A: For aesthetic reasons, we'd like the brick building we're designing to have raked mortar joints. Even though we've seen buildings with raked joints in good condition, we recently heard that raked joints don't weather well. Is this true? What styles of mortar joints weather best?
- The style of mortar joint does greatly influence the watertightness of a wall. As shown here, concave and vee joints are the most watertight and weather-resistant. Beaded and weathered joints also perform satisfactorily. Raked, extruded, struck, and flush joints, however, are not weather-resistant and should not be used on exterior surfaces.Materials and workmanship also influence the watertightness of mortar joints. Mortar should meet the requirements of ASTM C 270, Specification for Mortar for Unit Masonry. Brick generally should have an IRA in the range of 5 to 30 grams per 30 square inches per minute. Brick with an IRA greater than 30 grams per 30 square inches per minute should be wetted. Masons should set the units within 1 minute of spreading mortar, and they should not spread mortar beds more than 4 feet in front of the masonry. If they spread more mortar or take longer to set units, the wind and sun can dry the mortar before the units are placed. This creates a weak mortar-brick bond that allows water to penetrate. Masonry units also should never be moved after they are laid; any slight movement can break the bond between the unit and mortar.The mortar joints should be tooled when the mortar is thumbprint hard, not sooner or later. Tooling pushes the mortar back into contact with the masonry units, closing slight hairline cracks that form as the mortar hardens and shrinks. It also compacts the surface of the mortar. If the tooling is done too soon, the mortar only shrinks away from the masonry units again. If it's done too late, the mortar is too hard and can't be pressed tight against the masonry units. Because exterior joints must be watertight, all head joints should be filled, even if they are not exposed. And they should be buttered, not filled by slushing. Some factors that affect the watertightness of mortar joints are described in more detail in "The Most Important Property of Masonry," by Clayford T. Grimm, published in the September issue of The Magazine of Masonry Construction, page 254.