How to Form a Window Arch in Fibrous Plaster: Template and Assembly
An arched window opening is a decorative finish, not a structural arch. The timber frame or masonry carries the load; a thin band of fibrous plaster—a paper-faced plasterboard product—creates the curved soffit and face. That distinction matters because every cut, fixing and joint must follow one accurate curve.
A dependable result comes from controlling the geometry before handling the plasterboard. Decide where the curve begins at the springing line, how high it rises and how deep the window reveal will be. Build a full-size template, then fix the finished bands to a rigid former. Each stage depends on the same measured curve, from the initial set-out through the final skim.
Choose a Practical Forming Method
| Forming method | Best suited to | Construction | Main limitation |
|---|---|---|---|
| Timber rib former | Repeated arches or deep reveals | Segmented timber ribs, braced and skinned with thin ply or plasterboard | Strong and adjustable, but bulky to set out |
| Flexible plywood former | A single arch with a consistent curve | Thin plywood or slotted battens supported on a base frame | Faster to build, but more vulnerable to damage |
| Direct plasterboard band | Simple, shallow curves | A cut band fixed to prepared backing with reinforced joints | Fewer components, but little tolerance for uneven framing |
The three forming methods cover most residential work. A timber rib former provides strong, repeatable support, while a flexible plywood former suits a one-off installation. A direct plasterboard band can work for a simple curve with an even reveal.
Whichever method you choose, use the same geometry. Measure the opening at both springings and record the finished width, rise and reveal. Mark the crown, or highest point, and check that the template follows the exact intrados, meaning the underside of the curve. A window head may be segmental rather than semicircular, so never assume the springings sit at the centre of a circle.
Shape the Template and Check the Curve
Build a full-size template from thin plywood, stiffened ribs or a combination of both. For a semicircular curve, use a trammel or radius battens; for a shallower segmental arch, use the actual centre line and radius. Plywood needs controlled relief cuts on its reverse where the bend is tight, rather than being forced until it splits. Fit a clean, removable facing so the plaster does not bond to the former.
The NAPL explanation of making a running mould shows how a shaped guide and a firm former can produce a repeatable profile. Although it deals with cornice work rather than a window head, the same discipline applies: establish one reliable curve and check it against the opening before cutting.
- Mark the springing line, crown and finished reveal on both the substrate and template, then compare them with the window dimensions.
- Keep the template removable where access behind the arch will be needed, or where the finish may need renewing later.
- Use paper tape or a clean sacrificial skin on the template face to prevent the plaster from bonding to the former.
- Test-fit a scrap plasterboard strip before committing to full-width bands, especially at tight springings and around the crown.
Prepare the Opening and Fix the Supports
Clear the opening back to sound material, confirm the structural lintel and check the framing before installing the former. In many Australian homes, 90 mm studs and 70 x 45 mm battens are familiar sizes, but measure the actual wall rather than assuming. Add nogging or blocking behind each springing and enough temporary bracing to stop the curve moving while the plaster sets.
Preparation should be treated as a fixed stage rather than a finishing extra. The problems described in garage-door installation mistakes offer a useful parallel despite the different product: an opening that is measured poorly or supported badly becomes difficult to correct after the finish is closed up.
Plan the sheet layout around plasterboard commonly sold in 1200 mm and 2400 mm lengths, which can reduce waste but should never dictate the arch design. Older terraces in Sydney and Melbourne often have deep masonry or stone reveals, while modern stud walls in Brisbane may need extra blocking to keep the plaster curve stable.
Cut, Join and Finish the Plaster Bands
Cut the fibrous plasterboard into manageable bands, following the template rather than freehand. Develop the curve gradually with controlled sanding and, where necessary, shallow relief cuts on the back of each strip. Keep the paper facing continuous. Lightly dampen the reverse only if the manufacturer permits it; soaking can weaken the facing and cause the finished curve to pucker.
Fix the first band at the crown, then work outwards in balanced stages towards each springing. Use temporary blocks and fasteners that hold the plaster against the ribs without crushing the paper edge. Avoid placing every fixing at a joint: solid areas behind the band carry the load, while the joints are reinforced and filled afterwards.
Keep butt joins away from the crown where the geometry allows. Reinforce them with paper tape embedded in plasterers’ clay, followed by thin skim coats. Feather each coat beyond the edge, allow the product to dry as specified and sand only when the surface is firm. Do not use filler alone across a wide joint, as it will shrink and open a visible crack around the curve.
Build Your First Arch with Confidence
Make the first attempt a deliberate exercise, preferably on an offcut or a simple internal opening. Record the set-out, fixing intervals and drying times so the next job starts with better information. In Perth’s dry heat and Melbourne’s damp winters, those times can vary; follow the product data rather than forcing coats on early.
Use NAPL’s courses, eBooks and videos to extend the method, and check the site’s current listings for relevant trade contacts. Choose one opening, build and verify its template, then install the bands from the crown outwards. Start with a measured template and a rigid former, and carry that same system into your next job.