We explain the different types of nailing fins and clarify some common misconceptions as to what their real purpose is. You will also learn nail fin installation tips to help you avoid common window
What are nailing fins?
Nailing fins, sometimes called mounting flanges, are the thin strips installed on the exterior sides of a window. Unlike a “front flange” which is a decorative trim piece, a nailing fin is usually set back from the outer edge of the window frame and has fastener holes punched into it. The purpose of nailing fins is to secure the window to the wall sheathing and hold it in place while the shims and screws are being installed. Fins also work in conjunction with flashing and the weather resistant barrier (WRB) to prevent wind and water infiltration.
Not all windows have nailing fins
, sometimes called “inserts,” don’t have nailing fins because they are typically installed with the existing cladding still on the wall, which means there is no exposed sheathing for them to be fastened to. Replacement windows are fastened through the window frames into the sides of the window openings. Commercial windows are often installed into block or steel rough openings where a nailing fin would be impractical.