Nailing fins.
What a fin does, where it belongs, and what the installer has to get right.
Vistaza Engineering · Feb 9, 2023 · Updated Sep 20, 2026 · 3 min read
A nailing fin answers a hidden question: how the frame is fixed and sealed to the structure. It suits one kind of wall and one order of work. The visible side of the same joint, how it is finished against the facade, is covered in Exterior trims.
What a nailing fin is.
A nailing fin is a thin, flat flange that runs around the outside edge of the frame. It lies against the wall sheathing, and the installer fastens through it. It does two jobs:
- It anchors the window. The fin is the anchor point between the frame and the sheathing.
- It is part of the water seal. The fin works with the flashing and the weather-resistant barrier, so that wind-driven rain reaching the opening is turned back out.
Fins belong to one construction sequence: the window goes in before the cladding. That is the usual order on a framed and sheathed wall. Replacement windows, fitted inside existing cladding, do not use fins. Neither do windows set into a masonry or concrete opening. Those are fixed with clips or brackets, and the perimeter joint is sealed.
Integral or applied.
| Type | What it is | Worth knowing |
|---|---|---|
| Integral fin | Made as one piece with the frame. The corners are welded when the frame is welded | A continuous sealed edge around the whole window, with no fin-to-frame joint to seal |
| Applied fin | A separate strip fitted to the frame during or after manufacture | The corners, and the seam between fin and frame, must be sealed on site. Some types fold flat for transport |
An applied fin has one practical advantage on upper floors. With the fin folded, the window can be passed through the rough opening from inside the building.
In the Vistaza range the nailing fin is an option on Vi5 only; Vi6 does not take a fin. It is an applied, offset fin: it sits back from the outer face of the frame, so the frame stands a little proud of the sheathing and meets the cladding cleanly. We prefer the applied type. An applied fin is easier to repair, because a damaged fin can be replaced without touching the frame. It also suits windows that ship in sections and are assembled on site. Without a fin, the windows are fixed with clips.
What the installer has to get right.
Vistaza supplies the windows and the installation instructions. It does not install. These points are for the installer's method statement. They are the ones a field water test tends to find.
- Follow the installation instructions for that exact product.
- Prepare the rough opening. It should be clean, with no screws or nails standing proud, and flashed before the window goes in.
- Bed the fin. Run a continuous bead of flexible, non-shrinking exterior sealant on the back of the fin before it meets the wall.
- Shim the frame. Set shims between the frame and the framing, at the anchor points and under the sill, so the frame sits square, plumb and supported before any fastener goes in. The fin holds the window to the wall. The shims hold its shape.
- Seal the fin-to-frame joint on applied fins with self-adhesive flashing tape, and do the corners as well.
- Do not overdrive the fasteners. An overdriven fastener squeezes out the sealant and distorts the fin, and both open a path for water and air.
- Lap the flashing and the weather-resistant barrier over the fin in the right order, so that each layer sheds water onto the one below.
A fin is an anchor and a seal. It only works if the wall goes on after it.
Which walls take a fin.
| Wall | How the window is fixed |
|---|---|
| Framed and sheathed wall, cladding installed after the windows | Nailing fin, tied into the flashing and the weather-resistant barrier |
| Masonry or concrete opening | Clips or brackets, with a sealed perimeter joint |
| Existing wall where the cladding stays | Clips or brackets |
The nailing fin is an option on Vi5. The installation method is confirmed for each opening on the shop drawings.