January, 2007

A builder from North Carolina asks: What do you recommend for flashing windows?

By Steve Easley

Nearly 80% of construction defect litigation stems from water and moisture problems, and a majority of these cases come from water that has leaked around windows.

When flashing windows, I always recommend ASTM E 2112. This is the standard in the industry, and most window manufacturers use it as the reference for their window installation instructions.

The most recent draft of this installation standard focuses on integrating flashings and housewrap. For the first time, the material also addresses the use of newer, self-adhesive flashing materials. Unfortunately, the document is several hundred pages long. 

Here’s a brief summary of my recommendations for flashing windows:

Step 1 Wrap walls

Most builders wrap a house before installing the windows, though in some Western markets, windows may be installed before the housewrap. In either case, the critical detail is that window flashing must be integrated with the housewrap, with each layer lapped so that when water flows down a wall, it drains to the outside. In particular, the housewrap must lap over the top of any head flashing, and the sill flashing must lap over the top of the housewrap below the window.

Make sure that the housewrap is an approved water-infiltration barrier. Some wraps are only designed as air-infiltration barriers, and aren’t designed to exclude water.

When wrapping house walls first, do not “X-cut” the housewrap over the window opening. Instead, cut the housewrap straight across the top of the rough opening, and create a flap that can be temporarily taped up out of the way. This allows the top nailing flange on the window to be installed against the sheathing. This flap of housewrap will be folded over the top of the head flashing in the final step.

Step 2 Flash sill

Sill flashing is critical for draining any incidental leaks through windows to the outside. Best practice calls for creating a “boot” to protect the window sill with a flexible flashing membrane that can be stretched over the corners of the rough opening. This membrane should extend at least six inches up the sides of the opening, and the front edge must overlap the housewrap below the window.

Step 4 Flash down sides

After installing the sill flashing, the window can then be installed. The top and side nailing flanges can be bedded in caulk, but the bottom flange must never be sealed; otherwise, water diverted by the sill flashing cannot drain to the exterior.

Once the window is in place, the side jambs can be flashed. While ASTM E 2112 allows the window to be installed over the jamb flashing, this practice can leave gaps where water can enter. Best practice calls for installing the flashing over the nailing flanges.

Step 5 Flash across head

The head flashing should adhere directly to the wall sheathing, and each end should extend past the side flashing strips on each side of the window.

Step 6 Tape flap in place

Finish the procedure by pulling down the flap of housewrap from Step 1 along the head and taping it in place. Rather than using one continuous piece of tape to do this, use short pieces placed across the top of the window so water can drain out, should it need to.