In commercial construction, how should the horizontal leg of flashing at the base of the wall be terminated when the masonry veneer extends below grade?

ANSWER:

Masonry that extends below grade is in a potentially moist environment, so this design feature is not recommended.  When the veneer is designed to extend below grade, cells and cavities below the flashing should be filled solidly with mortar and/or grout.  With that in mind, four ways to terminate the horizontal leg of base flashing, including their advantages and disadvantages, are discussed below.

Sheet metal drip, hemmed and bent to 45 degrees or 90 degrees: A sheet metal drip, bent to 45 degrees with sealant below the drip, is the detail most frequently recommended by industry organizations because it sheds water from the face of the wall below.  However, exposed sheet metal edges (particularly at outside corners and laps) are sharp and pose a risk of injury to people who might be close to the building wall ([1]See MIM Bulletin).  Also, some people object to the aesthetics of exposed sheet metal.  Coating the metal with a color that matches the masonry units can overcome this shortcoming.

Sheet metal edge, hemmed and bent 180 degrees: The bend of the sheet metal should be placed at the outside surface of the masonry.  The sheet metal should be fully bedded in non-asphaltic mastic or adhesive or non-skinning butyl sealant.  This detail does not shed water away from the wall surface below.  This detail reduces the appearance of the sheet metal to a thin “line”.  ([2]See CSI magazine)

Flexible flashing cut flush with the outside surface of masonry: This detail minimizes the outside appearance of the flashing.  When this detail is desired, the flashing should be placed so that it protrudes from the face of wall and should be cut flush   When asphaltic flashing is used, this detail may result in unsightly black drippings.  With this detail, non-self-adhesive flashing should be fully bedded in non-asphaltic mastic or adhesive or non-skinning butyl sealant to prevent water entry below the flashing.  Some flashing materials cannot achieve perfect flatness and a wavy line may be visible at the outside surface.  ([3]See MIM Generic Wall Design details)

Flashing recessed 1/2-inch from the outside face of masonry: The advantage to this detail is that the flashing is not visible from the exterior.  Also, when asphaltic flashing is used, this detail is not likely to result in unsightly black drippings. Non-self-adhesive flashing should be fully bedded in mastic or adhesive or non-skinning butyl sealant to prevent water movement below the flashing.  However, when the flashing is placed recessed from the outside surface, there is risk that the flashing will be drawn into the wall by the weight of mortar droppings, resulting in the outside edge being recessed enough to expose the solidly filled masonry cores below.  Most importantly, though, is that this detail is not allowed by the Michigan Building Code [4].  Consequently, this detail requires building official approval in accordance with the ”Alternative materials, design and methods of construction and equipment” section of the Code [5].

References:

[1] Masonry Institute of Michigan, Exposed Metal Flashing Bulletin, 2016
[2] The Construction Specifier Institute Magazine, April 2016, Aesthetics versus Function
[3] Masonry Institute of Michigan, Generic Wall Design Details
[4] 2015 Michigan Building Code, Section 1405.4.2
[5]2015 Michigan Building Code, Section 104.11

 

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