Factors to consider when redesigning podium decks

Photos courtesy RJC Engineers
Photos courtesy RJC Engineers

By James Cooper, P.Eng., LEED AP

Podium (plaza) decks are structural slabs that extend beyond an above-grade building footprint and serve as roof slabs for below-grade levels. Often hidden by hard and soft landscaping finishes, these structural slabs can be forgotten and overlooked due to the relatively long service life of their protective waterproofing systems.

Podium decks are often the welcoming area of a property, providing access to primary entrances. They provide landscaped and naturalized environments, areas for rest and recreation, and a space to green-up environments in densely developed regions. A beautifully landscaped podium deck with well thought-out finishes can elevate a property, thus increasing value and demand.

While many waterproofing and structural components of a facility are fairly obvious—in a cursory tour of a property, one will see floor slabs, columns, walls and their surface finishes, parking levels and their waterproofing systems, and the roof can be accessed to check the installed systems—the podium deck waterproofing is hidden so as to blend into the surrounding on-grade areas. For the more visible and obvious structural and waterproofing components, it is easier to note visual cues of deterioration. However, the podium deck structure and waterproofing that are hidden beneath the landscaping finishes—while as important for overall building performance as roofs, walls, and parking garage systems—are often ‘out of sight,’ and therefore, ‘out of mind.’

Construction of podium decks

Podium deck waterproofing systems are typically protected by overburden. Since the waterproofing system is not wholly impacted by what is happening at the surface, anticipated service lives of the system can be a useful guideline to inform assessment and rehabilitation timelines. In unfortunate scenarios, owners may be unaware of early signs of deterioration, such as leakage into the below-grade areas, and may implement podium deck landscaping redesign without consideration for waterproofing age and condition.

Structural deterioration as a result of a failure of the podium deck waterproofing system.
Structural deterioration as a result of a failure of the podium deck waterproofing system.

Deterioration can occur in any of the components, and often happen in the various components at different times. Whether the podium deck is a lush, tree-lined oasis or a utilitarian, paved access point, it is subjected to aggressive environments that are similar to what a parking garage or above-grade roof encounters. Podium deck waterproofing systems and surface finishes are intended to protect the underlying structure from the moisture and chloride ions (salts) they are exposed to, handle freeze-thaw cycles, and withstand wear and tear. The overburden, landscaping finishes, and waterproofing systems have finite service lives. They require repair and, ultimately, replacement to maintain effective protection for the underlying structure. If podium deck moisture protection systems are neglected and remain past their period of effectiveness, moisture ingress into the structure can lead to costly deterioration that is invisible or unobvious, at least not as evident as a declining surface finish, such as a traffic coating in the parkade. Since the finished landscaping components are often expensive in comparison to the waterproofing system, it is important to ensure the latter still has an adequate service life prior to completing work on the overburden and landscaping above.

It is advisable to retain a specialist engineer to undertake a detailed assessment prior to significant podium deck landscaping redesign to ensure new finishes are not constructed on a waterproofing system near the end of its effective service life or a deteriorated structural slab. In fact, given the scale and potential costs of podium deck repair projects, an in-depth assessment is recommended before implementing any significant landscaping work.

As the podium deck waterproofing and overburden deteriorate over, potentially, a 25-, 30-, or 40-year effective service life, the podium deck assembly and types of finishes may become old or outdated. There may be a need for additional site parking and recreational space, outdoor seating, improved loading and unloading, waste management, or other changes.

An appropriate course of action

An accurate understanding of the current site conditions is vital for a successful project. An incomplete or inaccurate understanding of existing conditions prior to rehabilitation planning can spiral into larger issues and costs through design and construction phases. A more detailed scope of assessment and testing, despite additional initial costs, will help define and refine repair needs, and often result in multiplied savings over-and-above those initial expenses—accurate design drawings are the first way to save money (i.e. no costly change orders). Get the scope defined, make sure the details are correct, and one will not run into nearly as many changes.

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