Restoring and waterproofing split-slab roofs without demolition or disruption

Many large urban buildings constructed during the 1950s and 1960s have a split-slab assembly serving a dual role as roof and plaza deck. Split-slab assemblies were a popular mid-century alternative to solid-slab deck assemblies in which a single concrete element provides structural support and serves as the deck traffic surface.

In contrast, a split, or “sandwich” slab features a structural supporting slab, on which a topping slab is installed. In between, a waterproofing membrane is applied directly to the structural slab. Water that passes through the top slab at cracks, joints and perimeters collects on the waterproofing membrane and can be managed by drains.

Should drainage issues arise, however, the water that collects on the membrane can overwhelm the system and eventually find its way into the building. And unfortunately, the conventional fix for such situations is extremely costly, as it requires the demolition, removal and reconstruction of the topping slab.

Fortunately, today’s building owners can take advantage of a far less expensive and disruptive solution – the Henry® Pumadeq™ Waterproofing System. Rather than having to demolish, remove, and rebuild the topping slab, the Pumadeq™ System allows contractors to coat it with a cold fluid-applied waterproofing solution – one that cures in 30 minutes to provide a seamless, monolithic surface.

The Pumadeq™ Waterproofing System was recently proposed as an alternative solution for a split slab roof assembly atop an iconic mid-century skyscraper in one of the largest cities in the US.

Like many split slab assemblies from this era, the roof featured a structural deck on the bottom, a 6-inch concrete slab on the top, and a waterproofing membrane layer in between. But because the topping slab lacked a protective coating, years of mechanical wear and weather had created cracks through which water was entering and collecting on the interior membrane. The project goal was to prevent further water intrusion and surface damage.

Given the building’s busy urban location, the conventional approach of removing and replacing the topping slab was a daunting prospect. Demolition alone would involve cutting, jackhammering and transporting the old concrete down many dozens of stories to street level, causing severe disruptions to traffic as well as the building’s many corporate and retail occupants.

The far more preferable solution would be to resurface the topping slab with a waterproofing system suitable for pedestrian traffic. But because the system would serve as the roof’s surface, it had to provide a smooth, flat finish. It also had to withstand “vapor drive,” a process that occurs when sunlight warms the topping slab and causes the water trapped inside it to rise up through the concrete as vapor and cause blistering on the surface material.

As the architectural firm handling the restoration weighed its options, the Henry Building Envelope Systems® team delivered a presentation to them on the Pumadeq™ System.

The system features the proprietary Henry GC (“Green Concrete”) Epoxy Primer, which acts as a moisture mitigator that would help resist the vapor drive. The system also offers super elongation properties that would allow it to move with the concrete and the building to withstand long-term surface wear. And unlike the competitive systems that the architects were considering, the Pumadeq System doesn’t require fleece reinforcements, so its finished surface would be completely smooth, without any overlapping fleece lines that would be prone to wear.

The Pumadeq System presentation – which included a full-scale mock-up of the system application on the building roof – convinced the architectural firm. But before it could be installed, the firm had to implement a solution that would help alleviate the vapor drive associated with water trapped on the inner membrane. The architects designed a series of one-way vents placed at the intersections of several Henry Pumadeq™ System “looped” expansion joints that would eventually allow all of the water vapor to escape.

Pumadeq™ installed and warrantied for decades to come

Once these vents were in place, the contractors installed the first coat of the Henry GC Epoxy Primer. To make sure that vapor drive wouldn’t be an issue, a third-party team performed an EFVM® (Electric Field Vector Mapping) test to detect any vapor drive breaches. Any vapor leak pinholes were addressed before contractors applied the second coat of Henry GC Epoxy Primer.

Installation of Pumadeq™ Flex 31MV flashing and Pumadeq™ Flex 30SL was next, followed by a coat of Henry Deqcoat 50 in white to reflect sunlight and thus minimize vapor drive. With the guidance of Henry technicians, the contractors were able to complete installation of the Pumadeq™ System with no issues or delays.

This Henry solution helped the building’s owners avoid the extreme cost and disruption that a removal of the topping slab would have involved, and promises to protect the building for decades to come with a Henry long-term warranty. In time, the Pumadeq™ System, assisted by the escape vents, will help rid the building’s roof of any remaining trapped vapor, further contributing to the long-term waterproofing of the structure.

Assessing the Pumadeq System for your split-slab project

If your building has a split slab roof assembly and is facing water intrusion issues, the Pumadeq System may be able to provide a cost-effective, building occupant-friendly solution. To find out if the Pumadeq System is the right fit for your project, contact Henry using the form below to set up an on-site consultation.