Pella half-circle windows can be beautiful additions to any home, creating a look that sets the house apart from those using traditional rectangular windows only. Installing the half-circle window involves much the same installation process as the installation of those traditional windows. The primary difference is the need for an arched opening in the wall to hold the window in place. If there is no arched opening, you’ll need to create your own based on the chosen window’s dimensions. If an opening is in place, you’ll need to use the window opening’s size to determine the window that you can install instead.
Things You'll Need
- Tape measure
- Utility knife
- Flashing tape
- Carpenter’s level
- 1/2-inch drill bit
- 2-by-4-inch plank
- Circular saw
- 1-inch-wide, 1/4-inch wood shims
- 2-inch galvanized roofing nails
- 13/64-inch drill bit
- 1/8-inch drill bit
- Corrosion-resistant No. 10-by-3 1/2-inch wood screws
- Insulating foam sealant
- Foam backer rod
- Exterior-grade sealant
Framing the Opening
Measure the height and width of the window with the tape measure to determine the opening size needed to create your rough opening. Choose a rectangular window opening in the wall framing for the window placement.
Create a template of the window by placing the window onto a piece of cardboard and then tracing its outline onto the cardboard with a pencil. Cut out the template along the traced line with a utility knife.
Attach the template to the building sheathing with a piece of flashing tape, with the top of the window set against the header across the framed window opening. Use a carpenter’s level to make sure the base of the template is level so that your frame will be level as well. Mark the shape of the template onto the sheathing for framing.
Drill a 1/2-inch hole through the sheathing, touching the outside edge of the traced line. Cut out the template along the outside edge of the traced line with a reciprocating saw. Using the outside edge should provide you with about 1/4 inch of excess space around the window.
Place a 2-by-4-inch plank diagonally along the edge of the opening, running from the side wall stud to the header in a 45-degree angle. Mark the line of the header and stud on the plank for cutting the needed angle to fit the plank between the two pieces. Cut the plank with a circular saw using a 45-degree beveled cut on each end, and then nail into place running alongside the opening in the sheathing diagonally between the header and wall stud at the window opening’s side.
Installing the Window
Measure the window opening in your wall with a tape measure to determine the maximum dimensions possible for your window if a pre-cut half-window opening is already in place. Subtract 1/2 inch from the height and width measurements to leave a gap around the window for leveling it within the rough opening. Purchase your window using the adjusted measurements.
Cut the water-resistive barrier covering the opening if it hasn’t already been cut, using a utility knife. Cut the barrier across the top of the opening, following the line of the curve of the window. Make a second cut horizontally above the base of the opening. Make two additional vertical cuts starting 6 inches above the top of the opening, and 6 inches to the opening’s sides, stopping 6 inches above the base of the opening. Angle the side cuts inward toward the base of the opening at a 45-degree angle to meet the curved corners of the first cut across the opening’s top. Fold the bottom flap over the windowsill and then fasten it to the interior wall with flashing tape. Lift the top flap and tape to the exterior of wall above the window opening with.
Cut a strip of flashing tape so that it's 12 inches longer than the windowsill. Place the flashing tape along the exterior edge of the windowsill so that a 1-inch tab overhands the edge, and the ends of the tape rise along the sides of the opening 6 inches. You’ll need to cut inch-long notches at the corners with the utility knife where the base of the sill meets the sill sides to get the flashing to fit flush with the surfaces. Make the cuts 1/2 inch from the corner, both on the sill side of the tape and on the side of the tape that rises up the window opening edges. Fold the flashing snug into the corners of the window opening. Press the flashing tape to the opening firmly over the water-resistive barrier.
Cut a second length of flashing tape and place it along the inside edge of the windowsill so that it’s flush with the edge and overlaps the first piece of flashing tape at least an inch.
Set 1-inch-wide, 1/4-inch-thick wood shims onto the windowsill, 1/2 inch from each side of the opening. Cut the shims with a circular saw to match the depth of the sill. Set a carpenter’s level onto the shims to make sure that they’re level. Add thinner shims on top of the first for additional height if the sill is not level. If using a half-window made of two or more pieces, set a shim across the windowsill at any points where the multiple pieces meet.
Set the window frame into the prepared opening on top of the shims from the exterior side of the opening so that the jamb fin running around the window perimeter is flush with the opening edges. Center the window within the rough opening by measuring the distance between each side of the frame and the wall surface. The window is centered when all measurements are even. Hammer a roofing nail through the jamb fin at the first hole along the top of the fin sides to hold the window steady. Plumb the window by setting a carpenter’s level vertically against the front of the window and tilting the window in place until it’s vertically level.
Drill mounting holes through the window frame and into the framed rough opening on the interior side of the window with a drill containing a 13/64-inch drill bit. Place mounting holes 6 inches from the window corners, then 10 inches on center from the corner holes across the perimeter of the window. Drill the pilot holes 1 1/2 inches deep.
Drill a 1/8-inch hole 1 1/2 inches deep through the perimeter jamb fin into the opening frame every 12 inches. Hammer 2-inch galvanized roofing nail through the holes to hold the window in place.
Apply 1 1/2-long pieces of flashing tape to the corners of the windowsill over the jamb fin on the exterior side of the window. Place the first strip of tape along the base of the fin, and then set the second piece of tape along the side of the fin, overlapping the first. Set the tape so that it folds over the edges of the sill onto the frame of the window by 1/2 inch.
Cut two more strips of flashing tape to the height of the window plus 5 inches. Place the tape over the jamb fin along the exterior sides of the window, overlapping the frame 1/2 inch, and positioned so that the ends are 2 inches above and 3 inches below the frame.
Cover the top of the window jamb fin exterior with flashing tape, using multiple pieces to cover the curved edge. Work from the sides of the fin inward, overlapping the prior piece of tape with each new piece. Place a second layer of tape across the top of the first, overlapping the first by 1 inch and offsetting all of the seams to prevent creating channels for water flow.
Lower the exterior piece of water-resistive barrier back over the top of the window. Place flashing tape along the 45-degree angled cut from the corner, and then along the side cut, overlapping the corner piece by an inch.
Set additional shims along the base and sides of the window at each of the pilot hole locations. Drive corrosion-resistant No. 10-by-3 1/2-inch wood screws through the pilot holes and into the frame around the window opening. Do not use shims along the top of the window opening, as you need to leave space for header movement.
Seal the window from the interior using insulating foam sealant. Place the nozzle of the sealant bottle into the gap between the window and the wall until it's 1 inch from the outside edge. Spray a bead of foam along the gap, filling the space to within 1 inch of the interior window surface to allow for expansion of the foam.
Seal the exterior of the window by setting a strip of foam backer rod into the gap surrounding the window 1/4 inch beneath the window opening surface. Fill the last 1/4 inch of space with an exterior-grade sealant.
Tips & Warnings
- You will need 1 1/2 inches of solid wood blocking within the rough opening to install the window. If there is no wood frame already in place, then cut 2x wood planks the same depth as your window and attach the planks to the rough opening to create the frame before measuring for window size.
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