How to stop: Parapet and soffit construction solutions

Graphical representation of typical pressures experienced by the building enclosure due to effects of HVAC, wind, and stack effect.
Graphical representation of typical pressures experienced by the building enclosure due to effects of HVAC, wind, and stack effect.

Soffit case study

Soffits can be deceptively with several complex design and construction challenges, such as:

  • delivering the desired esthetic;
  • understanding the lines of air and thermal control;
  • accommodating structural movement;
  • considering service penetrations like ducts and lighting; and
  • identifying potential thermal comfort risks.

Soffits are also constructed by many trades over a protracted project schedule with, too often, few integrated design and delivery strategies.

A few years ago, the author was involved with an investigation of a deteriorated soffit and requested to develop remedial strategies with the aim to improve long-term durability and address occupant comfort concerns. While people generally think of soffits being located close to grade, often on the underside of an overhanging second or third storey, the soffit in this case study is located on the seventh floor of a newly constructed building. Regardless, the defects and conditions presented by this case are very typical and occur more frequently than one would expect.

Upon arriving at site, it was noted a large portion of the soffit had failed and fallen to a lower roof. Accessing the area by an elevated work platform allowed an up-close review of the failed components and of adjacent, intact assemblies. Immediately, a discontinuous air barrier permitting warm, moist, interior air to leak into the soffit space was identified. Once in the soffit space, the air condensed and led to moisture loading of the soffit sheathing. The coating on the soffit sheathing and inadequate venting of the soffit void prevented drying of the sheathing and, with material softening, contributed to failure of the panel retention system.

A couple of other potential contributing factors to this soffit failure were identified. They are as follows.

  • The suspension system was secured to the slab above and to the curtain wall. Since the built-up soffit assembly is supported from the floor above and the curtain wall is supported on the floor below, the soffit suspension system was subject to differential deflection it was likely not designed to accommodate.
  • The soffit slab was covered by a self-adhered bituminous air barrier/vapour retarder (AB/VR), requiring mechanical securement on any soffit condition. Further, the soffit insulation was restrained by impaling pins adhered to the membrane. As a result, a membrane not designed to take its own weight was also carrying the full weight of the insulation. Photographs from the site show the resultant delamination of the AB/VR from the concrete slab.

To resolve these issues, several measures were recommended. These included:

  • installing a continuous air barrier/vapour transition material from the curtain wall to the slab underside while accommodating structural deflection;
  • installing a new membrane where it had failed,
  • installing mechanical securement where the membrane remained intact;
  • improving insulation retention with mechanical attachment;
  • modifying soffit venting to improve drying; and
  • reviewing coating materials for the sheathing that would not trap moisture.

None of these items would have significant impact on the design concept or the construction budget had they been included. However, the absence of these measures resulted in gross soffit failure and a considerable repair bill.

Detailing for success

These examples highlight the need for attention at critical junctures, such as parapets and soffits. Detailing these assemblies requires an understanding of structural building movement, thermal performance requirements, air control, material limitations, construction technologies, and trade co-ordination. Transitions in these areas are frequently overlooked and misunderstood.

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