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How to Build a 90 Degree Box Bay Window

Updated July 20, 2017

Converting your bay window to a box bay window has its challenges. While most people veer away from the use of circular and mitre saws, some homeowners may be comfortable with them and thus, might still find that they are up to the challenges this project offers. Since the base of the project is already installed and ready to go (the existing bay window), this project is not that difficult beyond the tools required for its completion. If you are one of the brave homeowners that choose to build your own box bay window, you can bet that you will save a lot of money and have a big sense of pride every time that you see your handy work.

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Pry the nails out of the window frame of the existing bay window, using a pry bar.

Unscrew all fasteners that may be securing the existing bay window, using a screwdriver. Gently pull the window frame away from the window.

Measure the window frame's dimensions using a tape measure, and record the exact measurements of the frame onto paper using a pencil.

Place light pencil markings onto your plywood, using a yard stick or straight edge as your guide. This will help you create an exact template line for you to cut using your circular saw. Cut the plywood along the lines that you just created.

Use a caulk gun to place caulk on the edge of the window frame. Attach the window frame to the newly cut plywood piece, using a screw gun and screws. Place a screw every four to six inches to secure the pieces together.

Place a line of caulking inside the original window framing, from inside your home. Screw the new box to the window frame, inside of the house. Every twelve inches, place a new screw going around the entire perimeter.

Measure around the edges of the window frame where the plywood and house are joined. Cut the wooden moulding to these measurements, using a mitre saw set to a 90-degree angle. Only cut off the ends of the moulding with the mitre saw. Make sure to turn the moulding over, after cutting the first edge of a piece, then cut the other edge at the same angle.

Bead caulking to the box area and the house, using a caulk gun. Secure the moulding with screws and your screw gun. Place the screws twelve inches apart. Bead the corner of the plywood with caulking, using the caulk gun. Screw the corner moulding into place with your screw gun.

Prime the box bay window area. Allow the paint primer to dry for 24 hours. Apply two separate coats of paint to the primed area, after it has dried completely. Allow the first coat of paint to dry for at least 24 hours. Let the second coat dry for at least 48 hours, to complete the job.


Ninety-degree angle cuts on mitre saws are most useful when you are trying to put one piece flush with another piece; otherwise, it is usually best to try using a 45-degree angle on the mitre saw when doing this or a similar task. It helps to match the colour inside the box bay window to the paint scheme inside your house, and to match the outside portion of the box bay window to the colour scheme on the outside of the house.


Always wear protective gear when operating a power saw, and make sure that you are conscious of your surroundings while using power tools, to avoid accidental injuries.

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Things You'll Need

  • Pry bar
  • Screwdriver
  • Tape measure
  • Paper
  • Pencil
  • Yard stick (or other straight edge)
  • Circular saw
  • Marine-grade plywood sheet (1/2-inch)
  • Caulk gun
  • Caulk
  • Mitre saw
  • Wooden moulding (with corner moulding)
  • Screw gun
  • Stainless steel screws (self-threading)
  • Paint primer
  • Paint
  • Paint brush

About the Author

James Essence has been a professional writer since 2006. He writes articles on real estate, art, the music business and professional wrestling. Essence has been published in several online magazines, including Shive Magazine. He currently is a student at the University of Wisconsin-Milwaukee.

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