Windows and doors are an interruption in the wall’s house wrap or insulated sheathing drainage plane and thus are a vulnerable spot for water leakage. Many a homeowner has suffered from water intrusion around doors and windows, often not discovered until considerable damage has been done within the wall. The wall framing around windows and doors must be protected from any water that finds its way behind the siding at these locations and the water must be directed out to prevent damage to the rest of the wall. Properly installed flashing that is integrated with the other elements of the wall can help prevent water damage. 

Flashing materials may be installed by insulators, framers, or subcontractors or vendors hired specifically to install the windows and doors. This task should be included in the contract for the appropriate trade depending on the workflow at the specific job site.

Other Considerations: When designing the home, walls and doors should be located under overhangs or porch roofs whenever possible; these features protect against rain and snow and also minimize unwanted solar heat gain by blocking out high summer sun.
 

How to flash windows and doors

1. Cut the housewrap covering the rough opening in the shape of a modified "I." See Figure 1.
   • Fold the side and bottom flaps into the window opening and secure.
   • Above the window opening, cut a head flap. Fold it up to expose the sheathing and loosely tape it in place out of the way.

   

   Figure 1 - Cut an I in the house wrap over the window rough opening. 

2. Install back dam and self-adhesive flashing at sill. Tape the corners. See Figure 2.

   

   Figure 2 - Install self-adhesive sill flashing in the window rough opening. 

   For pan or sill flashing, use formable flashing, a stretchable self-adhered membrane that bends at corners so one continuous piece can be used to cover the bottom and sides of the sill.

   • First install a back dam consisting of a strip of wood or beveled siding nailed along the back (inside) edge of the rough opening (over the flap of housewrap). Sloped pans are required if the sill depth is greater than 6 inches according to ASTM E212207. 
   • Cover this with the membrane. Begin pressing in the middle of the sill and work toward the sides, removing adhesive covering strips as you go. Make sure to press the membrane tightly into the corners to avoid tears later when the window is installed.

   Other options for pan flashing include self-adhered non-elastic membrane (se Figure 3), which must be cut and patched at corners, two-piece rigid manufactured pan flashing, which comes with a built-in back dam that must be protected from breakage during window installation, or asphalt-based liquid flashing that is applied with a paint brush or roller.

   

   Figure 3 - Three types of window pan flashing with back dams. 

3. Caulk the outside edges of the head and side jambs and install the window. See Figure 4.
   • Do not caulk across the sill.
   • Install the window plumb, level, and square following manufacturer’s specifications.

   

   Figure 4 - Caulk the top and sides of the window rough opening, but not the bottom. 

4. Install jamb and head flashing. See Figure 5.
   • Install self-adhesive jamb flashing extending 4 inches above the top of the head flange and even with the bottom of the sill flashing.
   • Install self-adhesive head flashing extending 1 inch beyond the jamb flashing.
   • Unfold the housewrap and lay over the head flashing. Tape bottom edge of housewrap across the top window flange and tape down corner seams of housewrap. See Figure 6.

   

   Figure 5 - Flash the sides and top of the window. 

   

   Figure 6 - Tape down the housewrap head flap. 

5. On the interior side of the window, seal the gap between the window and the rough opening with backer rod or non-expanding foam and caulk. See Figure 7.

   

   Figure 7 - Air seal the rough opening around the window from the inside. 

6. Install trim that has been painted or primed on all sides. Above top trim, install cap flashing that extends past trim. Cover top edge of cap flashing with adhesive membrane strip. Cover top edge of membrane with sheathing tape. See Figure 8.

   

   Figure 8 - Install trim and cap flashing above the window. 

How to Flash a Door

1. Flash the door head and jambs as described above for windows or in accordance with the door manufacturer’s instructions.
2. For sill flashing, see the door manufacturer’s instructions.
3. For houses with concrete slab floors, form a seat in the concrete slab to act as a “pan flashing.” See Figure 9.

Figure 9 - A sea is formed in the concrete slab to act as pan flashing for a door. 

4. Install pan flashing that is integrated with the housewrap. ASTM E2112-07 recognizes several flashing materials and methods, including single-piece sill pans formed from rigid sheet metal (see Figure 10) , multi-piece sill pans formed from rigid sheet metal or plastic, flexible membrane, or combinations of rigid corners connected by flexible membrane.

Figure 10 - Sheet metal pan flashing for doors.