Arches are constructed over temporary wood centering. Architectural drawings show the basic height and width of the opening. LAY THE ARCH ON THE GROUND FIRST The best way to lay out an arch pattern is to place the centering on the ground and position the masonry units around it. WEDGE-SHAPED BRICK ARE EASIER TO LAY Arches are usually built of wedge-shaped brick so that the mortar joints are a uniform thickness. To form the curve with standard brick, the mason must make the joints at the bottom of the arch thinner than at the top. The compressive and bond strengths of the mortar must be high; usually only ASTM C 270 Type S and N are recommended for brick and concrete block arches. With standard brick instead of wedge-shaped units, the bond of the mortar is even more important. SHORE FOR AT LEAST 7 DAYS Hold the centering in place with wood posts or concrete masonry units with wedges inserted at the top that can be easily driven out. Keep the centering in place for at least 7 days after completion.