Waterproofing multifamily balconies: Common mistakes leading to major failures

by John Igo

All images courtesy Formulated Materials
All images courtesy Formulated Materials

Private balconies have long been a sought-after amenity in multifamily dwellings for renters of all demographics. However, apartment developers, builders, and designers often dislike balconies due to the added expense, complexity, and risk of failure from water. With careful attention to details, it is possible to design and build functional, aesthetically pleasing, and leakproof balconies.

Balconies involve many different trades, such as framers, painters, electricians, occasionally a plumber, ironworkers, concrete contractors, and waterproofers. Individuals involved with each of these trades do not necessarily understand what the other trades need to be successful on the construction of this important element.

Balconies increase the cost of every unit constructed. Every balcony on a project brings with it a host of issues, such as additional framing and doors, railings, sequencing problems, and jobsite safety considerations. Most importantly, these elements bring liability due to the risk of failure that is shared by almost every entity involved in the project from conception to delivery. Unfortunately, it is also shared by future tenants.

Six dead and seven injured in Berkeley

Six college students died when a balcony on the fourth floor of an apartment building collapsed in Berkeley, California, in June 2015. As a result of poor construction, wooden support beams that rotted due to water intrusion gave way at the worst possible moment, turning a celebration among friends into a tragedy. This singular event proves that when given the opportunity, water and time always win the battle. One only has to stand at the rim of the Grand Canyon to realize the absolute truth of this statement. It is the responsibility of professionals in the construction industry to both clients and future tenants to design and install systems in a way ensuring water does not have a fighting chance, yet still offer the amenity of a balcony to enhance apartment dwellers’ quality of life.

This author has spent the past eight years wrestling with the challenges of balcony waterproofing in multifamily construction as waterproofer, building materials supplier, and product developer. Throughout all of the research and conversations, it has become clear there is no consensus on the ‘right way’ to design and build a proper balcony. However, this author has, over the years, encountered many of the same mistakes pertaining to balcony design and installation. This has led to the conclusion balconies do not inherently require risk. Sound, proven practices can be employed to deliver leakproof balconies with the ability to stand the test of time.

Design mistakes

Figure 1 Stanchions placed so closely to the wall and each other will make proper shingling extremely difficult, if not impossible, to accomplish.
Figure 1: Stanchions placed so closely to the wall and each other will make proper shingling extremely difficult, if not impossible, to accomplish.

This article addresses many of the common misconceptions and mistakes this author has encountered over the years and provides consensus-building ideas for a very important amenity in the multifamily construction world.

Sloping trusses

One of the only reliable constants in construction is gravity. This force is known, quantified, and calculated into almost every aspect of engineering and design. For this reason, it is surprising wood-framed balconies are designed without slope built into the trusses or joists supporting the decks. Slope in the deck substrate should be the first line of defense to move water away from the building. Gravity is a friend to an architect in very few places, and balcony waterproofing is one of them.

A percentage of the deck in flat-framed balconies is always slightly pitched back to the building. The most effective way to defeat water is to limit the time it has to interact with the building. Sloping the substrate ensures water flows out of the weep system at the balcony face and spends as little time in interaction with the waterproofing system as possible. If the thinking is sloped concrete removes water from a balcony, it is important to note while water on the surface of the concrete is free, one will eventually pay dearly for any water trapped beneath the concrete. Providing proper slope in the framing reduces the risk of improper drainage.

Incorrect drain mats

The use of drain mats, or drainage composites, beneath concrete wear surfaces is growing in popularity, and in general it is a good idea. Unfortunately, most drain mats specified are engineered for below-grade drainage and designed to be high in compression resistance and provide a high flow rate when under heavy loads. Typically between 6.5 and 13 mm (¼ and ½ in.), these drain mats are too thick and this reduces the thickness of the concrete at the face of the balcony.

Balcony drain mats do not need to withstand high dead loads and are not required to provide a high flow rate. One does not need to filter 76 L (20 gal) per minute from beneath the concrete, but a path must be provided for water to exit once it arrives there. Specifiers should select appropriate low-profile drain mats (3.2 mm [1/8 in.] is suitable) that are a solid sheet of studded membrane bonded with a filter fabric to allow for moisture transmission. This low profile is the key to durability of the concrete along the perimeter of the balcony where the concrete meets the pour stop (usually an aluminum ‘T-bar’ integrating a drip edge along the perimeter).

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