Water leakage from faulty window and door flashing is one of the leading trouble spots in new construction. The damage, often hidden under the siding or within the wall cavities may not show up for several years.
Exterior leakage may first show up on the exterior of the house as peeling paint around or below windows or on the interior as water stains on the drywall. But it’s the hidden damage to the sheathing, framing, and insulation that can be the most destructive and expensive to fix. Damage from leaky window flashing has spawned a number of lawsuits by homeowners’ associations in large developments.
To prevent serious problems down the road, it’s critical to get window flashing installed properly when the house is built. Since the window flashing must be woven into the housewrap (water-resistive barrier), it is very hard to make corrections once the house is complete.
Roof overhangs. Not surprisingly the greatest leaks occur on windows exposed to a lot of rain. This is typically on the side of the house facing the prevailing winds during the rainy season. It is also more prevalent on walls with small or no roof overhang. In new homes, make sure your design has adequate roof overhangs on all sides. This is the first line of defense and will prevent against water leaks at windows and doors.
Flange-type windows. Most new windows have an integral nailing flange, either plastic or metal, that is nailed to wall to secure the window in place. The flange must be sealed to the surrounding walls and flashed to shed water. The flashing needs to be integrated properly with the house wrap and siding for the system to work properly.
There is still some debate about the best way to flash this type of window, and most window manufacturers publish their own detail. Most published details follow the general approach shown below. The detail relies on sealant and peel-and-stick flashing tape, both of which are adhered to a plastic housewrap, a material notoriously difficult to stick to (see Illustration).
The key to success with any flashing detail is to follow the shingle principle, with upper layers of waterproofing materials overlapping lower layers, shedding water to the building exterior. This will continue to provide protection even if the sealant or tape fail over time as is often the case.
The other key is good workmanship on the job site. In general, it’s easier to draw a tricky detail than to build it in the real world — with dirt, rain, time pressure, and workers who may not have the best training. The more fussy the detail, the greater the chance it will be messed up. This is one place where it’s worth extra supervision, inspection, or whatever it takes to get it done right.
Following the shingle principle, start at the bottom with a sill pan (membrane, metal, or plastic), install the window, then seal the top, always lapping upper flashing layers over lower ones, shingle style. For the sill, moldable flashing, such as Tyvek FlexWrap, is the best choice as it is made to bend up the side jambs without stressing the material. Standard flashing tape is usually cut part way and patched at the bottom corners, a potential leak point.
Flashing Tape and Sealant. While peel-and-stick flashing tape is very sticky stuff, it is not magic and will not stick to wet or dirty housewrap that has been exposed to the weather for weeks. It’s best to wipe off dirt, make sure the wood or housewrap is dry, and to press the flashing tape in place with a roller, as recommended by most manufacturers. Where caulk is required, choose a high-quality “sealant” approved by the manufacturer of the house wrap. Low-cost hardware store caulk will fail quickly. In the best of cases, however, you should not rely on tapes and sealants to stay stuck forever. The flashing should shed water even if the adhesives fail.
NOTE: Do not caulk any horizontal joints above or below the window or any flashing. These should be designed to shed water to the layer below. Caulking will trap water instead of allowing it to drain.
Window Cap Flashing. Before sealing up the top of the window, I prefer to install a traditional metal cap flashing. This helps direct water out away from window. In this type of installation, seal the cap flashing directly to the sheathing with high-quality sealant. Then place the top piece of flashing tape over the cap flashing and layer the house wrap over the tape. If you’ve added wooden trim around the window, the cap flashing goes over the wood trim. Pay attention to the ends of the cap flashing and make sure they extend slightly past the window molding.
A Better Way to Flash Windows
While the detail described above, if followed precisely, works pretty well most of the time, failures are still common. An expert who sees many of the failures is consulting architect Harrison McCambell, who gets called in to diagnose and fix buildings after they have failed.
Writing in The Journal of Light Construction, the leading technical magazine about residential construction, McCambell attributes most of the problems to a combination of poor workmanship, material limitations (like stretching flashing tape beyond its limits), over-reliance on tapes and sealants, and the detail itself even when followed diligently. The corner patches, often used at the two bottom corners, are problem areas. Also conflicts between code requirements and manufacturers’ instructions lead to confusion about what exactly to do.
McCambell often works on large residential complexes with 200 or more windows, so he needs to provide construction details that are easy to follow, durable, and reliable. His solution, which I would use on my own house, eliminates key weaknesses of the standard approach. Key details to McCambell’s approach (see Illustration below) are:
- The housewrap is cut flush to the opening on the side and bottom — no more flaps to wrap inward (sometime bringing water with them). The top flap is cut vertical at least 6 inches beyond the opening.
- Jamb flashings are bonded directly to the rough opening since the flashing tape protects the wood framing more effectively than the housewrap can.
- Flexible/moldable sill flashing is turned up the side jambs only 0ne inch (rather than 6 in.), and downward only 2 inches, which puts less stress on the corner points.
- The sill flashing laps over the housewrap or a separate piece of through-wall flashing needed for brick and other masonry veneers.
- While deviating from the window manufacturer’s instructions could potentially void the window warranty, McCambell feels (and I agree) the potential cost and liability of water damage far exceeds the dubious value of a manufacturer’s warranty.
You can read the full article at The Journal of Light Construction.
Harrison McCampbell is a consulting forensic architect in Brentwood, Tenn., specializing in moisture-related construction defects.