Trends in Urban Outdoor Amenity Spaces

In-demand roof deck systems add value to today’s buildings
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Sponsored by Unilock
By Elena M. Pascarella, RLA, ASLA
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Basic Componoents of Roof Deck Systems

There are primarily three construction systems used for amenity roof applications: the pedestal system, slabs on granular base, and slabs on permeable base. Although these three are the most commonly used, other construction methods, such as mortar set or sand over concrete, are also used in the appropriate applications.

The pedestal system is the most widely used, as it is easy to install and maintain and provides a system for leveling uneven roofs. Pedestal systems, however, have size limitations due to the load limitations, their limited size, and the required spacing of the pedestals. These systems start with the concrete structure of the building over which a waterproof membrane is placed. A protection board is then placed on top of the waterproof membrane, and insulation is placed on top of this protection board. The support pedestals sit on top of the insulation, and the roof slab tiles or units are anchored to the pedestals. Tiles or units can be made of wood, concrete, stone, or porcelain depending on the desired aesthetic. It must be noted that some tile or paver products may require a full base tray to support the product against breakage or cracking as a result of impacts or too heavy loads.

There are three basic types of pedestals: fixed height, adjustable, and stackable. The adjustable pedestal systems are the most commonly used, as they provide the greatest flexibility.

The pedestals are fabricated of a high-density copolymer polypropylene containing 20 percent post-industrial recycled material. They are impervious to water, mold, and freeze/thaw. They can support loads of 1,250 pounds per pedestal. The height adjustment ranges from 21/4 inches up to 36 inches, and the integrated leveler accommodates a 0 to 0.5 inch/foot slope.

Shown is the pedestal roof system at Fuse Condominium, Cambridge, Massachusetts.

Photo courtesy of Unilock

Shown is the pedestal roof system at Fuse Condominium, Cambridge, Massachusetts.

A slab on a granular base is the oldest and most widely used system and is easy to install. There are no size or shape limitations so these roofs can cover large areas. However, slabs on granular base are the most expensive systems to install and can be costly if repairs to the membrane are necessary. The system starts with the building’s concrete slab over which a waterproof membrane is placed. A drainage mat is then placed over the membrane followed by any potential insulation, if required. The drainage mat or roof drain has a geotextile fabric covering to prevent setting bed fines from clogging the drainage area. Roof tiles or pavers are placed on the sand setting bed and held in place with joint sand. Figure 1 provides a typical cross-section view of this type of system. In some newer systems, the insulation is placed on top of the waterproof membrane, providing more protection to the membrane from extreme temperature changes and physical damage.

This cross section of a slabs on granular base roof system shows sand setting bed with drainage mat and insulation.

Photo courtesy of Unilock

This cross section of a slabs on granular base roof system shows sand setting bed with drainage mat and insulation.

A roof system with slabs on permeable base (chips) can accommodate heavier loads than the pedestal systems. This system also provides optimum drainage, as the permeable chips or aggregates maximize water penetration. The construction for this system is shown in Figure 2 and consists of the building’s concrete structure, a waterproof membrane, insulation, a permeable setting bed and base, and finished with the roof slab tiles or unit pavers.

Shown is a cross section of a slabs on permeable chips roof system.

Image courtesy of Unilock

Shown is a cross section of a slabs on permeable chips roof system.

Existing wood roof structures can also be retrofitted with a roof deck system by adding a structural underlayment to the existing joist and beam structure. Spacer tabs of 1/8-inch thickness are set on the structural underlayment, and the roof tile units (porcelain, stone, concrete) are anchored on top.

Whether the slabs are set on a permeable base roof system or on a pedestal system, this top surface of the roof is still “permeable.” However, all of the above systems use a waterproof membrane in their construction to keep water from penetrating through the roof into the building.

There are opportunities and limitations with each paving system. With the elevated roof slabs on pedestals, utilities can be hidden under the slabs and allow easy access to roof drains and other maintenance. For some buildings, a pedestal system can be a very economical way to transform any exterior roof space. The main limitation of elevated pedestal slab systems is regarding layout. The most efficient size is on a 24-inch grid. However, this grid reduces the design flexibility. In contrast, the sand set and permeable over granular base systems allow for a more dramatic use of material sizes, shapes, and layouts. These systems expand the landscape architects creative abilities to work off the grid by designing more natural, curvilinear spaces. Certainly, with base material comes an increased cost, but the weight per square foot can also be a limiting factor. Confirming the structural loads is an essential step in the design process.

All three of these roof decking systems help to provide added insulation to the building’s interior spaces. With the added layers of these roof decking systems, building interiors remain cooler in summer and warmer in winter, thus providing energy savings.

Hardscaping Products for Roof Deck Systems

There are four popular products used as slab or paver units in roof deck systems, and these products provide designers with a range of colors, sizes, and finishes for roof deck surfaces.

Concrete slab tiles provide a range of sizes, colors, and finishes. They range in price from $3.50 to $11 per square foot depending on the size and finish. Concrete slab tiles must meet ASTM C1782, which is the standard specification for utility segmental concrete paving slabs. The primary method for determining suitability for spanning between two pedestals spaced 24 inches apart is the product’s flexural strength, measured by its modulus of rupture, which places a load in the center of a slab supported only on the corners. According to ASTM C1782, a product in this application support a minimum load of 650 psi. In addition, they must comply with ASTM C1645 for freeze-thaw durability and ASTM C418 for abrasion durability. For pavers set on a granular base and are 101 square inches or less, they need to meet ASTM C936: Standard Specification for Solid Concrete Paving Units.

The concrete slab tiles have quality controls that limit the modulus of rupture to an average of 725 psi. Modulus of rupture indicates the flexural strength or bending strength of the concrete. Concrete slab height tolerances are allowed to +/- 3 mm. The height tolerance ensures uniformity to the slabs and makes the installation easier, as the uniform height or thickness means less time is required during installation. The weight of these concrete slab tiles varies depending on the slab thickness. Units that are 13/4 inches thick weigh 19 pounds per square foot. Units that are 2 inches thick weigh 22 pounds per square foot. Units that are 23/4 inches thick weigh 31 pounds per square foot. Units that are 4 inches thick weigh 46 pounds per square foot.

Concrete slab tiles and pavers can be manufactured in the standard ‘thru-mix’ where the same aggregates are used throughout the slab. The only drawback is that over time, the concrete and pigment will wear, showing the large aggregate at the surface, and appear faded and worn. A newer two-step manufacturing process combines a base of coarser aggregates for high strength and durability, while concentrated color and wear-resistant finer aggregates form the top. This results in a surface that is almost fade proof and UV resistant.

The aesthetic options for concrete slab tiles and pavers include multiple surface finishes of standard, smooth, brushed, exposed aggregate, blasted, speckled, and mottled. Concrete slab tiles can be both wet-cast and dry-cast depending on the style. Two recently developed concrete slab tiles provide an exposed granite blasted finish and a flamed finish. These newer tiles use a process that is more like wet cast, only the water is squeezed out. These tiles and pavers can also be treated with an easy-to-clean, stain-resistant finish during the manufacturing process, eliminating or reducing the need to use aftermarket sealers.

A system of concrete pavers and slabs on permeable base is used at 233 N. Michigan Ave., Chicago.

Photo: Essi Ala-Kokko Photography

A system of concrete pavers and slabs on permeable base is used at 233 N. Michigan Ave., Chicago.

While slabs or tiles have more commonly been used in roof deck environments, traditional concrete unit pavers that are used on the ground are gaining popularity in roof deck spaces. This allows designers to further extend their creativity and create continuity from the ground level to the amenity spaces, while enhancing the flow, patterning, privacy, and division of amenity spaces. These pavers were used on the Hullmark project in North York, Ontario, Canada, and Fuse Condominium in Cambridge, Massachusetts.

Natural stone tiles for amenity spaces are available in three classification categories: igneous, metamorphic, and sedimentary. With each classification, there are a variety of stone options, with the most popular being granite, sandstones, and limestones. Many are available in a range of colors and textures as well as sizes. Natural Stone pricing can range from $8 to $44 per square foot depending on the stone variety, finish, and thickness.

Finishes include polished, honed, water jet, sandblasted, leathered, flamed, combed, riven, and antiqued. Sizes range from 12 inches by 24 inches up to 24 inches by 36 inches. Natural stone tiles are available cut into natural edge steps, natural edge copings, full nose copings, and stone pillar caps.

Stone thickness ranges from 7/8 inch to 21/2 inches or thicker if necessary. This low-profile thickness makes it well suited for many applications but in particular for a concrete or wooden deck overlay where there are constraints on the height or weight of the material. The thickness will be determined by the span of the stone between supports if not placed on grade. For granite materials, the Marble Institute of America recommends a minimum flexural strength of 1,200 psi per ASTM C880 with compressible strength of 19,000 psi per ASTM C170.

Sources for natural stone include countries in Asia such as India where you may wish to validate the ethical standards of the quarry. There are suppliers in North America aligned with quarries that only process to the strict standards of ethical trading initiatives, ensuring that no child labor is used, fair wages are paid, and safe working conditions are maintained. Quarried stone varieties from India and other countries in Asia, North America, South America, and Europe include sandstone, limestone, granite, basalt, and quartzite.

Natural stone is used as a roof deck walking surface at a private residence in Chicago.

Photo courtesy of Unilock

Natural stone is used as a roof deck walking surface at a private residence in Chicago.

Porcelain tiles also provide a wide selection of colors and finishes. They are available in a variety of sizes, however, three sizes are commonly used for roof decks: a 24-inch by 24-inch square, 16-inch by 48-inch rectangle, and a 16-inch by 32-inch rectangle. The porcelain tiles are also uniformly thick at 5/8 inch, and this low profile makes them ideally suited as overlay products, particularly where installations have height or weight constraints. The tiles can be used as an overlay application on new and existing concrete decks, porches, balconies, stairs, and wood decks. Finishes include stone finish, and wood grain finish and edge treatments can be specified as sawn or pressed. Pressed edge (other names include cushion edge, round edge, and soft edge) tiles have been manufactured using standard pressing processes in molds that result in a slight curving on the sides. The amount of curving will vary between manufacturers and processes. First quality tiles with pressed edges have batches with an allowable variation in size. Thus, a wider grout joint should be used to allow for this tile size variation.

Porcelain tiles meet the following standards: ASTM C373 for water absorption (which is very low), ASTM C1026 for freeze thaw, ASTM C648 for breaking strength, and ANSI A137.1 for slip resistance. Porcelain tiles are designed for pedestrian use only but are ideal for commercial applications. The colors are non-fading for long-term wear performance. They are priced on average about $8 per square foot. Porcelain tile is manufactured worldwide. The tiles are manufactured to size and well packaged to protect them during shipment. Porcelain tile and natural stone may, in some cases, require additional securing to prevent against wind uplift.

Wood decking units or panels are usually square and have minimal size variations. Structural wood tiles are available to provide decking on roofs where designers are looking for a different texture. Wood tiles or panels can be fabricated from foreign woods, such as ipe, cumaru, and garapa. It is important to make sure that these products are harvested using environmentally responsible methods designed to preserve rainforest hardwoods or are certified by the Forest Stewardship Council (FSC). Wood tiles can also come from thermally modified North American hardwoods. Wood decking panels have hardiness ratings ranging from 1,630 to 3,680. They come in two sizes: nominal 24-inch square panels that weigh 24 pounds and nominal 24-inch by 48-inch rectangular panels that weigh 48 pounds. All wood panels have a Class A fire rating. Pricing ranges from $12 to $14 per square foot. Even though these products resist fire, some restrictions may apply with local fire building codes.

The thermally modified North American hardwoods are domestically sourced from red oak and ash trees that are currently harvested in Wisconsin. Red oak and ash are the preferred species because of their consistency in durability and color. The natural durability of the red oak and ash is enhanced through the thermal heating process, creating a product that is a Grade A wood with a 25-year Class 1 durability level and a resistance to decay that is comparable to South American hardwoods.

The variety of available tiles for roof deck systems provides designers with a broad palette of materials in designing urban outdoor spaces for roofs. The variety of products to create amenities on building roofs extends vertically as well as horizontally.

These various surface treatments, wood decking, stone tiles, porcelain tiles, and concrete pavers are all materials that meet LEED criteria. Porcelain tiles, stone tiles, and concrete pavers can be obtained regionally. All of these surface materials have much higher reflectivity than standard black roofs, thus providing higher solar reflective indexes (SRI) for compliance with the International Green Construction Code (IGCC) and reducing heat island effect.


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Originally published in Architectural Record