Andersen Windows 400 Installation Instructions

Andersen has producing high-quality windows for over a century and its premier Series 400 windows offers a wider range of sizes, types, interior finishes and quality features than its other window series.
Among the Series 400’s many added features, they are equipped with a composite sill that is impervious to the threat of rotting so common with standard wood sills and the exterior cladding is a heavier gauge that is available in a wider range of colors. The installation procedure for an Andersen Series 400 window insures perfect alignment and operation, as well as a weather-tight seal against the elements.

Step 1

Perform a preliminary verification that the window’s rough opening in the wall is level and square. Check the sill of the opening with a builder's level and adjust if needed. Verify that the opening is square by measuring the opening diagonally, in both directions from the corners. The two diagonal dimensions must be within 1/8 inch for the opening to be square.

Step 2

Install the window’s base flashing, called the “sill pan,” by cutting a piece of flexible adhesive-backed flashing to the opening’s width plus 24 inches. Apply the flashing to the window sill following the manufacturer’s instructions printed on the flashing. The sill pan prevents water from penetrating the wall in the event the window’s seals are damaged or fail.

Step 3

Position beveled plastic window sill shims on the sill surface. Place one at each side of the window opening and additional shims equally spaced approximately 6 to 8 inches apart in between. Tack the shims in place using 6d nails.

Step 4

Insert and center the Andersen Series 400 window into the rough opening, with the window frame sitting on the plastic shims installed in Step 3.

Step 5

Adjust the window within the rough opening to where the gap between the wall framing and the window jamb are equal on both sides of the window using a pry bar to shift the window as needed. Nail both vertical exterior nailing flanges to the wall framing using 8d nails in each flange hole.

Step 6

Recheck the interior gap between the window jamb and the wall framing to verify it is equal and consistent on both sides. Complete the securing of the nailing flanges with 8d nails in each flange hole around the perimeter of the window frame.

Step 7

Cut two pieces of flexible adhesive-backed flashing to the vertical dimension of the window plus 10 inches. Apply one piece of flashing to each side of the window following the manufacturer’s instructions printed on the flashing. Make certain the inner edge of the flashing covers the nailing flange on both sides and extends beyond the window frame approximately 2 inches at the top, with the remaining excess overlapping the sill pan flashing applied in Step 2.

Step 8

Measure the width of the top of the window frame and cut a piece of flexible adhesive-backed flashing to this dimension plus approximately 24 inches. Apply the top piece of flashing following the manufacturer’s instructions printed on the flashing, making certain to cover the window’s nailing flange and extending equally past both sides of the window to overlap the side flashing pieces applied in Step 7.

Things You Will Need

  • Tape measure
  • Builders level
  • Utility knife
  • Hammer
  • Pry bar
  • Flexible adhesive-backed window flashing
  • Window sill shims
  • Andersen Series 400 window unit
  • 6d box nails
  • 8d sinker nails


  • Because the interior window frame adjustments must be made in conjunction with the exterior attachment, it is recommended that you enlist the assistance of a helper in undertaking this project.
  • Flexible adhesive-backed window flashing has a two-piece paper backing and application of the flashing is made easier when only one piece of backing paper is removed gradually to apply the adhesive side. The second backing piece is removed to complete the installation while stretching the flashing into place. The adhesive is very sticky and nearly impossible to separate if the piece should wrinkle or stick to itself.


  • You must stretch the flexible window flashing around corners as it is applied, rather than cutting the flashing, as cutting adds to the risk of water leakage and may void the warranty.

About the Author

Paul Massey has been writing since 2009, drawing on a 35-year career in the construction industry. His experience includes 15 years as a general building contractor specializing in architectural design, custom homes, commercial development and historic renovations.