The discoloration is likely “efflorescence,” which is an accumulation of minerals and salts on the surface of the brick due to repeated bouts of excess water in the material. When the brick is saturated with water, the minerals that naturally occur in the masonry material are dissolved and, as the water migrates to the surface of the brick and dissipates from evaporation, the mineral deposits are left behind as a thin layer of powder. Repeated saturation and evaporation cycles lead to a buildup of the surface powder. Efflorescence can also form on concrete block walls, as shown below.

   While the efflorescence is only a cosmetic problem in itself, its appearance on the wall indicates an ongoing water intrusion problem, which can lead to mold growth in adjacent building materials over time. Occasionally, efflorescence is mistaken for mold. Here’s a three ways to tell the difference between the two:

1) When a sample of efflorescence is pinched between the fingers, it will crumble into a powder. Mold will not.

2) Efflorescence grows on inorganic masonry materials, while mold does not; with the exception that a dirt/dust buildup on the surface of a moist masonry wall will sometimes grow mold.

3) Efflorescence will dissolve in water, while mold will not.   One common cause of minor efflorescence is a sprinkler head that sprays on the wall, like in the photo below. A pressure washer and/or a diluted acid solution is typically used for removal of efflorescence, with the surface promptly dried afterwards to prevent reabsorption of the water.

    Also, efflorescence can form on concrete that is repeated wetted, such as the garage floor in the photo below with an adjoining driveway that was incorrectly sloped to let water puddle at the garage door after every rain.

     A final note: a new masonry building will sometimes have a minor efflorescence bloom, as the moisture still in the material from the manufacturing process is evaporated away during the first months following construction. 

    Also, see our blog posts How do termites get into a brick house and What causes raised white lines of residue on a block wall that are crusty and crumbling?  and What causes brick stain or discoloration near the ground? and What causes the surface of old bricks to erode away into sandy powder?

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To learn more about exterior walls and structures, see these other blog posts:

• What is the difference between soil subsidence, heave, creep, and settlement? 

• How much ventilation is required for the under-floor crawl space of a home? 

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem? 

• What are the pros and cons of concrete block versus wood frame construction?

• Should I buy a house with a crawl space? 

• Why is my stucco cracking?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize serious structural problems in a house?

• What is engineered wood siding?

• Should I buy a house that has had foundation repair? 

• What is a "continuous load path”?

• Should I buy a house with asbestos siding?   

• How can I tell if cracks in the garage floor are a problem or not? 

• What do you look for when inspecting vinyl siding?

• Why is housewrap installed on exterior walls under the siding? 

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• Should I buy a house with structural problems? 

• What causes cracks in the walls and floors of a house?

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What are the common problems of different types of house foundations? 

• What are the warning signs of a dangerous deck?

• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

    Visit our EXTERIOR WALLS AND STRUCTURE and MOLD, LEAD AND OTHER CONTAMINANTS pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.