Behind every successful window-and-door job lies a deep understanding of flashing systems, water management, and evolving best practices. Overlooking these fundamentals can lead to air leaks, water damage, and costly callbacks. In interviews with seasoned builders, consultants, and remodelers across the country, a consistent message emerges: Precision, patience, and preparation are everything.

Here are some of the most common mistakes and how pros avoid them.

1. Getting Flashing Details Wrong

Improper or incomplete flashing remains one of the most frequent causes of failure in window and door installations.

“The biggest thing, especially doing remodeling, is when we take an existing window out, you can see why there’s such a high focus on flashing and the window flashing tape,” says Mike Whalen, lead carpenter at DBS Remodel in New York, who’s been in the business for 35 years.

Correct sequencing is crucial. “It’s that positive water flow that you would see on roofing,” Whalen says. That means starting at the bottom with sill flashing, wrapping up the sides, and finishing with the head flashing. He recommends flashing tape that overlaps the nail flange and house wrap, and a drip cap is a must, even if the house wrap is sealed well above.

2. Neglecting Sill Pans and Drainage

Skipping sill pans is a costly oversight.

“We do [use sill pans], especially on sliders,” Whalen says. “We’ll manufacture those ourselves out of aluminum.”

Mark Parlee, a 45-year residential construction veteran and owner of thebuildingconsultant.com, also stresses the importance of back-damming or angling the sill. “It’s good to tap up on your rough opening sill just a little bit to give it a tilt toward the exterior so that it can in fact drain,” he says.

3. Blocking the Drain Path

Water that gets in must be able to get out. One common mistake? Applying the foam sealant near the window flange.

“Now you don’t have a cavity,” Parlee says. “Also, the spray foam … doesn’t have a lot of movement capability.” If the foam bead is adhered on three side (to the window jamb, the window flange, and the rough opening) it can crack and an air leak will develop. Parlee prefers sealing from the inside with a backer rod and caulk at the jamb extension to retain a drainage cavity behind the window flange and allow flexibility. A sealant that joins only two sides can stretch in one dimension without failing.

4. Improper Leveling and Squaring

Parlee challenges the often-repeated phrase “plumb, level, and square.”

“You use the longest dimension only, then you square,” he says. “If your window is wider than it is tall, then put your level on the bottom or the top and level it and then square it … even if your window is out of kilter just a little bit, if it’s square, it’s going to operate.”

Straightening the edges is also critical—he frequently sees bowed or humped bottom rails because installers don’t check flatness after tacking in place.

5. Not Sealing the Interior Properly

Thermal imaging and blower door tests reveal that most air leaks occur at the top of the window, Parlee says. That’s why interior air sealing is critical.

“If you’re using drywall returns at these windows, you need to seal the drywall to the window frame to prevent vapor movement into the interstitial wall cavity,” he says. The risk: trapped condensation forming frost that later melts and causes brown staining from tannins in the framing materials.

6. Relying Too Heavily on Sealants

Sealants fail over time, particularly when not applied correctly. Yet many installers treat caulk as the primary line of defense.

Doug Horgan of BOWA, a high-end remodeling firm in Washington, D.C., cautioned against this mindset. “Overreliance on sealants that don’t last forever” is a major concern, especially when water management is improperly planned. Horgan stressed the importance of lapping layers shingle-style and integrating windows into the WRB with proper detailing—not just slapping on caulk and hoping for the best.

7. Fastening Mistakes: Guns vs. Screws

Gun nails and delicate vinyl flanges don’t mix.

“I see so many guys using gun nails on vinyl flanges and they blow through them,” says Parlee. “Hand nails or screws are the best.” But even hand nails come with risk. “You get a little caulking on the head of your hammer and it slides off,” possibly cracking the window.

Whalen’s team has switched to screws for added control and adjustability. “It’s also nice if you have to make an adjustment,” he says. “Then you can just unscrew them, make the adjustment, screw them back in.”

8. Skipping Quality Control or Failing to Train Subs

Even with written standards, a set of drawings isn’t enough.

“We keep a copy on every job site,” Horgan says. “But I’m not sure I’ve ever met someone in the field with tools on who’s ever seen them.” That’s why he demonstrates installations in person and walks crews through each detail. “There’s always someone there on site with that piece of paper who can review it with them.”

9. Underestimating Climate Considerations

Installation techniques should be tailored to local weather conditions. In regions prone to wind-driven rain or snow accumulation, extra precautions are essential. For example, even on second-story windows with decent overhangs, diligent builders continue to tape all flanges—not just for moisture protection. “It helps with air sealing as well,” Whalen says.

10. Ignoring Manufacturer Guidelines

Although many installers look online for better techniques, some overlook the basics found in product literature.

With modern windows, installation guides often include crucial spacing and compatibility requirements for flashing and sealants. Ignoring these can void warranties.

Final Word

Contractors aiming for airtight, watertight success must think beyond code minimums.

When in doubt, do more—not less. The job you do today could save thousands in callbacks down the road.