Newer vinyl or aluminum windows usually include a nail flange or vinyl nailing fin that can be included in the window frame or a separate part inserted into a kerf (slot) on the window side or front. Fixhome offers a rigid most applications. We currently offer the rigid offset nail fin PP107. It has a 1/4” offset and fits a 0.078” wide kerf.
Nailing fin holds the window in place to help it stay waterproof and airtight. The most common use is on the side where the kerf is set approximately 1” from the face of the frame. In these cases, the nail fin lies on the exterior sheathing so that the siding or stucco can go on top. This flange is almost always used in replacements and when the window is placed into a new opening.