Protecting a parking deck from water leakage

Photos courtesy Amir Hassan

By Amir Hassan, M.Sc., PE, P.Eng.

It is hard to imagine any urban setting without parking garages. Given the limited availability of usable areas in busy cities, parking structures efficiently provide stalls for vehicles in airports, train and bus stations, commercial buildings, hospitals, and malls. They are commonly built using cast-in-place concrete, precast concrete, post-tensioned concrete, structural steel, engineered lumber, or a combination of these materials. Structural parking decks typically consist of reinforced concrete slabs over precast supporting units. The initial slopes of the slabs, along with their structural deflections, are important to direct water towards the drains as per the building’s water management plan.

Surface protection and insulation

Parking decks are costly to build, repair, and replace. Water is the major cause of concrete deterioration and steel corrosion; therefore its management is vital to ensure proper durability. Designing, installing, and testing specialized systems are key steps to ensure water is diverted away from the structural elements of the deck. The waterproofing layer can be placed on top of the deck system, where it is easy to inspect and repair, however, the layer is protected from wear and tear resulting from pedestrian and vehicular traffic. The preferable approach to parking garages is to use a protected waterproofing membrane. As the name implies, the membrane will be protected from traffic, which can extend the service life of the deck. Nonetheless, the limited accessibility will make the assembly hard and expensive to maintain. Loose-laid membrane can also be applied, but full adhesion is the most common application of deck membrane.

The adhered system can be achieved through cold, hot, or sheet application. However, challenges with maintaining thickness and quality through cold and hot adhesion makes rolling out premanufactured rubberized asphalt membrane the most common application. The sheet membrane has many joints which may leak water if they are not sealed tightly. Creating a continuous plane throughout the deck—especially around penetrations—is essential for watertightness. Flashing and pressure bars are used to maintain membrane continuity at terminations. Cracks, expansion, and construction joints in decks require special attention and consideration. Regular inspection and timely repairs are vital to maintain the health of a building. The structural behaviour of the floor slab, including deflection values and locations, affects the performance of the waterproofing system.

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