How to Frame Out a Trapezoidal Window

Mark the top of the double wall header with the width of the trapezoidal window, plus any additional allowance or clearance between the window and the rough opening recommended by the window manufacturer in the installation instructions. Hold a spirit level vertically against each mark, so that one end of the level touches the header mark and other, the rafter above. Transfer the header marks to the rafter.

Measure the distance between the header and rafter marks to find the height needed for your studs. Add 3 inches to this figure to create an allowance for bevel cuts. Cut each stud out of 2-by-4 lumber on a miter saw. Hold a 12-inch spirit level against each piece in turn and hold the piece vertical under the rafter. Scribe the angle of the rafter on the end of the stud with a pencil compass. Bevel the end of the studs on the miter saw. Make a note of the setting of the gauge -- for example, at 40 degrees -- on the miter saw for additional cuts later.

Line up the beveled end of the stud with the bottom of the rafter and mark the other end where it crosses the header. Cut the studs square at this mark. Fit the studs between the marks made earlier and toenail them in place with 10d nails.

Measure down from the rafter to mark each of the two studs for the height of the rough opening of the trapezoidal window. Measure the distance between these marks and add 3 inches to create an allowance for beveled ends. Cut a piece of 2-by-4 lumber to this length; this will be your window header. Hold or clamp the horizontal 2-by-4 so that its bottom surface touches the marks on each stud. Mark with a speed square where to bevel the ends so that the window header fits perfectly between studs. Cut the window header on the miter saw. Return it to the studs and toenail it in place.

Cut short studs to fit between the window header and the rafter at either side of the rough opening. Bevel one end of a 2-by-4 to the angle obtained earlier in Step 2. Mark the short stud the approximate vertical distance between the rafter and the window header plus ¼ inch. Make another bevel parallel to the first at your mark. Hold the short stud in place with a 12-inch spirit level. It should be slightly long. Mark a final trim at one beveled end and cut the short stud. Test fit the short stud and continue trimming it if necessary.

Cut identical, additional short studs to fit every 16 inches on center. Mark the short stud locations on the side of the window header and toenail the short studs in place.