Balcony Collapse – Waterproofing Defects

The balcony collapse at an apartment building in Berkeley, California was a reminder that waterproofing defects can possibly lead to life threatening situations. In a memorandum dated June 23, 2015, the Building & Safety Division of the City of Berkeley’s Planning & Development Department concluded, “…the deck joist ends protruding from the exterior wall at Unit 405 appear to be severely dry rotted.” Unfortunately, the memorandum did not state the cause of the “dry rotted” deck joists, but this article provides an overview of typical waterproofing issues that can lead to deteriorated wood framed residential balconies.

Residential wood framed balconies primarily have two different types of surfaces: concrete topping or wood/composite decking. Wood decking or composite decking (also sometimes referred to by a brand name, Trex®) is attached directly to wood joists and allows water to pass through the edges of the decking and to the space below the balcony.

A concrete topping is the surface layer of a concrete sandwich membrane construction which is typically composed of the following: concrete topping, drainage mat, waterproof membrane, wood sheathing, and wood joists. In a concrete sandwich membrane system, the concrete topping is sloped to drain to the edge of the balcony or to a drain on the surface of the concrete topping which is connected to drain pipe.  This type of construction also has to allow for drainage at the waterproof membrane as the concrete topping is not a waterproof surface.  Water that does penetrate the concrete topping is directed to the drainage mat and the waterproof membrane; the water is then directed to the edge of the balcony where it can weep at a “T-bar” edge detail or to a “two-stage” internal drain.  In summary, water needs to drain at two levels: the surface of the concrete topping and at the waterproof membrane.

So, what are the issues that contribute to deteriorated wood framed residential balconies? Deterioration of structural components is typically caused by water intrusion into the balcony assembly.  Primarily, water intrusion occurs due to defectively designed and/or constructed details.  The following is a list of some of the issues that this writer has observed relative to balcony water intrusion and deterioration:

1.  Lack of ledger flashing – The ledger is the horizontal board that is attached to the wall and provides the surface to attach the ends of the wood joists. Typically, metal “L” flashing is provided over the top of joists, as illustrated in Figure 1. Without flashing at the top of the ledger, water may infiltrate behind the ledger and cause deterioration to the wood framing members.

Figure 1:  Wood framed balcony with composite decking (removed) at a single family residence that was undergoing repair.  The arrow points to the ledger flashing over the top edge of the ledger board.  The insert illustrates the ledger flashing on the opposite wall; note that this piece of flashing does not properly cover the top of the ledger board.

Figure 1: Wood framed balcony with composite decking (removed) at a single family residence that was undergoing repair. The arrow points to the ledger flashing over the top edge of the ledger board. The insert illustrates the ledger flashing on the opposite wall; note that this piece of flashing does not properly cover the top of the ledger board.

2.  Lack of weeps at the waterproof membrane – In a concrete sandwich membrane balcony, water that reaches the waterproof membrane needs a way to drain from the balcony assembly; this is accomplished by weeps at the perimeter of the balcony or an internal drain. Without a weeping mechanism, the water may migrate through weak points in the waterproof membrane such as laps and corners. Additionally, the water can contribute to concrete topping deterioration, as illustrated in Figure 2.

Figure 2 - Deteriorated Concrete at the Edge of the Balcony

Figure 2: Deteriorated concrete at the edge of the balcony. The arrow points to the T-bar which was not provided with any type of weep mechanism.

3.  Lack of counter flashing / reverse laps – Typically, waterproof membranes in concrete sandwich membranes turn up the exterior wall and terminate at a certain point. When a waterproof membrane turns up the exterior wall, some form of counter-flashing should be provided. Counter-flashing is typically provided by the weather resistive barrier beneath the exterior wall veneer lapping over the top edge of the waterproof membrane. This allows any water to be directed onto the exterior of the turned up waterproof membrane and not behind it; this is known as a positive lap (opposite of a reverse lap). Figure 3 illustrates a balcony where the top edge of the waterproof membrane is exposed; this is an example of a reverse lap.   This condition allowed water intrusion below the waterproof membrane and into the wood structure of the balcony assembly.

Figure 3 - Top Edge of Waterproof MembraneFigure 3: Top edge of the waterproof membrane (red arrow) where it turns up the exterior wall. Note the open gap (yellow arrow) that allows moisture to get behind the waterproof membrane and into the wall and balcony assembly.

4.  Lack of end dams / defective flashing details – Most often, water intrusion occurs at points of transition or termination of the waterproof membrane. Transitions at corners can be particularly problematic. Figure 4 illustrates a condition where the T-bar at the exterior edge of the balcony terminates at the exterior wall. This termination can be particularly challenging because the waterproof membrane must be properly lapped and adhered to three surfaces: the horizontal surface, the wall parallel to the T-bar, and the wall perpendicular to the T-bar. Additionally, the T-bar is outboard of the exterior wall; therefore, an end dam needs to be properly interfaced at the corner.

Figure 4 - T Bar at the edge of the Balcony

Figure 4: This photograph illustrates a T-bar at the edge of balcony that is outboard of the exterior wall. Note the gap (dashed box) between the T-bar at the exterior wall. The lack of proper flashing and an end dam at this location contributed to the deteriorated wood framing below the T-bar. The wood framing is white because the framing had been previously painted with a mold killing primer during a previous “repair” (as you can see, the previous “repair” did not address the improper flashing at this location).

More information about the Author: Jeremy A. Kozik, AIA, NCARB, LEED GA

Mr. Kozik is a licensed architect with experience primarily focused on the design, construction, and evaluation of light commercial, multifamily and single family residential projects.

Jeremy Kozik Website

 

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