I've seen many painted concrete masonry walls in warehouses and other single-story buildings. In cities, many brick buildings also are painted. Are masonry walls painted in order to correct problems or so that less expensive materials can be used for their construction? When do you recommend painting masonry?
Masonry walls typically are painted to change their appearance, correct a water-leakage problem, or as an attempt to correct deteriorating masonry. As a general rule, I do not recommend painting masonry walls. If the walls function as effective drainage walls, painting them will not improve their performance. Masonry often is selected because the surface is durable with integral color. Once masonry walls are painted, they must be repainted periodically, turning an otherwise low-maintenance system into a high-maintenance one. It is a common misconception that painted walls can be built with inferior materials or workmanship. The opposite more often is true. Painted surfaces tend to trap moisture within walls or slow significantly the rate at which moisture dissipates. This can lead to premature masonry deterioration. Painting walls that already are deteriorating often can accelerate this deterioration. A better approach in these cases is to repoint mortar joints and replace areas of deteriorated masonry. Single-wythe concrete masonry walls, however, generally are coated with a water-repellent or an elastomeric coating if exposed to weather. This is because uncoated conventional concrete masonry allows significant water penetration; water can appear on the interior by migrating through the masonry. Coating these walls reduces significantly the rate of water penetration. These systems should, however, also be constructed with flashing above openings and at the base of the wall, to collect water and direct it back to the exterior. When opaque coatings are used on masonry walls, they should be durable, flexible, and reasonably permeable to water vapor, so they don't trap moisture within the walls. Even the most durable coatings available today must be reapplied at regular intervals throughout a building's life. I generally recommend using elastomeric coatings in these cases. Many of these products elongate 500% or more without failure, permitting them to withstand small movements across cracks. Elastomeric coatings should "breathe" or permit the passage of water vapor. Many coatings have a vapor permeance of 10 perms or greater at their recommended dry-film thickness. Whenever masonry walls are coated, it is extremely important to detail the top of the wall and any openings carefully in order to prevent water from penetrating the masonry behind the coatings; even the most breathable coatings reduce the rate at which moisture dissipates.