An Architect’s Guide to Blindside Waterproofing

Photos courtesy Tremco Sealants

by John Trenta

Blindside waterproofing is common in Canada, but it is not without its challenges. Handling drainage and sealing penetrations are two critical tasks, among many, that must be addressed to ensure there are no leaks. When designing or specifying blindside conditions, it is best to not jump in blind. With numerous technologies available today, it is crucial to understand how they handle real-world applications.

Basics of blindside waterproofing

Waterproofing protects the integrity of a structure and its occupants by forming a barrier to prevent water from entering the building envelope. Blindside waterproofing is more complex than traditional below-grade waterproofing because the process is ‘reversed,’ and waterproofing is installed before the concrete/shotcrete is installed. This method is used when excavation outside of foundation walls is not feasible, such as in urban or high-density areas where a project is close to bordering structures or there is little room to manoeuvre heavy equipment.

The basic process of blindside construction involves:

  1. Excavation is shored with the lagging wall using methods such as soldier pile-and-wood lagging, soil-nailing, caisson walls, or shotcrete;
  2. Drainage mat is installed (when applicable);
  3. Waterproofing is applied to the shoring wall and detailed;
  4. Structural walls are placed against the waterproofing; and
  5. Foundation is poured.

Blindside waterproofing technology options

To eliminate potential sources of water ingress, the waterproofing solution must be continuous and have the ability to accommodate normal movements of the structure. Compatibility between the membrane and adjacent components is also important to avoid material failure. Design professionals should be aware of the features and benefits of prominent waterproofing technologies to help identify the best option for their application and to overcome job-specific challenges.

Spray-applied membranes

Spray-applied, liquid membranes provide a monolithic coating for protecting seams and sealing penetrations. The exothermic reaction during concrete cure builds chemical adhesion of the asphaltic membrane to the structure, creating a bonded waterproofing assembly, thus eliminating the potential for lateral water movement. For best results, it is recommended to spray the material onto a flat surface to allow for the correct mil coverage and consistency across the entire substrate.

Bentonite-based sheet membranes

Bentonite sheet membranes have a long history of success due to their controlled mil thickness and multiple components for waterproofing, such as an added high-density polyethylene (HDPE) material. When the bentonite is hydrated, it swells to create an impermeable barrier to water and chemicals. It can also be applied in cold weather and is tough enough to withstand shotcrete applications directly on the face of the membrane. Using this material can be labour intensive though, and requires a specialized trade. For all applications, care needs to be taken to prevent the bentonite from prematurely hydrating, so it needs to be covered with a plastic protection sheet if wet weather is imminent in exposed conditions.

Bituminous sheet membranes

Derived from the roofing industry, asphaltic sheets are a reliable blindside waterproofing option when a flexible material is needed or when applying in cold temperatures. They comprise a modified asphalt-polymer blend with reinforced fibre. Seams are heat-welded together, which can make the application more difficult.

Plastic sheet membranes

For high strength, it is advisable to consider sheet waterproofing membranes made with synthetic materials, such as high-density polyethylene, polymers, and resins. They are impact, chemical- and low-temperature resistant, but they can be more complicated due to the additional detailing involved with penetrations. Their fabric or adhesive layers can provide a mechanical or chemical bond to concrete.

Synthetic-rubber sheet membranes

Synthetic-rubber sheets include flexible and durable materials such as ethylene propylene diene monomer (EPDM). They have excellent water resistance and can withstand ultraviolet (UV) rays and high temperatures. They bond mechanically to concrete, and some product options may have the added benefits of small, self-healing penetrations and self-sealing overlaps.

Product selection factors

When deciding which waterproofing product to select, it is important to consider the following job-specific factors.

Water table height

If the jobsite has a high water table, the lower part of the waterproofing membrane will have immediate contact with water. An asphaltic spray-applied product may erode with long-term exposure, so the project will require a different chemistry in a sheet-applied membrane for the entire height of the structural walls, or a sheet-applied membrane toward the bottom and an asphaltic product on higher levels where the risk of degradation is minimal.

Hydrostatic pressure

Some areas have greater hydrostatic pressure than others. Most spray-applied membranes are unable to handle these forces. In these instances, an alternative technology should be considered. It is best to confirm with the manufacturer the selected membrane can handle the anticipated hydrostatic pressure.

Construction method

Whether the project is utilizing a cast-in-place or shotcrete method will dictate which technology can be used in blindside waterproofing. If insulated concrete forms (ICFs) are used for the foundation and above-grade walls, a peel-and-stick membrane is recommended to avoid penetrating the insulation with a mechanically fastened product.

Soil characteristics

EPDM should be avoided when soil has high-methane content. Instead, it is recommended to use a more robust solution such as an asphaltic-based, bentonite, or HDPE product for greater density to prevent the methane gas from entering the building.

The presence of hydrocarbons in the soil can also impact product choice. If there is high hydrocarbon content, EPDM and some spray-applied products must be avoided. The manufacturer may recommend a different technology or a greater mil thickness of the spray-applied product.

Saltwater in the soil, present predominantly near coastal regions, can impact bentonite membranes as it will not expand as anticipated with fresh water. Talk with the manufacturer about alternate saltwater-specific products for this condition.

Temperature restrictions

If the jobsite faces extreme high or low temperatures, check for application limitations of the below-grade waterproofing products. Sheet-applied membranes are often recommended as they offer design flexibility and do not have temperature restrictions.

Protrusions through the waterproofing membrane and seams must be carefully detailed.

Code requirements and regulations

Across Canada, there are varying code requirements. Some regions will necessitate a protection course over the waterproofing membrane for redundancy. It is important to review the codes with the design/build team to ensure compliance.

In some provinces, including Ontario, cities are taxing owners of buildings using drainage or weeping systems to pump out water into city sewers. The architect should have an alternative solution in mind should the owner demand it, understanding the completely watertight system may add hydrostatic pressure since the water has nowhere else to go. This different approach will be more robust and likely more expensive upfront, but will save the owner continuing tax costs.

In many urban areas, flooding is a major concern, so bioretention requirements are emerging to control and limit water runoff. Here, commercial projects may need to redirect drainage or install underground water retention tanks to collect water and control the exit flow into the city sewage.

Ease of installation

Every technology has its pros and cons and their ease-of-installation can be a large contributing factor. Bentonite sheets, for example, are heavy and require more manpower to move the rolls, while EPDM and other rubber sheet membranes are less dense and, therefore, lighter to work with. Spray-applied products need different equipment, but tend to be easier to install to fill voids and complex geometries.

General weather

The construction team should review upcoming weather conditions prior to installing a waterproofing membrane. After a period of rain or snow, some waterproofing materials can be applied to damp concrete, which can speed up construction. Others may need to wait until the substrate is completely dry, resulting in project delays. During high winds, bentonite sheet membranes can be difficult to work with and will prematurely expand with exposure to any precipitation, if unprotected. If bentonite is hydrated before the concrete is poured, the sheets will need to be removed and replaced.


Before selecting a technology or material, it is advisable to check adherence to ASTM testing standards to ensure it fits project requirements. Some important performance characteristics related to physical properties include:

• hydrostatic head resistance, per ASTM D5385, Standard Test Method for Hydrostatic Pressure Resistance of Waterproofing Membranes;

• low temperature flexibility, per ASTM D1970, Standard Specification for Self-adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection;

• peel strength of adhesion bond, according to ASTM D903, Standard Test Method for Peel or Stripping Strength of Adhesive Bonds;

• elongation/strength at break, per ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers—Tension;

• puncture resistance, per ASTM D4833, Standard Test Method for Index Puncture Resistance of Geomembranes and Related Products;

• permeance, according to ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials; and

• resistance to micro-organisms, per ASTM
D4068, Standard Specification for Chlorinated Polyethylene (CPE) Sheeting for Concealed Water-containment Membrane.

An installer mechanically fastens a sheet waterproofing membrane.

Blindside waterproofing challenges

Due to the complexities of blindside waterproofing, many design/build professionals recommend a combination of technologies. During the construction of a new university building outside of Toronto, a three-layered, below-grade waterproofing system was installed in a blindside application for enhanced redundancy.1 After the wood lagging wall was installed and approved, drainage board was attached to the wall with drains at the footing as well to direct the flow of water through pipes into the sump pit. From there, a layer of HDPE laminated to sodium bentonite was mechanically fastened. Lastly, an asphaltic membrane was spray-applied to create continuous protection of all the penetrations and seams and give the bentonite added weatherability. This combination offered the best of all technologies in terms of reinforcing durability, flexibility, and complete coverage. In the end, there were no reported leaks or callbacks.

Often, material performance alone is not enough to protect the building envelope. The installations themselves pose challenges that can negate the effectiveness of the waterproofing as a barrier to moisture intrusion. Penetrations of the waterproofing membrane by other trades are a common source of failure. The problem can be remedied if caught before the concrete is poured. If not, repair methods can be deployed to minimize water ingress, but they are not always a comprehensive fix. The injection of epoxy or polyurethane materials at various locations can try to stop the flow of water but lateral movement may still occur, making it difficult to find the source of the problem. If that does not work, cementitious or capillary treatments have shown some success, or the more expensive and destructive process of excavating may be necessary.

Pre-construction meetings

The purpose of the pre-construction meeting is to establish clear communication and collaboration between all parties involved, including design professionals, trades, and manufacturers. It ensures everyone understands their responsibilities and timelines to mitigate any ambiguities throughout the installations.

In the pre-construction meeting, the group should complete the following:

• review all project drawings and system specifications to determine if the proposed details can be constructed as intended by the design professional;

• confirm product selection and compatibility with each trade;

• examine construction details including tie-in areas;

• identify the sequencing of all trades with a comprehensive construction schedule; and

• discuss any other project-specific considerations.

In blindside waterproofing, sequencing is imperative to identifying repair protocol should a subsequent trade intentionally or inadvertently penetrate the waterproofing membrane. Having this conversation helps prevent conflict down the line and increases the chances of damage being caught and addressed before concrete is poured, thus ensuring the long-term integrity of the waterproofing installation for years to come.

Drainage boards are applied over a wood lagging wall.

Surface preparation

The surface to be covered must be smooth, firm, and free from mud, loose mortar, wires, fins, metal projections, or other substances preventing placement of membrane. With a wood lagging wall, no gaps greater than 25 mm (1 in.) are acceptable. Any protrusions or changes in plane greater than 6 mm (0.25 in.) should be made even with mastic, sealant, or plywood.

Pay attention to vulnerable areas

The lack of proper detailing and installation of proper gaskets or waterstops in the structure are the most common causes of ground water ingress. Special attention should be made to the following areas below-grade as they are particularly prone to moisture infiltration:

• seams, penetrations, and tiebacks;

• cold joints;

• expansion joints;

• foundation drains;

• structural connections;

• damaged membranes; and

• transitions and change of condition or plane.

Always check the blindside

For water management at below-grade conditions, the blindside process is a challenging but necessary and an increasingly common method. When designing waterproofing and drainage at this location, it is crucial the selected systems be effective for the unique jobsite circumstances, such as water table height, hydrostatic pressure, construction method, soil characteristics, and climate.

By understanding the intersectional factors tying into product application, construction professionals will be better armed to communicate their needs to the rest of the design/build team and ensure a quality and durable installation. Addressing the areas susceptible to moisture intrusion ahead of time can prevent callbacks, as it can be difficult and expensive to remediate below-grade leaks after construction.

John Trenta is application manager in the product management department at Tremco Commercial Sealants & Waterproofing. He has managed fluid and sheet-applied below-grade waterproofing product technologies, assemblies, and accessories, and also addresses the needs of construction professionals with the management of the Tremco Sealants and Adhesives product portfolio. He supports Tremco’s high-performance building envelope strategy with a focus on innovative product solutions and their connectivity across multiple building enclosure systems.

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