How-To

What Are Window Nailing Fins? Types, Installation Tips & More

Understand the details about proper window installation.

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.

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.

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.

How to Install a Window Without a Nailing Fin

When nailing fins aren't present — such as with replacement windows or commercial installations — windows are secured through the frame directly into the rough opening using screws. The window should still be shimmed carefully to ensure it's plumb and square before fastening. In lieu of a nailing fin creating a seal against the sheathing, a properly installed flashing tape system and weather-resistant barrier (WRB) integration becomes even more critical. Sealant should be applied around the perimeter of the frame on the interior and exterior to prevent air and water infiltration. Because the self-alignment benefit of a nailing fin is absent, careful measurement and leveling take on added importance.

Nailing Fins vs. No Nailing Fin: Which Is Better?

For new construction, windows with nailing fins are almost universally the preferred choice. The fin creates a physical barrier at the wall sheathing, simplifies flashing integration, and holds the window securely while shims and frame screws are set. For replacement projects where exterior cladding remains in place, a window without a nailing fin — or with the fin removed — is typically the practical and necessary choice. Neither approach is inherently superior: the right answer depends on whether you're working with exposed sheathing or installing into an existing opening.

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.

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.

Common Nailing Fin Installation Mistakes

Sealing the bottom fin. This is one of the most common errors. The bottom fin should remain unsealed to allow any water that infiltrates above the window to escape. Sealing it traps moisture and can lead to wood rot and structural damage over time.

Using nails instead of screws. Despite the name, screws are the better fastener — they're easier to adjust, less likely to cause damage during installation, and hold more securely over time.

Skipping the seam between non-integral fins and the frame. On wood, metal, and fiberglass windows, the connection between the fin and the frame must be sealed. Self-adhering flashing tape run over the fin and onto the frame is one of the most reliable methods.

Forcing the window into a twisted or out-of-plumb wall. Integral fins on vinyl windows are rigid and will follow the plane of the sheathing regardless of wall conditions. If the wall is out of plumb, the window will be too — leading to seal failures and operating problems down the line.

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.

Do Nailing Fins Affect Window Energy Efficiency?

Nailing fins themselves have a minimal direct effect on energy efficiency — their primary role is structural and weatherproofing, not thermal performance. However, improper sealing of the fin-to-sheathing connection can allow air infiltration around the window perimeter, which will impact energy performance. When fins are correctly integrated with flashing and the WRB, they contribute to the overall building envelope's air and water tightness, which does support energy efficiency indirectly.

FAQs

Q: What is the difference between a nailing fin and a mounting flange?

A: They're the same thing — just different names used interchangeably depending on the manufacturer or region. Both refer to the thin perforated strip that extends from the window frame and is fastened to the wall sheathing.

Q: Can nailing fins be removed or added after the fact?

A: On non-integral windows (wood, metal, fiberglass), fins are attached during assembly and can technically be removed, though this changes the installation method. Adding a fin after the fact is generally not recommended and not supported by most manufacturers. Integral vinyl fins are extruded as part of the frame and cannot be removed.

Q: What type of screws should be used with nailing fins?

A: Always follow the specific manufacturer's installation instructions for fastener type and size, as requirements vary by window and wall assembly. Generally, corrosion-resistant screws appropriate for exterior use are specified, and the length should be sufficient to penetrate the sheathing and reach the framing behind it.

Q: How do nailing fins work with different types of siding?

A: The fin attaches to the wall sheathing beneath the siding, and the siding is then installed over the top of the fin. Flashing is typically integrated at the head of the window and layered over any housewrap or WRB to direct water away from the opening. The type of siding — lap, stucco, brick veneer — can affect flashing details, so always reference the manufacturer's installation guide for your specific wall assembly.

Q: Can nailing fins be used on a block or masonry wall?

A: Generally, no. Nailing fins are designed for wood-framed walls with sheathing. For block, brick, or masonry rough openings, windows are typically anchored through the frame with masonry screws or set in clips, and the weatherproofing detail is handled differently than with a fin-based system.

Q: What happens if a nailing fin gets damaged during installation or transport?

A: Minor bends in non-integral fins can often be carefully straightened. More significant damage — cracks, breaks, or compromised seal areas — should be evaluated against the manufacturer's guidelines before proceeding with installation. A damaged fin that compromises the weatherproofing connection to the sheathing could create long-term water infiltration risk.

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