Arched windows and doorways are among the most striking architectural features in a home, but they demand a level of craftsmanship that goes beyond standard trim work. The challenge is producing curved casing that matches the profile of the straight casing used elsewhere in the house, while maintaining a smooth, professional appearance at every joint. Strip lamination is the go-to technique for professional finish carpenters, and when done correctly, the results are indistinguishable from factory-made curved molding. For a solid foundation in basic practices, explore our detailed guide on trim carpentry techniques before tackling curved work.
Planning and Pattern Making for Curved Casing
Before any wood is cut, accurate patterns must be made for every arched opening. Even windows that appear identical can vary by small amounts due to framing inconsistencies or drywall buildup, so each arch must be measured individually.
Methods for Capturing Arch Profiles
- Kraft paper tracing: For simple radius arches, press kraft paper against the window frame and trace the interior edge. Trim the paper to the line and test-fit against the opening. This is the fastest method for straightforward curves.
- Flexible strip method: Clamp a thin, even-grained ripping of wood (about 1/8 inch thick by 1 inch wide) against the inside of the arch. Add crosspieces with brads to lock the curve shape, then transfer the strip profile to kraft paper.
- Beam compass layout: For large-radius arches, tack a piece of plywood against the window frame. Use a beam compass to find the radius center points by trial and error, swinging arcs until one matches the opening exactly.
- Mathematical calculation: For segmental or elliptical arches, calculate the arc geometry from measured chord lengths and rise heights. This method requires careful field measuring but produces highly accurate patterns.
Pattern Preparation and Organization
Once the arch profile is captured, cut the pattern from stiff paper or thin cardboard and test-fit it in the opening. Check for gaps at the jambs, at the crown, and at any transition points. Extend the pattern at both ends by at least 2 inches to allow for cutting the final miter joints to length. Label every pattern with the room location, window number, and wood species. This organization saves hours of confusion when working on multiple arches across a large project.
Building Bending Forms for Strip Lamination
The bending form is the heart of the strip-lamination process. A well-built form produces consistent, repeatable curved casings. The form must be rigid enough to hold the clamped laminations without deflecting, and accurate to within 1/16 inch of the finished arch profile.
Constructing the Form Base and Ribs
Start with a sheet of 3/4-inch plywood as the base, cut slightly larger than the arch pattern. Trace the arch profile onto the plywood, then cut several curved ribs from 2x stock on a bandsaw. For round-top arches, a circle-cutting jig on the bandsaw table produces accurate ribs quickly. The jig consists of a plywood extension with a pivot screw; the 2x stock rides on a carriage that rotates around the pivot point.
For larger arches with gradual curves, extend the pivot point using a long 1×3 stick anchored at one end, allowing arcs with radii of 6 feet or more. Space the 2x ribs every 6 to 8 inches along the arch and screw them to the plywood base from underneath. For very large arches, screw the ribs directly to the subfloor to prevent movement during clamping.
Form Surface and Production Planning
Attach strips of 1/4-inch hardboard to the faces of the ribs to create a smooth clamping surface. Sand the form smooth and seal it with shellac to prevent glue from sticking. Apply paste wax before each use for easy release. On jobs with many arches, prioritize building forms for identical windows first so you can batch-produce those casings, then tackle unique arches one at a time.
| Arch Type | Form Material | Rib Spacing | Key Consideration |
|---|---|---|---|
| Round top (single radius) | 2x stock + plywood | 6-8 inches | Circle-cutting jig is essential |
| Segmental (shallow curve) | 2x stock + plywood | 4-6 inches | Extended pivot arm needed |
| Elliptical (changing radius) | MDF or plywood ribs | 3-4 inches | Must plot multiple radius points |
| Gothic (pointed arch) | 2x stock + plywood | 4-6 inches | Two intersecting radii |
Strip Lamination Technique: Step by Step
With the forms built and patterns verified, the strip-lamination process itself is straightforward but demands attention to detail at every stage. The goal is curved casing that matches the straight casing profile perfectly, with no visible glue lines or flat spots.
Selecting Stock and Ripping Strips
Choose straight, even-grained casing stock free of knots, wane, and other defects. For painted trim, poplar or MDF are excellent choices. For stain-grade work, select clear pine, cherry, or oak with straight grain. A typical 3/4-inch-thick casing requires at least 5 strips at 1/8 inch each. Set up the tablesaw with a sharp thin-kerf blade and rip the casing into strips of equal thickness, alternating strips from two different pieces of casing. This alternation is important: each strip retains one side of the original profile, and alternating them reassembles the full profile.
Gluing and Clamping Sequence
Use a high-quality wood glue with good gap-filling properties and extended open time. Spread glue evenly on both faces of each strip using a small roller. Assemble the stack against the form, starting from the outside of the curve and working inward. Apply clamps every 4 to 6 inches along the arch, alternating between top and bottom of the form. Use cauls between the clamps and the work to prevent denting. The clamping process follows these steps:
- Position the first strip against the form and clamp it at the crown.
- Apply glue to the second strip and place it against the first, working from the crown outward.
- Add clamps progressively along the curve, flexing strips into full contact with the form.
- Repeat for each additional strip until the full stack is assembled.
- Install a backer strip on the inside of the curve to prevent the innermost strip from lifting.
- Check alignment with a straightedge across the face of the laminations.
- Allow glue to cure for at least 24 hours at 70 degrees Fahrenheit before releasing clamps.
Post-Lamination Work
After the glue cures, remove the curved casing from the form and clean off glue squeeze-out with a chisel or scraper. Sand the curved piece smooth, following the contour of the arch. Cut the ends to length with a coping saw or bandsaw, and cut miter joints using a miter saw with the casing held at the correct angle. Learning the proper methods for casing a door is helpful when planning how the curved section transitions back to straight casing at the spring point of the arch.
Installation and Finishing of Curved Casing
Installing curved casing is the final test of your craftsmanship. The curved piece must fit seamlessly against the straight casing and the window frame.
Dry Fitting and Fastening
Fit the curved casing into the opening without glue or nails first. Check the fit at three critical points: the crown of the arch, the spring points where the curve meets the straight side casing, and the mitered joints. Mark any high spots with a pencil and sand them down. Use 18-gauge brad nails or 16-gauge finish nails driven through the curved casing into the window frame, spaced every 6 to 8 inches. On tight-radius arches, pre-drill nail holes to prevent splitting. Set nail heads below the surface and fill with wood putty.
Curved Extension Jambs
The same strip-lamination technique works for producing curved extension jambs on arched windows. Rip the jamb stock into strips and laminate them around the same bending form. The finished jambs install before the casing and provide a complete, custom-fitted frame. For more on similar bending techniques, see our article on installing curved trim on curved porches, which covers lamination principles in an outdoor application.
Common Problems and Troubleshooting
| Problem | Cause | Solution |
|---|---|---|
| Flat spots on the curve | Insufficient clamping or gaps between ribs | Increase clamping to every 3-4 inches; add more ribs to the form |
| Strip breakage during bending | Wood too dry or has cross-grain defects | Use thinner strips (1/8 inch max for tight radii); select straight-grained stock |
| Glue-starved joints | Too little glue or slow clamping | Use a roller for even coverage; work in pairs |
| Casing profile mismatch | Strips not alternated correctly | Alternate strips from two identical casing pieces |
| Casing pulls away after install | Insufficient fasteners or spring-back | Add adhesive behind casing; increase nail density on the curve |
Prime the curved casing before installation if it will be painted, or apply stain and sealer for natural wood finishes. Touch up nail holes and joints after installation, and caulk the seam between the casing and the wall. A high-quality finish applied in multiple thin coats produces a look indistinguishable from factory-made trim. For a broader overview of interior trim possibilities, our complete guide to trim work and millwork installation covers baseboards, crown molding, and window casings in depth. The strip-lamination techniques described here are also directly applicable to creative trim details and custom millwork projects, making this a versatile skill for any finish carpenter’s toolkit.