December 1, 2012
By Scott Wylie
The restoration of the Lieutenant F. Campbell, VC, Barracks in Meaford, Ont., required brick façade replacement to its military residence. The owner, Canadian Department of National Defense (DND), identified masonry deterioration due to the spalling of brick at the base of foundations and at lintels over most windows. The primary reason for this situation was the through wall flashings had failed to provide effective drainage from the cavity.
Deterioration was also evident at the parapet wall—a fact attributed to moisture accumulation from air pressure differentials, such as stack effect, fan pressurization, and wind direction. The inability of accumulating moisture to drain effectively from the cavity needed to be addressed, and the building’s airtightness had to be improved. Structurally, the masonry wall also had to be replaced.
The performance criteria to correct the existing problems called for a stainless steel metal drip edge, combined with cavity drainage and mortar control. This was a departure from the traditional methods of mitigating moisture from the cavity.
The cost-effective solutions specified were improvements to the overall design. Despite research expounding the warnings of not incorporating a metal drip edge and mortar control device, many continue to allow the status quo—standalone plastic or rigid rubber and asphaltic flashing sheets without any mortar-control device.
The fundamentals of a through-wall flashing specifically designed to excel in migrating moisture away from the wall is typically an afterthought by many designers. Some design professionals and building envelope specialists prefer to conceal the front edge of the flashing for esthetic reasons. Besides, most flashing sheets are not resistant to ultraviolet (UV) light.
Leakage from moisture, moving off the concealed front edge of the flashing, can easily return underneath the flexible flashing, resulting in wall deterioration. A metal drip edge can reduce this design dilemma, while also preventing veneer stains.
In the case of the barracks, site inspections revealed the flashing materials had failed in numerous locations, particularly at transitions; this had allowed moisture to move laterally. Overlaps, tie-ins and end laps using peel-and-stick, primers, and mastics can prove difficult to properly install, especially at end-dams.
A mortar-control device that suspends mortar droppings at unequal heights can allow moisture to drain from the cavity and assist in airflow within the cavity. Unfortunately, this had been omitted in the initial design.
The specification for the amelioration project outlined a cost-effective system that combined through-wall flashing with a mortar-control, metal drip edged assembly. A termination bar to keep water from getting behind the flashing and a stainless steel drip edge were combined. The ease of installation, the ability to overlap the end laps, and installed pre-moulded end-dams were taken into consideration.
Scott Wylie is principal of Wytech Building Envelope Solutions. He can be contacted at firstname.lastname@example.org.
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