Building Cladding to Podium Deck Transition Design

cover

Building Cladding to Podium Deck Transition Design

Alon Gold, P.Eng., LEED AP, Stephenson Engineering Ltd.

This article was originally published in the Spring 2017 edition of Pushing the Envelope Canada, which is published twice per year by Matrix Group Publishing Inc. for the Ontario Building Envelope Council. It has been reproduced with permission.


Designing waterproofing transitions between building cladding and podium deck systems requires a unique level of knowledge, experience and understanding. In new construction, a lack of continuity between the building cladding and podium deck moisture protection systems, or a break in the membrane’s continuity, can often result in water ingress to interior spaces or premature failure of an entire waterproofing system.

Waterproofing systems are only effective when they are continuous, regardless of whether they are buried within the cladding assembly or multiply protection systems applied on structural slabs. The challenge for the designer is to develop a continuous waterproofing system that is effective. In situations where the building cladding and podium deck waterproofing systems are designed by different parties, there is an additional challenge to ensure design coordination.

There are many waterproofing systems available to designers. Each system is dependent on several factors, including the building cladding system in question, performance requirements, client needs and cost limitations. Waterproofing systems can be comprised of the membrane, sheathing, reinforcement, protection board and other components. Building envelope systems are typically divided into roofs, walls and foundations. Waterproofing systems tend to be specific to these broad categories. Designing waterproofing transitions between these categories requires a unique level of knowledge, experience and understanding.

PODIUM DECK WATERPROOFING

The City of Toronto defines lot coverage as “the portion of the lot that is covered by any part of any building or structure on or above the surface of the lot.” In some urban developments, the building footprints occupy a portion of the total available lot space, with an underground structure directly below the footprint of the building. In other developments, the building’s underground structure exceeds the footprint of the building above it, forming a roof deck (or podium deck). Podium decks are typically constructed of reinforced cast-in-place or precast concrete and have provisions for protection against corrosion and water leakage.

The CSA has several publications specifying design requirements for concrete and steel structures. CSA S413 is a design standard for parking structures. According to CSA S413, all structural slabs (such as parking structure roofs/podiums) must be protected from corrosion and leakage. One of the most prevalent podium deck waterproofing systems is hot- or cold-applied rubberized asphaltic waterproofing systems. Perhaps not coincidentally, many rubberized asphalt waterproofing systems are compatible with moisture barriers found in cladding systems.

CLADDING MOISTURE BARRIERS

There is a vast array of cladding systems which can be installed in multiple configurations.  There are several reference publications that specify cladding design, performance and operational requirements, such as ASTM International and the Ontario Building Code (OBC).  The Canadian Housing and Mortgage  Corporation (CMHC) is an organization that publishes best practice guides for cladding systems including details, specifications and installation guidelines.

The details published by the CMHC provide designers with a guide for various cladding transitions such as windows and roofs, and junctions between cladding systems (such as EIFS to brick masonry).

The CMHC also provides termination transition recommendations at grade, such as:

  • Brick masonry and EIFS systems specify a 150mm and 200mm clearance between the underside of the cladding and grade, respectively;
  • Curtain walls termination at grade indicate a waterproofing transition onto a foundation wall; and
  • Flashing systems specify a 150mm termination above grade.

However, in each of the CMHC guidelines, the cladding terminations at grade do not take into account below-grade structures, such as podium decks.

CLADDING TO PODIUM DECK TRANSITIONS

Given the lack of best practice guidelines, it is up to the designer to establish the cladding to podium deck transition detail and to ensure the waterproofing membrane’s continuity. Over the course of many podium restoration projects, we have exposed poorly designed transition details.

Figure 1  shows a low-rise condominium building in Toronto constructed in the late 1970s. The brick cladding extended approximately 600mm below grade and terminated onto the podium deck slab. The exposed bricks were found to be badly cracked and spalled at some locations. This was likely caused by moisture entrapment in the bricks. Brick drainage was nearly impossible due to the waterproofing membrane being in-stalled directly overtop the brick face.

Figure 2  is from a high-rise commercial office building in Toronto, constructed in 1991. The curtain wall cladding system terminated approximately 300mm above the podium deck slab. Immediately below the curtain wall pressure plate is a 100mm void space (previously concealed with metal flashing but removed in this photo) and a 200mm tall concrete curb. Hot-applied waterproofing was installed on the podium deck slab and terminated flush with the surface of the asphalt pavement (removed in this photo). There was no waterproofing membrane joining the curtain wall to the concrete curb.

CONSIDERATIONS

As previously discussed, improperly detailed cladding to podium deck transitions can result in premature cladding failures, water and moisture infiltration and premature podium waterproofing failures. The CMHC guideline for brick masonry does, however, provide some relief.

Figure 3 and Figure 4  indicate transitions between brick masonry systems to inverted flat roofs (on concrete roof slabs). In both details, the waterproofing membrane is continuous, from the brick masonry to the horizontal roof slab. At the 90-de-gree turn, where the waterproofing would normally be exposed to weather elements and direct sunlight, the water-proofing system is protected by rigid insulation or metal flashing.

In my opinion, this is an ideal detail and circumstance. A similar detail should be prepared for cladding to podium deck transition. In general, cladding systems should never finish below-grade and there should never be void spaces between the cladding system and the podium deck waterproofing system. Continuity of the waterproofing systems must be maintained. However, most membranes are not UV-stable and cannot be left exposed to environmental conditions. A potential solution is integrating concrete curbs to protect membranes.

PROPOSED DETAIL

Concrete curbs provide protection for waterproofing membranes without compromising the membrane’s continuity. The designer needs to take into account podium deck finishes (asphalt pavement, granular overburden, landscaping, etc.). The height of the curb should be at least the depth of the overburden plus minimum grade terminations, as specified in CMHC guidelines.
Figure 5 is a sample detail for a brick masonry termination to a podium deck slab. The podium slab is protected with a hot-applied waterproofing system, then concealed with compacted granular and a concrete topping (not shown, for clarity). The curb extends approximately 150mm above the concrete topping and protects the continuity of the waterproofing membrane behind it. Each building will have unique circumstances. It is up to the building designer to verify the adequacy, practicality and implementation of their transition details.

Although this may seem trivial in the grand scheme of building design, restoration and building science consultants frequently face challenges in solving waterproofing failures and difficult-to-find leaks. In some cases, the issue can be resolved by improving or correcting an existing detail. In other cases, cladding systems need to be locally modified or replaced due to poor base building design. It is the intention of this article to identify the challenge in designing effective cladding to podium deck transitions and I hope that designers will consider this issue more carefully in future developments.

References:
1. www1.toronto.ca/city_of_toronto/city_planning/zoning__environment/files/pdf/guide_lot_coverage.pdf
2. CSA S413, Appendix A
3. CMHC best practice guide for brick, detail 4.1, 4.7a and 4.7b, http://pub-lications.gc.ca/collections/collec-tion_2016/schl-cmhc/NH15-132-1-1997-eng.pdf
4. CMHC best practice guide for EIFS, detail 10, http://publications.gc.ca/collections/collection_2016/schl-cm-hc/NH15-424-2004-eng.pdf
5. CMHC best practice guide for curtain walls, detail 17, http://pub-lications.gc.ca/collections/collec-tion_2016/schl-cmhc/NH15-428-2004-eng.pdf
6. CMHC best practice guide for flashing, detail 2.1, www.civil.uwat-erloo.ca/beg/ArchTech/CMHC_ flashing_BPG%20Compact.pdf

 

Alon-b&w

Alon Gold, P.Eng., LEED AP is a senior project manager at Stephenson Engineering Ltd., in the Toronto office. He specializes in the review, analysis and restoration of buildings, particularly parking garages, podium decks, balconies and other structural building components. Alon is a graduate from McMaster University and has been working in the construction and consulting industries for almost 10 years.