An arched window is the opening people notice first on a facade, and the one most trim jobs get wrong. Arched window trim has to follow a curved line exactly, carry the same profile from the head down into the legs, and land on a sill without a visible seam or change in width. Done well, it softens the facade and makes the window look taller, and it can reframe an existing window without replacing it. Done poorly, the arch looks like a patch.
The short version: Arched window trim is casing made to follow the curve of an arched window, in the same profile as the straight legs below it. Half round, segmental, eyebrow and elliptical arches each need trim cut to their own radius or template, so send the window width, rise and a photo. Mouldex cement-coated foam arches are CNC-cut to the radius rather than bent, so stepped profiles stay crisp around the curve and match the straight casing and sills.
This guide covers arch types, how arched trim is made in wood, PVC and cement-coated foam, matching the arch to straight casing and sills, measuring, and detailing the head on stucco, EIFS and brick, for residential and commercial projects.
Types of Arched Windows and the Trim Each One Needs
Every arch is defined by two numbers: the width of the opening at the springline (the chord length, where the curve starts) and the rise (how far the top of the arch sits above that line). Those two numbers set the radius, and the radius decides how the trim is built.
Half round (full round) arches
A half round window, also called a half-moon, fanlight or sunburst window, is a half circle: the rise is half the width, so the radius equals half the width. Half round window trim is a single constant radius, the simplest arch to design. Half rounds often sit above a door as an arched transom, with the trim continuing down both sides as a full surround. A full circle (oculus) window uses the same geometry all the way around, with the diameter as the only measurement you need.
Segmental and eyebrow arches
A segmental arch is a shallow slice of a larger circle: wide and low, with a rise much smaller than half the width. It is the most common arched head on new homes and commercial buildings because it adds softer lines without changing the window size. An eyebrow window is a very flat segmental or elliptical arch, often on a dormer or above a wide picture window. Eyebrow window trim needs a large radius and a clean transition into the legs so the curve doesn't read as a sagging straight line.
Elliptical and pointed arches
An elliptical arch is tighter at the ends and flatter at the top, so there is no single radius. Gothic or pointed arches are two arcs meeting at a point, seen on churches and some Tudor and Victorian homes. Both are custom pieces made from a template or drawing of the actual window.
How Arched Window Trim Is Made: Wood, PVC and Cement-Coated Foam
Curved molding can't be bent out of a straight piece of most materials, so the method used to create the arch is the biggest difference between products.
Wood: laminated strips or cut from flat stock
Wood radius trim is made one of two ways. In bent lamination, thin strips are coated with glue, bent around a form that matches the window radius and held with a clamp every few inches until the glue cures, then shaped with a router or router table. The alternative is cutting the arch from wide flat stock, MDF or a sheet of plywood with a router trammel, usually in two pieces or more laid out on a story pole with the radius and miter angles from a construction calculator, because a half circle won't fit on one board and the grain runs out at the ends. Either way it is skilled shop work, and outdoors the joints and end grain are where wood starts to rot. That is why radius head casing indoors is often still wood or MDF, while exterior arch window trim, which has to stand up to weather exposure, is increasingly made in other materials.
PVC: heat bending
PVC trim can bend into an arch after it is heated and set on a form. It suits simple flat profiles, but stepped profiles can distort as they bend, and the finished piece still expands and contracts with temperature, which opens the joints where the curved head meets the straight legs. Wrapping a curved wood head in aluminum coil is another shortcut that rarely looks crisp on a tight radius.
Cement-coated foam: CNC-cut to the radius
Mouldex trim takes a different route. Each piece has an EPS foam core that is CNC-cut, then coated with a cement-based finish (thickness about 3/16 in), reinforced with fiberglass mesh. Because the core is cut rather than bent, the arch is drawn to the window's radius and cut as one smooth curve: a half round, a segmental head or an elliptical template curve are the same job to the machine. Curved sections are available on request for our standard profiles, and our custom foam cutting and coating services handle new or matched profiles. Arches are CNC-cut from digital files, with lead times typically 7 to 10 business days from design approval, depending on profile and quantity.
The finished trim is moisture-inert, won't rot or warp, is less susceptible to cracking and is much lighter than precast, so a large arched head goes up without cranes. Near-zero joint expansion keeps the joint tight where the arch meets the legs.
One Profile Around the Arch and Down the Legs
The rule for good arched window trim is continuity. The same profile should run around the curve and down both legs, with the same face width, projection and steps, so the eye follows one unbroken frame. Where the other windows on the facade are square, use that profile on their straight casing too, so the arched window looks like part of the family.
In practice, choose the profile from the straight casing range, then order the arched head in that profile. Arched window casing works the same way as straight casing: a stepped Victorian surround such as the Nash, a flat modern casing such as the Iris or a Craftsman profile like the Lawrence arched window trim can each be carried around a curved head. The arched section is made to the window's radius, the straight legs are sold by the piece and cut to length on site, and the joint falls at the springline, where the curve ends and the straight run begins, so there are no corners to miter on the curve. The same approach works for arched doorways.
For matching profile to building style, see our exterior window trim ideas, from arched tops to pediments.
Keystones, Sills and Other Details
A keystone at the top of the arch is the classic way to mark the centre point, especially on Georgian, Colonial and Mediterranean facades. At Mouldex, keystones are specialty orders, sized to suit the arch and the trim profile.
At the bottom, the legs should land on a projecting sill a little wider than the surround. Pair the arched head with one of our exterior window sills so the base of the window carries the same weight as the head. On a full facade, a band molding at the floor line and a cornice at the roof line tie the arched window into the elevation.
How to Measure an Arched Window for Trim
Most arched trim problems start with a guessed radius. To size arched window trim, send us:
- Width at the springline, measured to where the inside edge of the trim should sit (outside of the window frame, or the edge of the reveal).
- Rise, from the springline straight up to the highest point of the arch.
- Leg length, from the springline down to the top of the sill.
- A photo of the window and, for elliptical, pointed or irregular arches, a full-size template traced from the opening on paper or 1/8 in hardboard.
For a circular arch, radius = (width squared divided by 8 times the rise) plus half the rise. A 48 inch wide window with a 12 inch rise has a 30 inch radius; with a 24 inch rise it is a half round with a 24 inch radius. Measure the width at more than one point and double-check the rise, because masonry and stucco openings are rarely perfect. For architects specifying arched surrounds across a building, CAD details and specifications are available to members of our Trade Program.
Detailing the Arch Head on Stucco, EIFS and Brick
Arched trim is decorative and non-load-bearing; it doesn't replace the window's head flashing or the lintel above a masonry opening. Install the window, flashing and weather barrier to the wall system's details first, then apply the trim over the finished surface.
- Stucco: adhesive is usually enough, with mechanical fasteners such as screws added depending on the weight of the piece. Seal the top edge of the arch per the installation guidelines so water at the head is shed, not trapped. Our guide to the best exterior window trim for stucco homes covers profile sizing for stucco walls.
- EIFS: the head detail follows the EIFS manufacturer's details for terminations and penetrations. Our comparison of EIFS vs. stucco explains how trim works on each.
- Brick and stone: arched trim can frame the window inside an existing brick arch, or give a new wall the look of a stone arch.
Indoors, arched windows are a finish carpentry job: interior radius casing is usually wood or MDF, nailed with brads and set flush to the drywall with jamb extensions (extension jambs) where the wall is deeper than the window jamb. Our arched trim is for the exterior.
Arched Window Trim on Homes and Commercial Buildings
- Custom homes and renovations: a half round transom over the front door, segmental heads across a Mediterranean stucco facade or an eyebrow window on a dormer.
- Multi-family and mixed-use: segmental arched surrounds on the ground or top floor give a large building rhythm without heavy precast.
- Hotels, restaurants and retail: arched entrance surrounds, dining room windows with keystones and arched storefront heads.
Mouldings are primed, then painted after installation with exterior latex or a textured brush coat, so the arch and the rest of the trim finish as one surface.
Arched Window Trim FAQ
What is a half circle window called?
Usually a half round window; you'll also hear half-moon, fanlight or sunburst (when the glass has radiating muntins). Its trim is one constant radius, half the window width.
What are eyebrow windows called?
Eyebrow windows are low, wide arched windows, called eyebrow dormers when they sit in a curved roof. The arch is a very flat segment or ellipse, so the trim is made to a large radius or from a template of the opening.
How do you measure a half-round window for trim?
Measure the width across the bottom at the springline. For a true half round, the rise is half the width and the radius equals half the width. Check the rise anyway: many "half round" windows are actually segmental.
How do you make arched molding?
In wood, by gluing thin strips around a form or cutting the arch from wide flat stock with a router; window arch molding in PVC is heat bent. Cement-coated foam trim is CNC-cut to the radius from a digital file, then coated, so the profile stays the same around the curve.
What is the brick arch above a window called?
A brick arch, built from wedge-shaped bricks called voussoirs; the centre one at the top is the keystone. A flat brick arch is called a jack arch.
What is the typical width of exterior window trim?
Exterior casing commonly runs about 3 to 8 inches wide, and our standard profiles span that range, from 3 inch Colonial casing to 8 inch modern trim. On an arched window, use the same width as the straight windows so the facade stays consistent.
Ready to frame an arched window? Browse our exterior window trim profiles, order a free sample kit or send us your project with the window width, rise and a photo.
