Cut and pry carefully to avoid damaging the window jambs or any part of the surrounding wall. Spread out your force by placing a piece of wood under your pry bar. This will lower the risk of damaging the surrounding surfaces. If you’re starting from scratch with a blank drywall window opening, you can get straight to measuring and cutting your new casing.
Rather than simply recording the length and width of the opening, make sure you measure each side separately. There may be small discrepancies in the dimensions of the opening. Round off to the nearest ⅛” (. 32cm) to give yourself a little room for error.
For example, if you’re adding trim to a window that’s 28” (71cm) wide and you want the sill to come out 3” (7. 6cm) on either side, the total length of the sill piece will be 34” (86cm). A sill that’s slightly wider than the window opening will create the appearance of support across the bottom.
To avoid mistakes, make each cut individually, stopping the saw and repositioning the board after each line.
Test the fit again before nailing the sill into place. You may need to do this several times. Don’t rush, since you want a good fit.
Recess the nails into the wood using a nail set. This will make the nail holes easier to fill and paint later. A nail gun can save you a lot of time in the long run compared to a traditional hammer. [6] X Research source
It’s common for the upper casing to be about an inch (roughly 2. 5cm) shorter than the sill on either side. If you’re using the same casing for the top and sides of the window, the distance that the top piece extends on either side should be equal to the width of the trim. [8] X Research source
Mark the length of the top piece from the end of the casing to save yourself an unnecessary cut.
Before you cut your sloped casing, mark your window measurements on your trim piece. Then, cut your 45 degree angle moving away from the measurement lines on both sides of your trim. The shorter part of your head casing piece needs to match your window measurement. The edges of the finished top piece should now fit together perfectly with those of the side casing. If you don’t have access to a miter saw, you can also use a miter box and a normal handsaw to achieve a precise angle. [10] X Research source
Use a level to confirm that the top piece is even before nailing it down. [12] X Research source The reveal line will lend the appearance of added depth to the finished window. If your side and head cases look a little misaligned, you can fix this with a long finish nail. Drive the long finish nail into the top of the side casing and into the head casing, which should straighten them.
Be sure to mirror the cuts on each piece so that they’ll fit together with the head casing. [13] X Research source
Perform a couple mock-ups before you secure the casing permanently. If possible, have someone else hold the trim so you can inspect it from a distance. A little adhesive will help hold the trim in place while you fasten it.
To make mitered casings extra secure, fasten them together from the outer edge at the point where they intersect. [16] X Research source
Most wood fillers will take between 15 minutes and an hour to dry completely, depending on the depth and number of holes. [18] X Research source
As long as you’ve measured, cut and nailed the casing carefully, there should be no need to caulk.
Use just enough force to wear down swelling, splinters, and other inconsistencies and prepare the materials to accept paint. If you caulked around the new trim, cut or fold the sandpaper into a small strip and go over it until it’s no longer visible.
Use painter’s tape to mask the surrounding wall and avoid stray drips and streaks. Choose a shade that matches the rest of the trim throughout your home. Be prepared to give each coat of paint 4-6 hours of drying time before reapplying.
