How to Install Circle-top Windows

October 11, 2010

Due to the half round shape of the glass, installing circle-top windows may seem more difficult than installing regular picture windows. This is not necessarily the case, especially if you learn how to install circle-top windows the right way. Make sure to follow the specific instructions included with the window you have purchase and familiarize yourself with these common installation guidelines:

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  1. Start by preparing a level and plumb rectangular window opening which is approximately 1/2 of an inch wider and taller than the circle-top window frame.
  2. Temporarily place the window into the opening and center it on the sill. Do not nail the window in place, but rather have a helper hold the window in place from the outside. From the inside, determine the size of the two framing members which are needed in the upper left an right hand corners of the opening. These framing members will be made from 2x4s or 2x6s(depending on the thickness of your home's walls) and are the support which the curved nailing flange of the window will attach to.
  3. Remove the window from the opening and install the two newly measured framing members and some plywood sheathing to match your home's existing sheathing(usually 1/2" thick). You should now have a rough window opening with 6 straight sides.
  4. Wrap the edges of the opening with building wrap, working from the bottom to the top so water can shed down the wall of the house without running beneath the building wrap.
  5. Apply a bead of silicone caulk around the perimeter of the window opening, making sure the window nailing flange will cover the caulk, thus forming a weather tight seal.
  6. Install the window into the opening and center it on the sill.
  7. Using 2-inch roofing nails, secure the window in place by setting nails through the holes in the window's nailing flange.
  8. Cover the nailing flange with self-adhesive flashing tape, sealing the bottom edge of the window first and then the curved part of the window.
  9. From the inside of the house, fill the gap around the window frame with low expanding foam insulation.
  10. Circle-top windows require radius trim pieces both inside and out, and these are usually available from the window manufacturer.

If you are still unsure how to install circle-top windows, then you should hire a window installer or contractor who can help you complete the job.

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