The glaze on glazed brick must make the brick virtually waterproof. Why is it that many glazed brick are actually less durable than unglazed brick?
Although the glaze prevents water from penetrating the masonry unit directly through its face, most water enters masonry walls through separations between mortar and brick. Water also enters masonry walls as vapor, both from the exterior and interior. Once water enters the wall, the glaze slows the rate at which the water can evaporate or leave the wall. A second problem is that glazed brick units meeting ASTM C 126 may not be suitable for exterior weathering. Glazed brick meeting this specification do not have any restrictions concerning absorption or saturation coefficient. However, glazed brick can perform reasonably well. The units should be specified to meet both C 126 and C 215 Grade SW. An air space should always be provided immediately behind the glazed brick veneer. Proper flashing details are essential to direct water out of the masonry wythe. The mortar joints should be completely filled and well tooled to prevent water from entering the wall. For further recommendations concerning the design of glazed brick walls, consult the Brick Institute of America Technical Notes 13.