Interior trim styles for nail-fin windows

Window trim is highly specific to time period and place of origin. Capturing the style and period of your home is vital to make things feel cohesive.

The trim pictured below is a variation on a rosette and casing window, with some beautiful embellishment. The style is ubiquitous to 1910-1920s St. Louis, Missouri when the city was still growing rapidly. This trim was mass-manufactured and its target market was the new, burgeoning middle-class.

On the other hand, most builder-grade homes built after ~1970 (ours included) make use of nail-fin type windows. The nailing fin is a 1-inch projection around the perimeter of the window typically made of metal that acts as a flange for the resulting gaps the rough window opening will have after shimming the window in place. This type of window is installed after the studs are raised, but before siding, insulation, and drywall are installed. While the ease of installation factor is high for new construction, the replacement and modification of window openings is near-impossible for brick or stone exterior finishes, and very labor-intensive for traditional wood or cement fiberboard siding (e.g. removal of siding, drywall, and utilities to accommodate the new nail fin). Luckily, homeowners can still remove nail fin windows and replace them with custom-sized insert windows to get the look they’re after.

The builder of our house chose aluminum nail-fin windows, and finished the interior with a simple window sill and drywall around the side and top jambs. Since nail-fin windows aren’t full depth, the drywall around the side and top is about 4-inches deep (whereas a tilt-in insert window might have an inch). This means that the window can’t be cased like a traditional window.

A typical nail-fin window. Perspective is as if the viewer were standing in the middle of the wall. Attached to the exterior plywood sheathing with nails hammered through the fin, the window has a ~1” solid metal flange on which to finish sealing with flexible/tarred tape. Siding is applied over this and the house is water-tight.

A tilt-in retrofit. Perspective is as if the viewer were standing in the middle of the wall. This type of window relies on the existing nail-fin to seal the opening, while the bottom and top of the tilt-in window frame mates up to the nail fin, sealing the surface. Pre-1970s homes may have this type of window at an original installation, but with additional casement trim and flashing to seal the exterior.

Types of trim

There are several classic patterns of trim, all drawing on different period elements functional purposes. For these to work with our windows, we needed to keep in mind that our jambs are 4-inches deep. This can make some styles look out of place, since they were designed for period windows.

Below is another trim example, this time coming from a 1920 bungalow built in the historic Houston Heights neighborhood. A little variation on a simple casing and mission-style sill

We wanted to add some additional character to the windows, but still retain the “modern craftsman” vibe of our home, so we ended up going with a very plain casing and something of a modified mission-style sill. It doesn’t extend to the outer perimeter of the casing. This choice was made mostly so that we didn’t have to get new sills, but rather just modify the existing sills by cutting them down to fit the interior window width. As long as most everything matches throughout the house, it is relatively safe to do something non-traditional.

A neat trick we learned was to off-set the casing by 1/4-inch around the window opening to give the illusion of a fancier style casing due to the shadow it creates. This is probably a useful trick for nail-fin windows with any style trim, as it makes it easier to hide windows that are slightly out of square with caulk.


To make exactly the same style trim as we installed you’ll need:

  • Primed 1×3 stock

  • Existing window sills (though you can make your own with 1×6 stock and a router with a 1/2-inch roundover bit)

  • Caulk

  • Drywall mud (to patch holes)

This will depend on your trim style, just make sure your stock/moulding fits the scale of your window.


  • Miter saw

  • Finish nailer

  • Measuring tape

Replacing trim in three steps

First thing we did was demo out the existing trim. Since we’re re-using the sill, I saved that, removed the nails, and then cut it down to the correct size. This involved cutting the “overhang” on either side down to a 1/4” overhang, so that it would mesh perfectly with the trim we would be installing with a 1/4’ offset from the edge of the drywall encasing the window framing. I used the finish nailer to nail this piece back in place.

For the trim we measured the inside of one of the long sides of the window (from the bottom of the sill to the inside top edge), then added 1/4” since there is no offset underneath the sill. Then we cut one end of one of the long sides at 45-degrees, and then laid out the next cut by measuring from the inside edge (not pointy) of the cut side to the total measured length. That set the inside edge of the cut. I then scribed another 45-degree angle with my combination square and pencil. Cutting on this mark gives the correct dimension. Next I nailed up this piece, making sure to maintain the 1/4” offset from the long inside edge of the window to the piece of trim.

I repeated this process three more times for the other long side and two short sides of the trim. Because windows are rarely exactly square, you may have to play around with the miter saw to get everything to fit well (angles needed could be 44- to 46-degrees — outside of this range, its better to just mud the joints, as the angles will make the trim look weird). The last step is to mud joints, caulk seams, and then prime and paint the trim your desired color. We chose a very basic while, Valspar “Base A,” in an eggshell finish (though satin will also work fine).

This post was written for the  Collected Eclectic blog.

Collected Eclectic was a passion project focused on recording the process as Grace and Michael van Meurer transformed their builder grade home in to something special.

124 blog posts were published between 2018 and 2021. Explore the complete Collected Eclectic archive here

Learn more about the project here

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