Plaza Deck Restoration: Principles of drainage and waterproofing

Multi-layer membrane installation can be designed to provide redundancy performance. This membrane combination consists of a loose-laid welded thermoplastic membrane system installed directly over a reinforced hot-applied rubberized asphalt membrane fully adhered to the deck.
Multi-layer membrane installation can be designed to provide redundancy performance. This membrane combination consists of a loose-laid welded thermoplastic membrane system installed directly over a reinforced hot-applied rubberized asphalt membrane fully adhered to the deck.

Installation challenges
One of the most common challenges in restoring the waterproofing on a plaza deck is the height limitation of existing flashings and penetrations. Typically, drainage for adjacent curtain walls, masonry wall weep holes, doors, and other penetrations are located at or within a few inches of the existing structural deck surface. This will make it difficult to modify the plaza assembly to provide subsurface drainage or tapered topping slabs. Facing such a dilemma, the designer may have to ‘compromise’ and reduce the slope of a tapered topping slab to maintain existing flashing heights or door levels.

On many retrofit projects, the challenge is not simply the flashing heights, but actually access to the existing conditions. Many plaza decks connect to adjacent curtain wall or cladding systems with precast panels or stone. Typically, these building components were put in place after the original waterproofing system’s installation. Therefore, in many instances, these existing building components will have to be removed to accommodate the installation and proper detailing of the new waterproofing system. One should not interrupt the waterproofing system to accommodate architectural elements. Instead, one should specify and install the waterproofing to be a continuous course under and behind these elements to provide proper restoration of the flashings and terminations.

In many cases, the existing concrete structural slab will require repairs. Depending on the conditions, these can take from a few days to several weeks to implement surface preparation using shot-blasting or similar methods.

When mechanical means are used to perform surface preparation, care should be exercised to avoid over-roughening the substrate, as irregular texture in the substrate can result in undesirable membrane thickness variations. After surface preparation, it may take many more days for the deck to properly cure before the new membrane can be installed. If the new waterproofing membrane is fully adhered, then moisture release issues from the newly repaired areas should also be considered. In some instances, the entire surface will have to be ground or scarified to remove the existing system. This is typically a time-consuming and expensive process that does not allow removal and installation of new membrane in the same day.

Moisture in concrete decks can result in pinholes through a fluid-applied membrane. Therefore, before membrane application, one should verify the deck is dry.
Moisture in concrete decks can result in pinholes through a fluid-applied membrane. Therefore, before membrane application, one should verify the deck is dry.

Waterproofing the planters on the plaza should be a separate specification and work item. One should specify an engineered expansion joint material to provide a watertight seal at all expansion joints. Reliance on waterproofing membranes to provide a seal at the joints is risky and should be avoided. Waterproofing membrane can be installed to the expansion joint sealant’s exterior side to provide membrane continuity and possibly enhance the seal. It is good practice to elevate the expansion joint cross-section above the primary deck plane by means of concrete curbing and to design deck slope that drains the water without it passing over the expansion joint.

In addition to these issues, contractors and designers must consider logistical problems associated with rehabilitating plaza decks at occupied buildings. Every project has its own set of logistical and access issues ranging from surface preparation to temporary weather protection to environmental issues.

Quality assurance
A pre-installation meeting for the waterproofing work should be scheduled before the work proceeds. The architect, owner (or representative), inspector, manufacturer’s representatives, and general contractor should attend this meeting, along with subcontractors for waterproofing, concrete, backfill, formwork, and excavating.

The owner should also be required to retain an independent inspector for full-time monitoring with daily submission of reports on work completed with the location noted on drawings, photographs, and weather information.

Further, one should insist the waterproofing subcontractor be an approved applicator of the waterproofing system manufacturer to comply with warranty guidelines. One should have the contractor submit applicator certification at the time of bid. This timely certification requirement is to deter the general contractor from accepting a low bid from a non-approved installer that may not have any experience installing the specified system.

A best practice is to have all mechanical penetrations and drains installed in final set placement before the waterproofing application. However, if this sequencing cannot be achieved, the mechanical and electrical subcontractors are required to notify the waterproofing contractor, in writing, where the waterproofing has been breached or damaged during their work so the system can be repaired before backfilling or topping material placement.

Overlap seams of this PVC membrane are fused together with conventional hot-air welding to form a seamless water barrier. Insulation and the concrete wearing course were then installed over top.
Overlap seams of this PVC membrane are fused together with conventional hot-air welding to form a seamless water barrier. Insulation and the concrete wearing course were then installed over top.

On completing the plaza deck waterproofing, the membrane system should be tested for leaks before the topping wearing course being installed. This can be achieved by means of electronic low-voltage mapping (ELVM). The former uses water as an electrically conductive medium that can quickly and accurately detect the point of water ingress. Unlike conventional water flood testing, ELVM can indicate breeches in the waterproofing system that are not actual active leaks, but instead breaches that could develop into active leaks during the structure’s life. A limitation of ELVM is that not all waterproofing materials are compatible with the test method due to electrical resistance properties, so one must consider the suitability of ELVM with the specified waterproofing materials.

Specifications should clearly communicate requirements under which all individual materials or systems are properly transitioned into adjacent construction or building envelope systems. Further, quality specifications should include termination requirements for each individual system or component, especially at building envelope material transitions. Project specifications must detail these requirements rather than leave the design to the discretion of contractor shop drawing submittals that may address terminations at material transitions by noting ‘work by others.’ However, proper waterproofing selection and a comprehensive specification with details provided by the specifier is an important part of ensuring waterproofing success.

Conclusion
The designer is faced with many challenges when rehabilitating an existing plaza deck. Premature deterioration and subsequent restoration of many plaza decks can be avoided with proper drainage and a waterproofing system. One should design the plaza with a proper drainage slope at both the wearing course and subsurface at the waterproofing membrane level, and provide enough drains with hardware not prone to clogging.

The design team should be wary of value engineering (VE) the waterproofing system––simply put, the amount saved by using an inferior waterproofing system can quickly evaporate and turn into enormous expenses when repairs are required. Yet, it is prudent the design team research, select, and specify a waterproofing system that meets the owner’s performance and cost expectations suitable with the site conditions and project construction.

Stacy Byrd, LEED AP, is the national products manager at CETCO and has 20 years of experience in waterproofing application design and fieldwork. He is a member of ASTM International Committee D 08 on Roofing and Waterproofing. Byrd can be reached via e-mail at stacy.byrd@cetco.com.

Control the content you see on ConstructionCanada.net! Learn More.
Leave a Comment

Comments

Your email address will not be published.