High-Performing Fill Material Supports Rooftop Swimming Pool

According to data from the U.S. Census Bureau, the Provo-Orem metropolitan area of Utah is the ninth fastest growing metropolitan area in the United States. This statistic isn’t surprising, given the significant gains in net migration to the state over the last decade as families flock to suburban communities. That said, with a burgeoning population comes an increased demand for housing.

Predicting the city’s need for housing, The Ritchie Group, a second-generation real estate development company, purchased an unfinished mixed-use development in Orem in 2015. Dubbed “Midtown 360,” the massive 265,000-square-foot complex was designed to accommodate both residential and commercial real estate needs; retail shopping and dining accompanies 245 contemporary apartment units and abundant underground parking. An added perk for residents of the luxurious complex? An outdoor swimming pool on the rooftop of a sub-grade parking garage. 

To bring the pool from concept to reality, The Ritchie Group enlisted Harris Architecture, an Orem-based firm specializing in residential and commercial design. From the start, sourcing an effective fill material to support the rooftop swimming pool proved difficult, considering the sub-grade garage was not designed to accommodate the high gravity loads imposed by the weight of so much water.

To meet this challenge, Harris specified expanded polystyrene (EPS) geofoam (GF) by Insulfoam. An engineered fill material, EPS geofoam is widely known for its lightweight composition, strength and customization capabilities. The effective fill solution is employed to address a variety of geotechnical challenges, including lightening loads on structures and serving as a structural void fill for concrete or landscaping applications. EPS geofoam was an ideal choice for use over the Midtown 360 parking garage because it offers incredible compressive strength, and yet, it’s light enough to sideline the risk of the pool floor sagging, cracking or leaking water onto the parking garage underneath. Had the project team utilized traditional fill materials such as concrete or soil, the owners of the Midtown 360 may have risked premature building system failure.

Lightweight Composition and Strength to Boot

Weighing approximately 0.7 to 2.85 pounds per cubic foot, EPS geofoam’s ultra-lightweight composition is one of its defining qualities. Incredibly, the rigid material is approximately 20 to 30 times lighter than concrete, and 100 times lighter than most soil types. This extreme difference in unit weight makes EPS geofoam an attractive solution over traditional fill materials. Because it weighs so little, large earthmoving equipment is not required during installation. Instead, the fill material can be installed easily and efficiently by hand. The custom-cut blocks are designed to lock in place, much like stacking blocks or assembling a puzzle.

Despite its lightweight nature, EPS geofoam is also designed to exhibit exceptional compressive strength and load-bearing capacity. As an engineered product, manufacturers can customize the material to offer values ranging from about 317 to 2,678 pounds per square foot at a 1 percent strain. Assuming combined dead/live loads do not exceed the 1 percent strain designation, the material will not creep or experience plastic yield. And because the material is engineered to disperse loads evenly across the roof deck, it minimizes post-construction settling, which supports a more stable foundation. This is not the case with soil and related fill materials, as their inconsistent compositions can lead to non-uniform load transfer and differential settlement. This can cause dangerous, uneven settling and lead to permanent structural damage. By employing the EPS geofoam approach to support the rooftop swimming pool, the project team would proactively avoid these risks and ultimately extend the lifespan of the complete building system.

Customization Capabilities

Blocks of EPS geofoam, which are typically 40 inches by 48 inches by 96 inches in size, can be employed to fill massive volumes, reducing upfront material and labor costs. For custom projects, like the Midtown 360 rooftop swimming pool, the rigid foam material can be cut on the factory floor to accommodate virtually any shape, curve or slope. This level of customization typically eliminates the need for separate concrete pours for vertical wall sections and topping slabs. Should the EPS geofoam need to be altered on site, crews can easily trim these oversized puzzle pieces using hot wire cutting tools.

In 2019, the pounding of hammers at the Midtown 360 was replaced with the splashing of water on the garage structure’s newly minted rooftop pool. Supported by a high-performance void fill from Insulfoam, the swimming pool and surrounding area quickly became a coveted destination for residents.

About the author: Tom Savoy is the technical director for Insulfoam, a division of Carlisle Construction Materials. He has worked in the EPS Industry for 33 years and in construction materials (manufacturing and testing) for 38 years. Tom actively participates in many trade organizations including ASTM, SPRI and EPS IA. He can be reached at [email protected].

TEAM

Architect: Harris Architecture, Orem, Utah, harris-architecture.com

Contractor and Project Manager: Cornerstone Concrete, Lehi, Utah, cornerstoneconcrete.us

General Contractor: Big-D Construction, Salt Lake City, Utah, big-d.com

MATERIALS

Fill Material: 13,720 cubic feet of InsulFoam GF EPS19 at 2.25 feet, 3.5 feet, 5 feet, insulfoam.com