Nail Fin, Flush Fin, and Block Frame Windows: Which Frame Goes Where

Order a replacement window in California and you are choosing two things. One is the window itself: the material, the glass, the way it opens. The other is the frame type, which is how that window attaches to the wall. There are three, and installers use the names constantly without explaining them.

This page explains all three, where each belongs, and what each means for keeping water out. It is part of our California window guide.

The three frame types in one table

All three can be the same product line with the same glass and the same energy ratings. What differs is where the water seal is and how much of the wall gets opened up to make it.

Nail fin: the frame a new house gets

A nail-fin window has a thin flange running around the outside of the frame. When a house is built, the window goes into the rough opening before the siding or stucco, the fin is fastened to the wall, and strips of flashing are lapped over it so water running down the wall is carried past the opening.

The industry standard for this is AAMA 2400, published by the Fenestration and Glazing Industry Alliance, which covers windows with a mounting flange in new construction. It describes the flashing going on in "weatherboard fashion": sill strip first, then the jambs, then the head, so that each layer laps over the one below it. Milgard's installation page tells installers to follow AAMA 2400 for new-construction applications.

That detail is why a nail-fin window is the most robust way to waterproof an opening. The seal is behind the cladding, integrated with the water-resistive barrier, and it does not rely on a bead of sealant exposed to the sun.

It is also why a nail-fin window is rarely the default on an existing house. To reach the layer the fin attaches to, the cladding around the opening has to come off. On lap siding with trim boards, that is a modest job. On stucco, it means cutting the stucco back around every window and patching it afterward.

When nail fin is the right call on an existing house

• You are replacing the siding or re-stuccoing anyway. The cladding is already coming off, so every window can be flashed properly at little extra cost. Doing windows and siding in the right order is covered in how installers handle corners, windows, and trim.
• The old frame is rotted, bent, or leaking into the wall. A retrofit covers an old frame; it does not fix one.
• The opening is changing size, or a window is becoming a door.
• You want the largest possible glass area and the original proportions, with no old frame left visible.

Flush fin: the California retrofit

A flush-fin window has no nailing flange. Instead it has a flat flange on the exterior face. The installer removes the old sashes and any center bars, leaves the old perimeter frame in the wall, slides the new window in from outside, and the flange laps over the old frame and the edge of the stucco. Sealant between the old frame and the flange makes the primary seal. A second bead where the flange meets the wall finishes it.

The standard that covers this is AAMA 2410, "Standard Practice for Installation of Windows with an Exterior Flush Fin Over an Existing Window Frame." Milgard's installation page names it as the standard for replacement applications.

Why this method took over in California

A large share of California's housing was built with aluminum-framed windows set into stucco walls. Those aluminum frames have their own fins, and the fins are buried under the stucco, lapped into the building paper. Taking one out means breaking stucco around the whole opening.

The flush fin was the answer to that. It replaces the glass, the sashes, and the thermal performance of the window without touching the stucco, the paper behind it, or the interior drywall returns. Milgard's flush-fin field instructions, written for its sliding doors, which retrofit the same way, make the reasoning explicit: they tell installers not to remove the head or jambs of the original frame, because doing so may damage the weather-resistant barrier.

So the method is not a shortcut invented by installers. It is a published practice built around the fact that, on a stucco wall, the old frame is already part of the waterproofing. Our retrofit windows page covers what the job looks like from the homeowner's side.

What flush fin assumes

AAMA 2410 is honest about its limits, and you should know them. The practice states that it assumes the seal between the existing window and the wall or water-resistive barrier is adequate, and that the wall around the existing window is structurally sound. It also requires a pre-installation inspection: if there is evidence of water penetration, corrective measures are to be taken before the new window goes in.

Read that as a rule for when not to use it. If there is staining under a window, soft drywall at the corners, or a stucco crack running from a corner that takes in water, a flush fin will hide the cause. That opening needs to be opened up.

The details that separate a good flush-fin job from a bad one

• Sealant on the old frame, not just around the edge. The standard calls for a continuous bead on the old frame's mounting surface across the head and down the jambs, so the flange beds into it. A bead run only around the outside edge afterward is the secondary seal doing the primary's job.
• Weep paths left open at the sill. The standard calls for gaps in the sill sealant in line with the old frame's weep holes, and repeats the instruction not to block them. A sill sealed solid traps water between the old and new frames.
• The head flashing left working. The flange must not cover or interfere with existing head flashing.
• Fasteners through the head and jambs, not the sill. Screwing down through the old sill can open a path through the water-resistive barrier.
• Low-pressure foam only. Expanding foam that is not rated for windows can bow a frame. The standard points to foams tested to AAMA 812.

Block frame: no fin at all

A block-frame window is a plain box. It has no nailing fin and no exterior flange. It is set into an opening, shimmed, screwed through the jambs, and sealed at the perimeter. Manufacturers use different names. Andersen calls the version that goes inside an existing frame an insert window, and describes a flangeless full-frame unit as one installed through the jambs and sealed on the interior and exterior.

On the West Coast you see block frames in a few specific situations:

• Wood windows with a wood frame worth keeping. The old sashes and stops come out, and the new unit sits in the pocket where the sashes ran. Interior casing and exterior trim both stay. Andersen notes that an insert needs an opening that is plumb, level, and square, and a minimum jamb depth of 3 1/4 inches between the stops.
• Openings in brick, block, or concrete, where there is nothing to nail a fin to and no flat stucco face for a flange to land on.
• Houses with wood exterior trim around the windows, where a flush fin would look wrong and new trim or a stop can cover the joint instead.
• Recessed stucco openings, where the window sits back from the wall face and the stucco returns into the opening.

The waterproofing on a block frame is the most dependent on workmanship. There is no fin behind the cladding and no flange lapping over the old frame, so the perimeter sealant joint, and any sill pan or flashing that can be added inside the opening, carry the load. Done carefully, with backer rod and a properly sized sealant joint, it works. Done with a thin smear of caulk, it is the frame type most likely to leak first.

Waterproofing: where each type keeps the water out

The California Residential Code sets the rule all three have to satisfy. Section R609.1 requires windows to be installed in accordance with the manufacturer's written instructions and flashed in accordance with Section R703.4. Section R703.4.1 then says flashing at window openings is to be installed per the window manufacturer's installation and flashing instructions, and where those give no detail, pan flashing goes at the sill with protection at the head and sides.

In practice that means the manufacturer's instructions for the specific frame type are the code requirement. Here is how the water management differs:

The detail of how flashing is layered is on our window flashing and waterproofing page.

How to choose for your house

For most stucco houses in Sacramento and the Bay Area with original aluminum windows and no sign of leaks, flush fin is the sensible default. It is the method the standards were written for, it leaves the stucco and the paper behind it alone, and it avoids a patch that may never quite match. We go through the stucco-specific options on replacing windows in stucco homes.

Choose nail fin when the wall is being opened anyway, when there is water damage to find and fix, or when you are changing the opening. Our comparison of retrofit versus new-construction windows covers that decision in more depth.

Choose block frame when the house has wood frames worth keeping, wood trim that a flange would fight with, or masonry openings.

It is common to mix them. A house can have flush fin on most openings and a full nail-fin replacement on the one window that has leaked. A good quote lists the frame type for each opening rather than applying one word to the whole house. When you are comparing bids, our guide to comparing window quotes line by line starts with exactly this item.

What the frame type does not change

• The energy requirement. The U-factor and SHGC your windows must meet come from your climate zone, not from the frame attachment. See what Title 24 requires of replacement windows.
• The permit. Whether a permit is needed is a question for your building department, not a function of the fin. See permits for retrofit windows in California.
• Safety glazing and bedroom egress. AAMA 2410 itself reminds installers that a retrofit still has to respect the rules on safety glazing and emergency escape openings. A smaller frame inside an old one can reduce the clear opening of a bedroom window, and that needs checking before the order goes in.

Sources

• 2025 California Residential Code, Section R609.1 (installation per manufacturer's written instructions)
• 2025 California Residential Code, Sections R703.4 and R703.4.1 (flashing at window openings)
• FGIA, AAMA 2410-13, Standard Practice for Installation of Windows with an Exterior Flush Fin Over an Existing Window Frame (licensed copy published by Anlin)
• AAMA 2400-10, installation of windows with a mounting flange (licensed copy published by Milgard)
• Milgard, Installation Information
• Milgard, Field Installation Instructions for Vinyl Sliding Glass Doors, Flush Fin
• Andersen, How to assess windows for replacement and How to measure windows
• ASTM E2112-23, Standard Practice for Installation of Exterior Windows, Doors and Skylights

Working with Heritage Exteriors

We look at each opening from inside and outside before recommending a frame type, and our quotes name the frame for every window so you can see where the stucco stays and where it does not. If an opening shows signs of a leak, we will say so rather than cover it. Licensed in California, CSLB #1055919. Call (916) 546-2991.