Last summer, the homeowners of a project we were trying to wrap up made a request for a “simple, plaster look” on a pair of small, arch-top windows in their newly renovated second-story bathroom. We had built a number of arch-top windows (and interior doors) on their remodel and finished them off with either wood trim or tile returns. But the request for plaster returns proved to be a bit of a head scratcher—how were we going to make the transition from the drywall to the window without trim?

The homeowners requested a plaster-look interior finish for a pair of small, arch-top windows in their bath remodel.
The homeowners requested a plaster-look interior finish for a pair of small, arch-top windows in their bath remodel.

We briefly considered installing corner bead, but the windows had a tight radius that would have entailed making many cuts with snips to form the bead to the arch. Also, there would be some buildup of metal along the edge that could affect the bonding of the mud. Instead, we decided to install fiberglass reinforcing mesh around the window opening and key the joint compound into the mesh, similar to old-school lath-and-plaster work (see illustration, below).

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Site-built jamb extensions, comprising three straight runs (for the jambs and sill) and a site-laminated arched top, were needed bridge the gap between the window frames and the face of the drywall. To fabricate the arch tops, we marked the inside and outside radii of the new extensions on a piece of sheet stock, then set a series of finish screws along the lines. We cut pieces of clear pine stock to the jamb depths, then ripped them into thin strips on a table saw. (When we demoed the walls, we found one of the exterior walls was framed with 2x6s and the other with 2x4s, which necessitated two different jamb depths.) We glued up the ripped strips and bent them along the radius lines; the finish screws held the laminations in place. We ran the strips long and later cut them along the arch’s spring line. After gluing and screwing the four pieces of the extension jamb together, we glued and screwed the final assemblies through the front edge of the extension jams and into the window frames.

A “lath-and-plaster” solution. Prior to mudding the windows, we installed temporary screed boards made from ripped-down clear pine run about a 1/4 inch proud of the extension jambs. We wrapped the outer edge of the screed boards with plastic tape to act for easy release after the drywall mud cured. We then brushed on Hydro Ban waterproofing membrane to the face of the extension jambs to prevent moisture from the joint compound from being sucked into the wood, possibly cracking the compound. Next, we tacked a large piece of fiberglass reinforcing mesh over the openings with stainless steel T-50 staples and cut out the window opening.

For the first coat, we used Durabond-90 joint compound. We pushed the mud through the mesh about 1/2 inch thick at the gap between the jamb and the drywall, and ran it up to the screed board about 1/8 inch. Durabond is a setting-type mud, and we used the 90 version to give us more time to work the material. Next, we applied USG Plus 3 pre-mixed joint compound—in three coats—using an oversized, 14-inch drywall knife to float the material from the screed boards out to the edges about 2 feet. Blending and floating the edges is easier with USG Plus 3.

After the joint compound cured, we removed the temporary screed and used a fine, 220-grit sanding mesh to break the edge of our “plaster” corner. The painters primed and painted the window return, and it blended in nicely.

I recently ran into the homeowner, and she said the trimless returns have held up, and she’s happy with the look.

Photos by New Dimension Construction and Tim Healey; illustration by Tim Healey