Masonry Wall Flashing

By Bob Wilkin, P.E., Waterproofing/Building Envelope Specialist

Few people realize the amount of water that can penetrate a brick or concrete block wall during a rain storm.  This water needs to be managed in order to prevent leakage and damage to the building wall and interiors. 

Older buildings were constructed with a thick masonry wall construction that absorbed the water through the face of the masonry before it reached the interior.  Newer building construction utilizes a cavity wall arrangement with brick veneer, block stone, etc.; with a narrow cavity between the veneer and the interior wall construction.  Water penetrating this veneer must be collected and directed back to the exterior.  This is done with a water resistant membrane covering the back-up wall and a through wall flashing at the bottom of the wall cavity. 

While most of this information is well known in the construction industry, CBI has observed many disturbing details on newer constructed buildings during water leakage investigations.

On two buildings constructed in the last ten years CBI found “rope weeps” left in the walls above the through wall flashing membrane.  Rope weeps were common 20 to 30 years ago when clothes line rope was buried in the mortar joints during the construction of the masonry. 

Commonly , the ropes were pulled out leaving holes in the mortar for drainage.  At some point someone had the idea to leave the ropes in place, maybe once they found pulling them from cured mortar was impossible.  The rope was intended to wick out moisture from behind themasonry and eventually rot out leaving holes at the flashing level for water drainage.  This detail never worked as intended and hasn’t been used in many years.  The next method of forming weep holes was to insert plastic tube weeps in the mortar.  CBI generally finds that the tubes get plugged with mortar or bugs and are usually unproperly placed, well above the proper flashing level within the cavity. 

These inadequate drainage methods for the flashing allow water to collect behind the veneer and buildup on the wall flashing.  The buildup of water can be 1” or more in height, depending on the height and condition of the veneer, which jeopardizes the water tightness of the flashing. 

Through wall flashing systems are intended to be installed perfectly water tight with waterproof end dams and seams.  This is not always the case.  Water collecting on the flashing will travel to the weak points and enter the building. 

 The important thing to remember when designing and constructing a masonry wall is to provide positive drainage from the cavity.  This should include the following:

  1.  Install a water resistant membrane on the wall sheathing that laps over the flashing system at the base of the masonry.  This membrane should also be an air barrier to limit the leakage of conditioned air through the wall.  It may need to be breathable or non-breathable depending on the location of the insulation in the wall and the location of the building in the country. 
  2. The Flashing at the base of the wall should extend out to the face of the masonry so water is directed out of the wall and cannot travel back under the flashing and into the wall below the flashing. 
  3. The ends of the flashing need to be properly dammed laterally to prevent water from running off the ends of the flashing.
  4. The weep holes need to be set directly on the flashing so water can flow freely from the flashing. 
  5. Some walls are built with vertical joints between bricks left unfilled with mortar to form weep slots.  This provides free drainage but also lets in insects and wind driven rain water to enter.  These joints should be filled with baffled or foam inserts specifically designed to fill the gaps and allow drainage of the flashing cavity.  The inserts should be set directly on the flashing not in a mortar bed. 
  6. A common problem with flashing cavities is the accumulation of mortar droppings at the base of the wall which fills the cavity and the weep holes with mortar and negating all the good work to make the flashing drain.  There are several products that can be inserted between the brick veneer and the backup wall during the start of the masonry work that reduces the mortar droppings at the bottom of the cavity, as the wall is constructed, and allows the wall to drain. 

Masonry construction can be made to perform well for many years through good detailing and workmanship.