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One of the key measurements that has to be taken before ordering replacement windows or sash (or old-fashioned storm windows, for that matter) is the angle of the existing sill. An accurate measurement ensures a good, tight fit and discourages water infiltration. The method shown here, from SpencerWorks, a maker of wood storm windows, relies on common tools—a square and a ruler—and basic trigonom­etry, which we've already done for you.

1. Place a square against the outside stop, or the sash's stile, as shown.

2. Using a ruler or tape measure, measure the vertical distance between the 2-inch mark on the square and the top of the sill.

3. Find the angle on the table below, which includes the most common sill angles. If the measurement falls in between the numbers on the table, that means the sill angle is in between, too.

Gap Angle

⅛" 4 degrees

¼" 7 degrees

⅜" 11 degrees

½" 14 degrees