by Wendy Talarico, LEED AP
Plaza decks suffer from an identity crisis. They have multiple definitions and names—are they roofs? Floors? Driveways? Courtyards? Vegetative decks? Located over parking areas, arcades, or water storage reservoirs, they may be large (the size of a city block) or small (just a square meter or two). Sometimes, plaza decks are well above grade, while AT other times they may be sunken several meters below. Regardless, they present design complexities.
Plaza decks are vulnerable to water and freeze-thaw damage, but they are often excluded in the design and construction of a building’s waterproofing layers. Draining water from the surface is a challenge, as slope is minimal or sometimes non-existent. The transition between the vertical wall (or planters) and the horizontal waterproofing should be carefully designed and built to avoid the water infiltration, but this critical detail too frequently becomes an afterthought.
Most plaza decks are constructed with cast-in-place concrete, often post-tensioned. Precast concrete is also used, as are wood, metal, and combinations thereof. Dead and live loads can be impressive when you consider the weight of the overburden, pedestrians, vehicles (e.g. buses and fire trucks), construction loads, and the weight of the deck itself. Saturated soil and the weight of planters and foliage can also be significant.
The National Institute of Building Sciences’ (NIBS’) Whole Building Design Guide defines a plaza deck as “any supported slab that provides green-scape, tree planters, and/or vehicle and pedestrian movement over occupied space.” Recent projects feature unoccupied space below—often storage.
Plaza decks are hybrids—the water protection system combines the features of roofing and waterproofing. The Whole Building Design Guide’s reference on plaza decks, as well as those of some manufacturers, calls installations where the waterproofing is below the insulation a “protected membrane” or “inverted roof membrane assembly.” In these systems, protecting the waterproofing layer from temperature extremes, ultraviolet (UV) radiation, and mechanical damage is the priority. As a result, the underlying structure, adjacent spaces, and whatever space (occupied or otherwise) is located below are also protected.
Ideally, the system—particularly the drains, expansion joints, and flashings—should be accessible for maintenance. Repairs to an existing deck are expensive, perhaps more so than the cost of the initial installation. Fixes may be done in damaged areas, or the entire top layers may be peeled away for full access.
Regardless of the definition, the trick to a successful plaza deck is good detailing, planned well in advance.
First step: The deck structure
A plaza deck is a layer-cake of materials: the waterproofing membrane provides the base layer and the rest—protection layer, drainage course, insulation (or sometimes a fill layer of sand or gravel), and wearing or traffic surface—is ‘overburden.’ How those layers are designed depends partially on the design of the slab itself, which, in turn, depends on factors that include the loads to which it will be subjected, local construction techniques, climate, and what is located below.
Walls, deck, and supporting columns should provide positive slope for drainage. Water in contact with the membrane creates hydrostatic pressure and, in most climates, freeze-thaw cycling. A minimum two percent (1/4 in. per 1 ft) of slope-to-drain is recommended. (A steeper slope may be required by code and may provide better drainage and resistance to ponding.)
The wear surface, or top layer, is the first line of defense against moisture infiltration and all the attendant problems. It is not enough to slope just the wear surface, or simply use tapered insulation to create slope. Positive drainage of all elements is the best policy. Accumulated water beneath the wear layer evades detection and may be the downfall of an otherwise solid deck.
When water dodges the top layer, it needs to be directed away from the lower layers and infrastructure. Drain basins should offer a two-stage assembly:
- an upper grate for surface moisture; and
- a lower grate for the structural slab.
Both stages must be clear and functional. Maintaining these drains means the difference between a long-lasting deck and one that ages fast. Using an insulation board with drainage channels is a good idea.