What You Need to Know About Window Nailing Fins

Have you been installing windows the right way?

July 14, 2022
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 installation mistakes.

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

Replacement windows, 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.
Rendering of Integral Nailing Fin on Marvin Window

Integral nailing fins

There are two types of window nailing fins, integral and non-integral. Integral nailing fins are called “integral” because the nailing fin and the window frame are extruded as one solid piece. When the windows are assembled, all four corners are melted/welded together creating a sealed shell around the entire perimeter of the window. Integral fins are only found on vinyl windows.

Rendering of Non Integral Nailing Fin on Marvin Window

Non-integral nailing fins

Non-integral nailing fins are added to wood, metal and fiberglass window frames during the assembly process. Both the area where the fins meet at the corners and the seams between the fins and the frames needs to be sealed during the window installation. Non-integral nailing fins have several advantages. They can be folded down which makes them less susceptible to damage during storage and transport. Folding fins also allow a window to be pushed through the rough opening from the inside, which means it doesn’t need to be carried up a ladder or scaffolding, an especially important feature when installing large windows on upper floors.

The stiffness of integral fins forces a window to follow the plane of the sheathing even if the wall is severely twisted or out of plumb. This can cause weather strip misalignment, poor operation, and put stress on glass, which could cause seals to fail prematurely. The flexibility of non-integral fins provides more adjustment options during the installation and creates a buffer when a house settles.

Nailing fins are only one part of the installation

Nailing fins do add structural integrity to a window, but proper shim location and the frame-screws are as, if not more, important than the fins. In fact, many manufacturers consider nailing fins nothing more than a guide to the installation process and a way to hold the window in place while the shims and screws are being installed.
Rendering of how to properly install flashing tape

Nailing fins need to be sealed

The back side of all nailing fins should be sealed to the wall with an exterior-grade, non-shrinking sealant that will remain flexible. Non-integral nailing fins also need to be sealed to the frames they’re connected to. This can be accomplished by running self-adhering flashing over the fin and up onto the window frame (above). Do not apply sealant or flashing to the bottom nailing fin. An unsealed bottom fin will create an escape route for any water that finds its way past the building envelope somewhere above the window.

Rendering of the proper way to secure nailing fins for window installation

Screws are better

The name “nailing fin” would suggest that nails would be the fastener of choice, however, screws work better and not just because they have more holding strength. Future adjustments are easier to make with screws and overtightening is easily avoided. Also, installing screws eliminates the possibility of damage caused by wayward hammer blows.

Rendering of Non Integral Nailing Fins with required Corner Gaskets for Marvin Window installation

Non-integral nailing fins require corner gaskets

Windows with non-integral fins are shipped with gaskets to seal the corners. If the gaskets get lost or damaged, it is acceptable to create your own gaskets out of self-adhering window flashing.

Follow the instructions

Sealant requirements, flashing procedures and nailing patterns can all vary from one window to another. Always follow the installation instructions created for the specific windows you are installing.

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