Centering: Design, Construction, and Installation

What Is Centering and Why It Matters

An arch cannot support itself while its mortar is wet. During construction, every voussoir (wedge-shaped unit) must be held at the correct angle and height until the keystone is set and the mortar reaches adequate strength. The temporary wooden framework that performs this job is called centering (sometimes spelled centring). Without an accurate centering form, the arch profile will deviate from the intended geometry, joints will be uneven, and the completed arch may carry loads unevenly or fail.

Because centering is removed and reused, building it precisely is an investment. A form that is accurate to within 1/8 inch at every point of the curve will produce consistent, professional results across multiple arch openings.

Anatomy of a Centering Form

A basic centering form has three parts:

• Ribs – curved plywood pieces cut to the exact arch profile. Two ribs are nailed together face-to-face (doubled) to give the assembly enough stiffness to carry the weight of wet masonry without deflecting.
• Lagging boards – 1×4 lumber pieces nailed across the curved top edges of the ribs at regular spacing. They form the bed on which voussoirs rest. Gaps between lagging boards are acceptable; they allow mortar droppings to fall clear and give you purchase when striking the form.
• Spreader blocks – short pieces of scrap lumber nailed between the two ribs near the base and mid-span to maintain consistent spacing and prevent racking.

Laying Out the Curve

The arch radius you calculated in the previous lesson determines the rib shape. Use a trammel bar (a long straightedge with an adjustable pin at one end and a pencil at the other) or a beam compass to scribe the arc directly onto 3/4-inch plywood.

Steps for layout:

1. Draw a horizontal baseline across the plywood equal to the clear span of the arch.

2. Mark the midpoint of the baseline. Measure down from the midpoint a distance equal to the radius to find the center point of the circle that generates the arc.

3. Set the trammel bar to the computed radius. Anchor the pin at the center point and swing the pencil through the arc from one springing point to the other.

4. Mark the spring line (the horizontal line at the base of the curve) clearly. The rib will be cut between the baseline and the curved top edge.

Double-check by measuring from the center point to five evenly spaced points on the scribed arc with a flexible steel tape. All five measurements must equal the specified radius within 1/8 inch before you cut.

Cutting and Assembling the Form

Cutting: Use a jigsaw to cut along the scribed arc. Work slowly and keep the blade perpendicular to the plywood face. A jigsaw can follow a tight curve; a circular saw cannot and must not be used on the curved cut. The circular saw is appropriate only for straight crosscuts on lagging boards.

Doubling the ribs: Cut a second rib identical to the first. Lay one rib on top of the other, align the edges carefully, and nail them together with common nails driven every 6 to 8 inches around the perimeter. Doubled ribs resist bending under the load of wet masonry.

Attaching lagging: Stand the doubled rib assembly on edge. Nail 1×4 lagging boards across the curved top, keeping them perpendicular to the ribs. Space them evenly, leaving narrow gaps (roughly the width of a nail) between boards. Do not leave gaps wide enough to allow a voussoir to tip.

Installation: Setting and Striking

Setting the form: The centering rests on temporary adjustable shores or wood wedges placed at the base of the wall opening. Wedges give you fine vertical adjustment. After placing the form, check it with a 4-foot level along the bottom chord (it must be level end-to-end within 1/8 inch) and with a story pole to confirm it is at the correct height in the opening. Check that the form is plumb across its face — a form that leans will produce an arch that is out of plumb.

Striking the form: Once the arch is complete and the mortar has cured sufficiently (follow the project specifications and mortar manufacturer guidance — do not rush this step), the form is removed by slowly tapping out the wedges from both ends toward the center simultaneously. This lowers the form uniformly. If one end is removed first, the form can rotate and apply lateral thrust to the uncured arch, potentially cracking joints. Lower the form straight down; never pry it sideways.

Safety Requirements

• Eye protection (safety glasses) is mandatory whenever operating the jigsaw or circular saw. Flying wood chips and sawdust are an immediate eye hazard.
• Hearing protection is required when operating power saws. Prolonged exposure to saw noise can cause permanent hearing loss.
• Keep fingers clear of the jigsaw blade path. Always wait for the blade to stop before setting the tool down.
• When setting shores, ensure they bear on solid, level floor — never on debris or unsupported material. A collapsing shore under a loaded arch form can cause crush injuries.
• Secure plywood on a stable work surface before cutting. An unsupported sheet can shift and bind the blade.
• Follow your program's lockout/tagout procedures for any stationary shop equipment used to pre-cut lumber stock.