Voussoir Cutting and Skewback Preparation

Why Voussoir Cutting Matters

An arch carries load through compression that travels along the curve of the ring and exits at the skewbacks into the supporting piers or abutments. Every voussoir—each wedge-shaped unit in the arch ring—must be cut so that its bed joints, when assembled, converge toward the center of the arch (the center of radius). If any unit is cut to the wrong angle or taper, a gap or hinge point opens in the ring and the arch loses its ability to transfer load uniformly. Accurate cutting is therefore a structural requirement, not just a cosmetic one.

The Skewback

The skewback is the inclined surface at each end of the arch on which the first voussoir rests. For a semicircular arch the skewback angle is 90° measured from vertical—meaning the bearing surface is horizontal at the spring line. For segmental or other arch forms the angle differs, so always work from your calculated springing-line angle before touching the saw. The two skewback bricks must be mirror images of each other; mislabeling them causes the arch to lean to one side.

Setting Up the Masonry Wet Saw

Before every cut, inspect the diamond blade for cracks, missing segments, or warping. A damaged blade must be replaced before the saw is operated. Confirm the water delivery system is flowing freely—wet cutting suppresses silica dust at the source and is the primary dust control method. Even with continuous water flow, wear your N95 dust mask any time you are within the saw's work zone, because water coverage can be incomplete. Always wear safety glasses under a full face shield, hearing protection, and nitrile gloves during cutting operations.

To set the cut angle:

1. Loosen the table-tilt or fence-angle adjustment on the saw (the method varies by saw model—check the manufacturer's guide).

2. Use a digital angle finder or protractor to set the table or fence to the calculated skewback angle.

3. Lock the adjustment firmly and verify the angle a second time before cutting any production brick.

4. Make a test cut on scrap brick and measure the resulting angle before committing to production units.

Secure the brick against the fence or with the rip guide so it cannot shift mid-cut. Feed the brick steadily through the blade in a single smooth pass; do not back the brick up while the blade is spinning, because that can cause kickback.

Cutting Voussoirs

For a standard semicircular arch built with standard modular brick, most voussoirs are cut with a taper—narrower at the intrados (inner curve) and wider at the extrados (outer curve)—rather than with an angled bed. The taper angle for each voussoir is calculated from the number of units in the half-arch and the rise-to-span geometry. Once you have that taper angle, transfer it to the saw setting as described above.

After cutting, use a lumber crayon to label every unit on its top bed with its position number (for example, "L-1" for the first unit from the left spring, "K" for the keystone). This labeling survives the dry layout and prevents confusion at the mortar stage.

Dry Layout and the Taper Gauge

Before any mortar is mixed, arrange all cut voussoirs and skewback units on the centering form in their intended positions. The centering form gives you the curve; the voussoirs must follow it without rocking. Check every joint with a taper gauge: the gauge slides into the joint and reads the opening at the intrados and extrados. The difference between those two readings is the joint taper. Joint width at any point must not exceed the maximum width specified for the mortar type you are using—standard references limit most mortar joints in brick masonry to a maximum of ¾ inch and recommend a nominal ⅜ inch; always follow the project specification. If a joint is out of range, re-cut the offending voussoir before proceeding.

Straighten any unit that rocks on the form by checking whether the cut face is truly flat. A rocker indicates a slight warp in the cut, often from feeding too fast or from a worn blade segment. Dress the face lightly with a rubbing brick or return it to the saw for a corrective pass.

Tolerance and Acceptance

The acceptance standard for this lab is an angular deviation of no more than one degree from the specified skewback angle, measured with a digital angle finder on the cut face. This mirrors the precision expected on a real job where arch units must close tightly enough to carry load without point loading on high spots. Units outside tolerance must be re-cut; attempting to compensate with excess mortar creates a weak joint and undermines the structural intent of the arch.

Silica Dust — Critical Safety Note

Cutting brick generates respirable crystalline silica dust, which causes silicosis—an irreversible and potentially fatal lung disease. OSHA's silica standard for construction requires engineering controls (wet cutting, local exhaust ventilation) as the first line of defense. Wet cutting must be active throughout every cut. An N95 respirator is the minimum respiratory protection when working in the saw area. Never dry-cut without additional engineering controls and a higher-level respirator. Never blow dust off cut units with compressed air.