I am replacing the flashing on a brick veneer wall. I am removing the brick above the shelf angle in alternating 8-foot-long sections. After I complete the flashing repairs and replace the brick, how long should I let the new masonry set before removing the rest of the masonry?
The length of time required depends upon the height of the wall and the rate of mortar hydration. The rate of mortar hydration depends on several factors: the outdoor temperature, the temperature of the materials, the outdoor relative humidity, and even the type of cement or additives used. The Brick Institute of America (BIA) in Technical Notes 7F recommends letting the replaced masonry sections age 5 to 7 days before removing intermediate sections. This recommendation applies to flashing replacements in all types of masonry walls and therefore is necessarily conservative. Most veneer walls supported on each level are stressed to less than 1% of the masonry compressive strength (ignoring safety factors.) When half of the supporting masonry is removed, the compressive stresses in the remaining masonry will double. However this value is still very small. BIA Technical Notes 35 summarizes several studies concerning early strength gain in brick masonry. A plot of the results from this research suggests that 2-day strengths may be as high as 70% of the 28-day strengths. Based on the above information and conversations with several masonry contractors who have performed this work, it may not be necessary to wait the 5 to 7 days recommended by the BIA. Reducing the time too much, however, may risk cracking or other damage to remaining masonry.