The deleterious situation of the pool structure

by brittney_cutler_2 | January 24, 2022 8:30 am

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Early co-ordination is essential for a safe and functional rooftop pool

By Patrick St. Louis, LEED GA, Lauren Millman, PhD, PE, and Ken R. Maschke, PE, SE, P.Eng., LEED AP

Once a luxury amenity, the residential high-rise pool is now as essential as indoor plumbing.

It has become increasingly common to locate a pool structure on top of an occupied space, such as a rooftop or garage. However, if not properly curated, this lavish and esthetically pleasing setting can come with long-term fiscal and physical liabilities.

Pool chemicals and chlorides create a harsh environment for the pool structure. Unintentional seepage of highly chlorinated water can accelerate corrosion of the embedded steel reinforcement. Typically, this results in pit (or localized) corrosion.

When pit corrosion occurs, repairs are costly and can indefinitely restrict use of the pool. The movement of water through the concrete structures could also lead to efflorescence and staining of building finishes and vehicles parked below. Therefore, project teams must be strategic in their design, construction, and maintenance to ensure success and minimize the risks.

Design and construction teams

The integration of an amenity pool should not affect the roof’s primary function, which is to ensure the structural integrity and watertightness of the space below.

Watertightness, or waterproofing, is not limited to the installation of flashing or liquid-applied products. It is a systematic approach involving multiple players.

Collaboration between design professionals and contractors during the early planning phase can address the unique challenges with rooftop pools and ensure the execution of a successful product.

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Since it is rare for a single professional to have complete knowledge of all aspects of design and construction, it is crucial to consult with engineers, architects, contractors, subcontractors, and specialists to build a structurally sound, watertight, visually appealing, and fully functional rooftop pool.

The plumbing contractor and designer can ensure the correct layout for the drainage system to minimize water ponding and moisture intrusion into the structure below. Water intrusion can originate from a variety of systemic breakdowns, such as improper waterproofing, leaks from pipe connections, and other unsealed separations in penetrating components. They will also confirm the pool’s shell is designed and constructed for watertightness.

The designer should then work with the client to select smooth, equally impermeable finishes. Higher resistance against water infiltration and chloride penetration will increase the life cycle of the pool structure.

Pool assembly

A rooftop amenity pool usually includes a raised deck surrounding the top of its shell. The deck has an elevated or surface-mounted paver or tile system. Underneath lies the roofing system on the structural deck slab. The shell is supported from below by a reinforced concrete structure. Typically, this is where the pool is incorporated into the building’s structural system.

Roof/pool deck

Many drainage complexities come with the construction of a low-slope roof deck. Incorporating the pool requires an extra level of co-ordinated effort between design and construction teams.

Communication between the owner, pool contractor, and other plumbing professionals is key when arranging the construction associated with roof drainage and layout of related systems.

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Like a standard roof, a pool deck is required to shed water and avoid ponding. Not only is it visually undesirable, but it also impedes foot traffic in communal areas. By most industry standards and roof material specifications, ponding should not occur for more than 48 hours.

The deck should slope away from the pool. If the surface consists of a poor or minimum slope, dirt and debris left by ponding water may wash in. Evaporation tends to leave behind additional residue that can stain deck finishes and cause contamination, which may shorten the life cycle of the filtration system.

The unsightly appearance of surface mould, silt, and dirt produces a slippery safety hazard at the pool edge. Slip resistance and compliance with industry standards should be considered when selecting surface coatings or finishes.

Coping caps

The edge of a pool will usually have coping within the design. The coping caps prevent water from getting behind the shell and shed water back into the pool drains.

An alternative to coping is the placement of deck material with a slight overhang above the water’s surface. This method serves a similar function, as contaminated water is sloped away from the pool and bathers.

Deck drains

Beyond the perimeter, the rooftop deck should be a system of valleys, with multiple slopes directed toward specific drains. These include, but are not limited to, spot, strip, or French drains. They are strategically placed by design professionals and the drainage pipe is encapsulated within a waterproofing and flashing system to remain watertight.

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Waterproofing

The waterproofing system is one of the most hidden, integral components. It ensures the watertightness of the pool structure and spaces below.

Waterproofing should be straightforward. However, client ambitions tend toward a complex design and require the need for early collaboration. A continuous multi-layer waterproofing system is fully adhered to the pool structure and is designed to be completely integrated into the coping and drainage system.

The drainage system is designed to minimize the restriction of water flow and limit blockages. A waterproofing system with properly terminated penetrations reduces the risk of moisture intrusion. Conditions and procedures addressing encapsulation of the entire surface area can ensure necessary construction order is carried out with warranties preserved.

Crack suppression

Due to high chloride content, water that reaches the steel reinforcement in the concrete can compromise the integrity of the structure. A waterproofing system with a high chemical chloride resistance and hairline crack bridging capabilities should be co-ordinated with the design-construction team to ensure consideration of essential details, penetrations/tie-ins, and separations.

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Design professionals, architects, and engineers should also collaborate in the placement and specification of the expansion, control, isolation, and construction joints. This will contribute to crack suppression, ensure watertightness, and serve as an artistic expression to the finish surfaces.

An important addition to a well-thought-out waterproofing system is the design and installation of waterstops within the concrete pool shell. Waterstops help obstruct the incidental passage of water through concrete joints. They should be placed systematically between the vertical and horizontal structural elements and should be able to withstand a chemically aggressive environment.

Admixtures can also be used to improve durability performance. They waterproof the concrete by growing a non-soluble crystalline structure within its pores and capillary paths. This process helps seal hairline cracks, which contributes to watertightness.

During the early planning stages, the concrete mix design should be co-ordinated with an admixture manufacturer’s representative to ensure proper dosage and strength. To avoid underdosage, it is recommended to specify the minimum percentage by weight of cementitious material requirements to meet its high-performance standards.

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The modified concrete will be treated as typical concrete, meaning there are no differences when applying decorative finishes or additional waterproofing tie-ins. However, the installer of the surface-applied material is responsible for taking the necessary measures to ensure compatibility and bond to the treated concrete.

While industry standards recognize reinforced concrete structures will have cracks to some extent, the pool’s concrete structure requires an approach that is more stringent and limits cracking.

Adding to the complexity of the engineering design, placement above a parking garage or unoccupied space limits the possibility of uniform placement of vertical structural supports. The spacing and frequency of these supports is ideally engineered to avoid deflections that would contribute to the cracking of the pool’s concrete shell structure or the finishes above. Spans exceeding safety guidelines put the pool structure at risk and could lead to costly repairs and extensive service interruptions.

While engineering codes and guidelines have provisions for complexity of design, non-uniform spans, and other challenges, they only address the structure’s immediate integrity—not its longevity.

Implementing additional measures during the design and schematic phases could help reduce deflections. For example, early co-ordination between the plumbing and structural engineer could reduce the number of penetrations (and those abandoned) and help with reducing overall congestion. The routing of plumbing, service lines, and pool equipment during design would lend to a shell structure that could further resist deflections and unanticipated stresses.

Electrical and lighting

Early collaboration between the electrical contractor and pool designer will ensure the proper equipment is specified, such as pool pumps, heaters, and whirlpool jets. These specifications should not only address pool health regulations and general code requirements, but also include considerations about energy efficiency and maintenance. Energy-efficient equipment may help offset the owner’s costs and minimize maintenance scheduling in the long term.

Bonding

While proper specifications and electrical connections are important, electrical safety should not be overlooked. The pool contractor and electrician should be well-versed in electrical bonding and grounding. All metallic components associated with the pool structure should maintain equal resistance. For instance, ladders must be electrically interconnected and grounded to the earth.

Additionally, structural and pool engineers should ensure steel reinforcement is bonded. Typically, electrical and metallic components of the pool are joined with a wire to form a non-resistive path between them. This is to prevent an electrical potential from existing. The goal of bonding is to connect, contain, and prevent the transmission of electrical voltage that would endanger a life or damage pool equipment. Bonding should not be an afterthought.

Grounding

The goal of grounding is to ensure the bonded components redirect harmful excess electrical current to the ground. The pool’s electrical network and panel must be grounded to dissipate excess electricity. Without a proper grounding system, people or pool equipment could become the conductor. Without both bonding and grounding, the risk is too high and repairs will be costly and timely.

Lighting

Ambient lighting specifications and layouts tend toward the vision of the designer. However, certain building codes and local municipalities have governances to protect wildlife. If the client wants lighting for night swimming, certain requirements need to be met, including those imposed by the health department. Early and frequent co-ordination will help promote safety and performance, while maintaining esthetics.

Exposed concrete within a garage

It may not exist in all configurations, but the underbelly of a rooftop pool may occupy a garage structure below.

Cars exhaust various toxic and non-toxic gases. Governing bodies within the mechanical building code, or industry standards such as those set by the National Energy Code of Canada for Buildings (NECB), have mandated venting requirements that address the carbon monoxide emitted from vehicles.

However, the carbon dioxide exhausted by vehicles idling or travelling within the garage can react with freshly placed concrete to produce a soft chalk residue on the surface, through a chemical reaction known as carbonation. Carbonation of reinforced concrete is a form of deterioration and contributes to the corrosion of reinforcement.

Other contributors to the corrosion of embedded steel are oxygen from the atmosphere and moisture from humidity or leaks from the pool above. Depending on the conditions, exposed and unprotected concrete may require an anti-carbonation coating to protect against the ingress of harmful elements, while still allowing the concrete to breathe.

Conclusion

Early co-ordination between design professionals and the construction team is necessary. During co-ordination and design, certain preventative measures should be taken to extend the life of the pool, maintain its structural integrity, avoid imminent danger to personnel, prevent damage to the property below, and hinder unnecessary future repairs. The pool system is complex, but attention to detail, specifications, and the interaction between trades can yield a safe, functional, and beautiful rooftop amenity pool.

Patrick St. Louis, LEED GA, is a project director with Thornton Tomasetti, Inc. (TT) in the Fort Lauderdale, Fla., office. Mr. St. Louis has been with TT for over eight years with a primary focus on forensic architecture. He has a bachelor’s degree from Florida Atlantic University. Mr. St. Louis is reachable via email at pstlouis@thorntontomasetti.com.

Dr. Lauren Millman, Ph.D., PE, is an associate with Thornton Tomasetti Inc. (TT) in Fort Lauderdale, Fla. Dr. Millman has been with TT for eight years and specializes in forensic structural and materials engineering. She has a bachelor’s degree from the University of Florida and master’s and doctorate degrees from the University of Miami. Dr. Millman can be reached via email at lmillman@thorntontomasetti.com.

Ken R. Maschke, PE, SE, P.Eng., LEED AP, leads the renewal and forensics practices for Thornton Tomasetti Inc. (TT) in Toronto, Ont. He has over 17 years of experience in building failure investigation, structural rehabilitation, recladding, and adaptive reuse projects. Since his college days at the University of Michigan, Ken has enjoyed sharing his deep enthusiasm for the engineering profession. He has participated on the American Society of Civil Engineers’ Committee on Pre-College Outreach, the Sustainable Energy Initiative (SEI) Sustainability Committee, and the editorial board of the journal of Leadership and Management in Engineering.

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