Park Seventeen Roof Garden

Challenge

Many challenges exist in the design of a roof garden atop a structure. At seven stories above street level, the weight and load of the Park Seventeen landscape and hardscape elements were key considerations. The impact of wind on comfort and function was also important, especially since the wind load would be exacerbated by the “funnel” effect created by the curving building facades.

Solution

The design team overcame weight/load challenges through careful coordination. First, designers ensured that there was adequate clearance in the parking structure below to accommodate the extensive package of amenities above. The use of varying depths between beams allowed for the placement of the pool and large specimen trees in deep areas, with turf or paving in the shallower areas able to support lighter loads. For wind load, design solutions included the use of canopy trees in areas where they could help dissipate the force, anchoring vertical elements (including trees) to the structural slab below, selection of weighted furnishings and amenities, and using binding agents within aggregate surfacing.

• Pinpoint structural coordination allowed for placement of specimen trees, such as redbud, magnolia, and bamboo, providing shade and an aesthetic complement to the outdoor leisure spaces.
• The use of artificial turf eliminated the need for mowing equipment, thereby reducing fuel consumption and emissions. Use of the turf also reduced weight (and subsequent cost) while accommodating outdoor functions.
• Small grade changes accommodated various soil depth requirements over the structure below. Engineered soil mixes were used in the intensive areas for trees, which have a 3-ft soil depth, and the semi-intensive areas for shrubs, which have a 1.5-ft soil depth.
• Warm-toned concrete and landscape materials increase reflectivity on the roof, which helps to reduce surface and air temperatures. The albedo of lighter-colored concrete is 0.7-0.8, while the albedo of darker-colored concrete is 0.35-0.4. A higher albedo means that less sunlight is absorbed by the material.
• A bamboo grove creates secluded spaces that are shaded and protected from the wind. Bamboo is well-adapted material in terms of wind mitigation and water consumption, and it was planted in contained planters to control its invasive nature.
• Of the total number of plant species, approximately 45% are well-adapted plants, which were used to accommodate low water consumption and also adapt to more extreme wind and heat issues on the elevated deck.

• The total size of the roof deck is 32,670 sf, and the cost to provide waterproofing, landscape, irrigation and pedestrian hardscape and amenities within this space was approximately $3,173,579, or $97.14/sf. In comparison, to acquire land for an at-grade park of the same size in uptown Dallas would cost approximately $63.54/sf or $2,075,852 in 2012 land costs. Considering the cost of landscape, irrigation, pedestrian hardscape and other amenities, the total cost of developing an on-the-ground park would have been higher than the cost of the roof garden.

• Municipal codes prohibited live turf on the roof garden, and so the designers used artificial turf to create green spaces with low maintenance requirements. However, this articifical turf unexpectedly contributed to increased temperatures in the roof garden. On a July day, the average mid-afternoon surface temperature of the artificial turf in the sun was 151.0 °F, while air temperature was 95.8 °F. These temperatures were equal to or above the temperatures of the parking lot below the roof garden. These findings could provide evidence to support future Dallas code variances for roof gardens to allow real turf, especially drought-tolerant varieties.

• Microclimates on rooftops can be extreme. According to the July 2012 survey about residents’ perception of the Park Seventeem roof garden, high temperatures and wind hindered many users’ level of satisfaction with the space. To create more pleasant and usable rooftop spaces, the impact of orientation and wind load should be key considerations in the building design, and roof garden designs should strive to create shade and wind breaks.

Exterior lighting: Drafted by Scott Oldner Lighting Design, LLC
Decorative, permeable aggregate: New Mexico River Rock; 4”-8” size Crushed Granite
Artificial Turf: ForeverLawn Select LX
Brick Veneer: Interstate Brick - Arctic White, Emperor, and Norman
Flatwork: L.M. Scofield Autumn Beige integral color concrete with #05 W.R. Grace surface retarder
Wood Decking: 5/4 Ipe with hidden fastener
Planters: Kornegay Designs - Dune and Square series 
Planters: The Chandler Co. - Rangoon and Nexus Low Bowl
Planters: Gainey Ceramics - Morgan Tall series
Pavers: 2’x5’ Granite pavers and 2’x5’ Travertine pavers, matched to architecture, flamed finish;
Pavers: Pavestone - City Stone III, Schlegel Tan
Pavers: Stepstone, Inc. - 2x6x36” Narrow Modular Paver, Almond #1806 with light sandblast finish

Project Team

Owner / Developer: Granite Properties
Residential: Gables Residential
Landscape Architect: TBG Partners
Architect: Good Fulton Farrell
Structural Engineer: L.A. Fuess Partners, Inc.
MEP Engineer: JJA, Inc.
Civil Engineer: Pacheco Koch Consulting Engineers
General Contractor: Austin Commercial

Role of the Landscape Architect

The landscape architect was responsible for the roof garden’s design as well as the coordination between different consultants. For example, the landscape architect coordinated with the structural engineers to address loading concerns for tree placement on the rooftop.