Mortar Application and Voussoir Laying

Mortar Application and Voussoir Laying

Building an arch requires more than cutting the right wedge-shaped units. The way you butter, place, and seat each voussoir determines whether the finished arch carries its load safely or develops cracks and movement after the centering comes down. This lesson walks you through every step from setting the first skewback unit to tooling the intrados joints.

PPE and Safety Before You Touch the Mortar

Mortar is caustic. Portland cement can cause skin burns and, with repeated exposure, sensitization or chemical burns to eyes. Wear safety glasses at all times on the mortar board. Wear work gloves rated for masonry work whenever you are handling mortar or freshly laid units. Keep a sponge and bucket of clean water within arm's reach so you can rinse skin contact immediately. Do not rub your eyes with gloved hands. If mortar contacts your eyes, flush with water for at least 15 minutes and report to the instructor.

The centering form holds the weight of all units until the arch reaches full cure. Never stand on, lean heavily against, or remove wedges from the centering during the lay-up. Inspect the form for cracked or loose supports before you begin each session.

Starting Right: The Skewback Units

The skewback is the angled bearing surface at each springing point where the arch meets the supporting wall or pilaster. Set both skewback units before anything else. Apply a full mortar bed to the correct skewback angle—this angle matches the geometry you calculated when you cut the voussoir set. Press the skewback unit firmly so the bed joint is fully filled with no voids, then check that its face is flush with the centering profile. Correct any deviation now; errors here travel up through every subsequent course.

Working Both Sides Simultaneously

The single most important rule in arch construction is to work up both sides at the same rate. If you lay three voussoirs on the left before touching the right, the centering form receives unbalanced loading and can shift or rack. Set one unit on the left, then one on the right, alternating until you reach the crown.

For each voussoir:

1. Apply mortar to both the bed joint surface on the form side (the concave intrados face of the previous unit) and the head joint surface that will contact the adjacent voussoir.

2. Press the unit firmly against the centering profile so no gap exists between the unit and the form. A unit held away from the form will dry in the wrong position.

3. Tap with a rubber mallet—never a metal hammer—to seat the unit and close any gap at the intrados.

4. Check plumb on the face of the unit with your 4-foot level or plumb bob. A voussoir that leans in or out creates a visible bulge at the arch face.

Mortar on Tapered Joints

The joint between voussoirs is tapered: narrower at the intrados (inside curve) and wider at the extrados (outside curve). This taper is what gives the arch its wedge geometry. A common and serious error is applying mortar only to the wider extrados side and leaving the narrow intrados side starved. Mortar squeeze-out at the intrados is the proof that the joint is full. If you do not see a small amount of mortar pressing out as you seat the unit, add more mortar and re-seat.

Keep mortar off the centering form itself. Mortar bonding to the form makes stripping difficult and can fracture units when the form is struck.

Tooling the Intrados Joints

Before the mortar reaches its initial set—typically within 30 to 60 minutes of placement in normal conditions, but always check the manufacturer's data sheet for the specific product—tool or point the intrados joints. Tooling compresses the mortar surface, closes micro-cracks, and produces the concave or flush profile that resists moisture infiltration. Rake the joint to a uniform depth at every course. Inconsistent depth is visible and indicates rushed or careless work. Do not tool joints that have hardened to the point where the jointing tool no longer leaves a clean impression; those joints must be raked out and repointed.

Measuring Joint Taper

After the arch reaches initial set, you will measure the actual joint widths at the intrados and extrados at every third voussoir using a joint-width gauge. Record both measurements for each joint in your lab log. The difference between extrados and intrados width is the taper. Your instructor will provide the allowable taper tolerance for the arch geometry you built. Calculate the average taper across all measured joints. Any individual joint that falls outside the tolerance must be documented—not hidden—in your log.

Checking Plumb and Profile

With a plumb bob or level, check that the arch face is plumb within 1/8 inch over its full height. Then hold a straightedge or run a string across the voussoir faces to verify that all units sit flush within 1/4 inch of the centering profile. Proud or recessed units indicate insufficient mortar, improper seating, or a unit that was not held against the form during setting. Document any deviation and discuss corrective options with your instructor before the mortar hardens.

Key Reminders Before the Check

• Always alternate sides to keep the form balanced.
• Full mortar coverage on the narrow intrados edge is not optional.
• Tool joints before initial set, not after.
• Measure, record, and calculate—do not estimate joint taper from memory.