How do I Add Fins to Replacement Windows in New Construction?

By Alexander Callos

Updated September 15, 2017

Items you will need

  • Tape measure

  • Snips

  • Paper

  • Pencil

  • Marker

  • Snap-on nailing fin

New construction windows come fully equipped with a nailing fin on all sides of the window. This is there so the fin can be nailed directly into the studs. For replacement windows, installation is done through the side jambs, and no nailing fin is involved. Oftentimes nailing fins are necessary for people who are not interested in purchasing a whole new window. The key to adding a nailing fin to a replacement window is knowing who manufactured the replacement window. From there, any homeowner can attach a nailing fin to a replacement window in no time.

Inspect the window to determine who the manufacturer is. Search the glass looking for any markings usually located in the bottom corners of the window. Write down any series of letters and numbers etched into the glass. Record any possible names of the manufacturer if they are listed.

Measure the size of the window to determine what size snap-on nailing fin will be used. Write down the measurements of both the length and width. Take the window information to a window supplier to check and see if they sell that type of window. Continue until the manufacturer is found and a snap-on nailing fin can be purchased.

Line up the snap-on nailing fin next to the window to see if the length of the pieces is correct. Mark on the pieces where cuts will need to be made and cut the fins to size with snips if necessary.

Line up the first side of the nailing fin running vertically along the window. Locate the accessory groove running along the outside edge of the window. Push the snap-on fin into the channel and lock it into place. Pull on it to make sure it is secure.

Line up the second piece along the top of the window and lock it into place in the exact same manner. Continue with the other side and the bottom snap-on nailing fin. Inspect the window to make sure everything is in proper working order and the fin is securely in place.


The accessory groove on the replacement window is about 1/8 of an inch wide and 1/4 inch deep. This may vary slightly depending on the manufacturer.