March 9, 2017
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:
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.
Perhaps the most important layer in the ‘cake’ is the waterproofing system itself. The system may be constantly wet, so it must resist leakage, hydrostatic pressure (often in the form of ponded water), and fungal growth. It must also be long-lasting, as repair costs are high. Resistance to chemicals is an important consideration. If there are plantings on the deck, chances are there will be fertilizers. Certainly, there will be cleaning products—and there is always the possibility someone walking over the deck will spill a soda.
Typically, the waterproofing membrane is adhered directly to the plaza deck. The deck surface provides a stable substrate and helps prevent leaks from migrating laterally. There are instances when the membrane can and should be loose-laid—for example, if there is no solid surface to adhere to, or if there will be exceptional deflection or movement in the deck. This technique is also very fast, making it ideal for transportation projects.
Some companies manufacture grid systems that simplify maintenance and repairs by allowing removal of certain sections of the assembly. These are ideal for some types of projects, but there are disadvantages, such as the cost of installation and the complications of sealing the edges of the grid.
The waterproofing should be highly vapor-resistant. A deck with occupied space beneath needs to control vapor drive from below as well as water from above, so the membrane is also the air/vapor barrier. In a wall assembly, permeable versus impermeable is worth debating, but in this case, the longevity of the plaza deck hinges on keeping moisture out.
Typical waterproofing systems include hot rubberized asphalt, modified bitumen, and cold liquid-applied membranes. Single-ply thermoset (e.g. ethylene propylene diene monomer [EPDM]) and thermoplastic (e.g. polyvinyl chloride [PVC] and thermoplastic polyolefin [TPO]) waterproofing membranes are also popular due to their low vapor permeance and water absorption. Each system has its own advantages and disadvantages, all beyond the scope of this article.
One group of membranes worth a look is cold-applied liquids. Introduced about 40 years ago, these materials are fairly new to the market, at least compared to old reliables like hot rubberized asphalt and modified bitumen. Made variously of polymer-based combinations, such as polymethacrylate (PMA), polymethyl methacrylate (PMMA), polyester, polyether, or polyurethane, they may go on alone, but most often they are paired with a reinforcing fabric like fiberglass or polyester for extra strength.
Some variations have had problems. There are specifiers who prefer the sheet variety because they are assured they will go on at the proper thickness; liquid variations require perhaps greater trust in the skills of the applicators. Additionally, the formulas are not particularly UV-resistant.
Liquids still offer some advantages, including the fact there are neither seams nor opportunities for fishmouths and wrinkles. These products go on quickly and easily; they can often be applied over green concrete, saving time and doubling as a curing agent. They may also cover asphalt, metal, and wood surfaces, as long as they are clean and free of coatings and sealers. One should check with manufacturers on how to apply liquids in cold temperatures. (Most require the surface be frost-free.)
Some formulations are low in, or free of, volatile organic compounds (VOCs,) solvents, and odors. All of these are an issue with plaza decks—they are often close to occupied space and HVAC systems. A worst-case scenario is an HVAC system picking up VOCs from a membrane and transmits them through the building.
Liquids often double as flashing materials. Working well in planters, they are highly adhesive and stick well to drains and joints. The joints in the substrate need to be pre-detailed with the material, sometimes with a reinforcing mesh, prior to the full membrane application. Most of all, it is convenient to use one material for so many different purposes on one jobsite.
These durable liquids do not melt in hot weather or crack in extreme cold. They may be double- or single-component, with advantages and disadvantages to either option. The two-component variety tends to have a longer shelf life, and some professionals consider them more durable. However, single-component liquids are easier to apply, and avoid problems with incorrect mixing.
Regardless of which membrane is used, all the components that follow—from flashing materials to caulks—should be compatible with the membrane and not cause delamination or debonding.
More layers and more details
After the membrane and flashing is installed and inspected, there must be flood-testing in accordance with ASTM D5957, Standard Guide for Flood-testing Horizontal Waterproofing Installations. All drainage locations have to be sealed before the test. After, sources of leaks are repaired, with the tests repeated until everything is sealed.
Protection of the membrane is mandatory. The protection course must resist crushing and transfer of concentrated point/impact loads to the membrane. This layer may also serve as a relief plane so expansion and contraction of the wear surface does not pull the membrane along and cause stress points and tears.
Sometimes the drainage mat alone will suffice for protection. It should be installed fabric-side-up, and must be continuous to prevent clogging. In other cases, the insulation may double as a protection course, which is not ideal. A purpose-designed protection course, often an asphaltic or cementitious board, is always the best idea.
In some parts of the country where rainfall is minimal, the drainage mat is located only in critical areas or skipped altogether. In these instances, the protection course alone sufficed, while in other cases, a protection course and a drainage mat are used in tandem. This aspect of the deck is left to the discretion of the designers.
When it comes to insulation—a very good idea to protect the assembly from thermal extremes and to insulate occupied spaces below—extruded polystyrene (XPS) is often the material of choice. It offers high compressive strength to protect the membrane and drainage layer. It should not be a substitute for a protection course, though it is often used as one in particularly mild climates. Further, seams in insulation layers should be staggered. The insulation layer is likely to be saturated if it is not drained from above and below.
The selection, specification, and design of traffic-bearing surfaces are the purview of the designer. Typical choices include concrete pavers, paving stones, rubber play tiles, cast-in-place concrete pavement, and artificial turf. However, the wear membrane, which may be colorful or convey a sense of nature or even direct traffic, is important to the deck’s function.
These systems are categorized as accessible or inaccessible. In the former, the wearing course is removable—pavers that rest on pedestals, for example. Inaccessible systems include a concrete or asphalt slab, or mortared pavers. Fountains fall into this category. The problem with inaccessible systems is they are expensive to repair, often requiring demolition.
The wear surface can aid or impede drainage. Accessible systems allow water to percolate through, but may also block flow. For example, sand and small stones can release from around larger stones, flowing into drains.
Concrete wear layers present another problem—rebar, wire mesh, and other elements can come into contact with the waterproofing membrane and damage it. Careful, supervised installation is the solution, ensuring the membrane is preserved for at each step.
How the ‘layer cake’ of materials is detailed—at the expansion joints, flashings, and where they meet different materials or vertical elements—determines the deck’s long-term performance.
The waterproofing is flashed against walls, as well as other termination points such as planters, fountains, and other vertical elements. Leaks occur most often at the flashing, not within the field of the membrane. The membrane must adhere tightly to all drains and flashings. Flashing must extend above the height of the final wear layer. If the overburden is above the top edge of the flashing, water can back up into the wall system. Ideally, flashing should be accessible for maintenance and repair wherever possible. Drains also need to be accessible for cleaning, maintenance, and repair.
Similarly, control and movement joints require careful planning ahead of the installation. Ideally, joints are located on raised curbs above the level of the waterproofing membrane. Better still, those joints should be in accessible locations, so they can be fixed.
Accessory items (e.g. electrical conduits, irrigation lines, and piping) should not be attached to a waterproofing system. It is best if these are also shielded from the membrane and accessible for repair.
Planters should be isolated from the membrane system and the sides of the building, if they abut. Many manufacturers require a root barrier—a good idea over any material, though many new cold-applied liquids do not require them. (For best practices, refer to the National Roofing Contractors Association’s [NRCA’s] Vegetative Roof Systems Manual.)
Railings, bike racks, planters, bollards, light poles, and other items are installed over the membrane system, and not integrated with it. This is easiest when the wear surface can support them, as in the case of a cast-in-place concrete slab.
Designers can find more information by consulting ASTM E2266, Standard Guide for Design and Construction of Low-rise Frame Building Wall Systems to Resist Water Intrusion. Further guidance is available through many other resources, including the aforementioned NRCA and NIBS’ Whole Building Design Guide.
|RENOVATING A PLAZA DECK AT SCOTTSDALE FASHION SQUARE|
Building a new plaza deck is complicated, but renovating an existing project brings additional challenges. There are the usual difficult logistics required to complete a project when a space is occupied. Often, the deck itself needs repairs, which is difficult to accomplish without removing the wear surface and violating the waterproofing system. Perhaps it was not built properly, has suffered excessive wear, or no longer meets the changing needs and priorities of the owners.
This was the case with the 1860-m2 (20,000-sf) plaza deck linking Scottsdale Fashion Square offices with the eponymous shopping center—the largest in Arizona. Positioned at the highly visible northwest corner of Scottsdale and Camelback roads in downtown Scottsdale, the original 1989 post-modern office building and deck needed to be freshened up to befit class-A office space.
The architect for the project, Gensler, integrated the six-level building and deck by harmonizing design details and “creating a consistent design language for the entire development,” said John R. Williams, AIA, associate.
It was also an opportunity to add a layer of waterproofing where there was none previously. (This was the norm for dry-climate areas, like that of Scottsdale, several decades ago.)
The deck, which shelters parking below, was also in need of some minor repair. Water was trickling through areas around the existing planters and vents.
“This is the kind of thing that just gets worse,” said Brian Whited, partner at AK&J Sealants (Phoenix, Arizona), subcontractor for the project, which was run by A.R. Mays Construction.
Care was taken to avoid hitting and damaging the post-tensioned deck’s tendons throughout the project. A peel-and-stick waterproofing was added to the edges of the garage, where accessible.
The architects and owners selected a cold-applied polyether waterproofing that could be rolled, troweled, or squeegeed onto the surface and emit next to no odor—important for occupied spaces. The proprietary material also functions as waterproofing for the planters. Since it can be lapped up the wall several inches above the topmost layer of the overburden, it serves double duty as a flashing.
Edges, cracks, joints, and corners of the deck were first protected with a layer of mesh embedded in polyether. A 1525-µm (60-mil) basecoat was applied, then a layer of spun-polyester reinforcing fabric. This helps prevent cracks in the substrate from telegraphing through, while adding overall strength. A second coat of the same thickness followed, for a total of 3050 µm (120 mil).
An asphalt-sheet protection course was added over the waterproofing, and drainage board was used under the planters. The cold-applied membrane was applied around the inside of the planters and at the base as a root barrier. These areas were not reinforced with fabric.
The deck features precast concrete hardscape in various colors denoting walkways. There are new desert-friendly plantings throughout, a shade structure, and a section of “very realistic-looking” artificial turf, Williams said. The work was done with an eye to creating an event space for the office and mall tenants, and also created a walkway from the street corner in front of the building into the mall. There is now also a loop-through drive for dropping off guests.
Wendy Talarico, LEED AP, is the architectural services specialist for W.R. Meadows. She is an instructor for Urban Green/New York State Energy Research and Development Authority’s ‘Conquering the Code’—an eight-hour energy class for architects and engineers. Talarico has also worked as a member of the engineering outreach team for the Brick Industry Association (BIA) and has more than 25 years of experience writing and editing. She is a member of CSI as well as the American Institute of Architects (AIA) DC High-performance Buildings committee. She can be reached via e-mail at firstname.lastname@example.org.
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