the vinyl J-channel as far away from the roofing as is aesthetically acceptable, having first ensured that there is sufficient flashing behind the J-channel to prevent water infiltration.
- Fasten the nail, screw, or staple that is closest to the roof line at the far end of the nail hem slot, to ensure that siding will expand away from the J- channel (Fig. 35).