Balconies are one of the most problematic ,elements of construction, and one of the leading causes of defect litigation and complaints.
In this post I will discuss and illustrate best practices for the construction of balconies, with focus on the most common types used in multifamily wood construction.
Traditional balcony design had the outside surface lower than the inside surface so as to prevent water intrusion.
However in most multifamily units, the balconies are required to be accessible by the International Building Code. This means that the inside floor surface and outside balcony surface must be at the same height, which makes the design more difficult.
For the purpose of this article, I will focus on the accessible design.
They also have two primary forms of waterproofing systems- Traffic topping (where the water barrier is also the wearing surface) and concrete topping, (where the water barrier is covered by a layer of concrete to protect it.
Traffic topping is more economical, but requires periodic maintenance and restoration.
There are two primary methods to shed water on balconies- One is to slope the surface and let the water sheet flow off the edge, and the other is to have an internal drain.
So with these parameters, I will present four basic designs:
Type A, traffic topping and sheet flow design;
Type B, traffic topping with an internal drain;
Type C, concrete topping with sheet flow design;
Type D, concrete topping with an internal drain;
For the purposes of simplicity, I will disregard railings and their attachments; These will be handled in a future, separate article.
TYPE A BALCONY
The design of the balcony begins with the framing. The illustration below shows a typical projecting balcony framing.
The framing configuration shown is Type III fire rated construction, where the interior floor framing is parallel to the exterior wall; The balcony is supported by outrigger beams which then support an edge beam, with the balcony joists spanning in between.
One important thing to notice- The most common form of multifamily floor construction uses a layer of cementitious floor topping, between ¾” and 1 ½’ thick;
Since the level of the balcony and inside floor must be the same, the framing of the balcony must be higher than the floor joists as shown in the illustration above.
We will return to study the threshold in more detail later in this article.
For the purpose of this article I have assumed the use of a typical traffic topping systems such as Tremco 350/351 or Pli-deck.
Most traffic topping manufacturers recommend terminating the topping at a metal edge flashing as shown in the manufacturers detail at right.
Although the manufacturer’s assembly is comprised of several layers of different materials and coatings, for the sake of simplicity I am treating their system as a single entity.
The chemical formulation of most traffic toppings is designed to create a tight bond to the metal flashing.
So when the manufacturer’s recommended flashing is applied to our design it will look like the image below.
Note the soffit drip edge; For this example I’ve assumed a Fry Reglet extrusion, but other shapes are possible. What is imperative is that there be some form of drip edge, to prevent water from pooling and working back along the soffit underside due to surface tension. I’ve also assumed the use of a metal casing bead but a wood stucco stop is also used.
So now lets look at the way the balcony interacts with the wall:
Again, if we start with the manufacturer’s recommended detail, we see that they recommend a 2″ gap between the deck and the plaster weep screed, and an ell flashing with a 6″ vertical leg.
Now let’s look a bit closer at the threshold for the patio door.
In this case the manufacturer is showing a step in height between the inside and outside. With most modern multifamily projects, the threshold must meet the accessibility requirements of the International Building Code which limits the TOTAL change in height to 1/2″ including the threshold itself.
For this article, I m assuming a threshold such as Pemko 2005.
The sill pan is one of the most critical and most often overlooked element of the waterproofing system. First, it must have end dams; If not used, the sill pan is useless since water just runs horizontally before reaching the end of the pan and entering the building.
Second, note the back turn up lip. This is recommended to be 1/2″, but in many cases will be limited to the space allowable under the accessible threshold.
CRITICAL NOTE!
Although most waterproofing designs assume that water always flows downhill, this is not the case. A differential in air pressure between inside and outside such as occurs during storms and high winds can cause water to be driven back under the threshold and upwards, sometimes as high as an inch or more.
Since this will obviously overtop a 1/2″ upturn, the design of a balcony which uses both traffic topping and an accessible threshold is not recommended for situations where this may occur, such as in high rise buildings or areas subject to frequent storms.
Deck to wall transition:
Note that most manufacturers will recommend turning the entire system up the wall, usually by 6″.
The last area to discuss may be the most important of all.
This is the re-entrant corner, where the side of the balcony joins the wall of the building.
What makes it so important is that it is rarely discussed. Traffic topping manufacturers offer guidance for their systems, but not outside of it. Conventional construction details focus on vertical cuts through walls and decks, but not the horizontal or three dimensional areas such as this. Worse, the subcontractor responsible for the traffic topping system is often different than the subcontractor installing the overall weather resistive barrier.
So the re-entrant corner falls into a scope gap, without any guidance to the trades people in the field.
Here is a view of the various components of the flashing at a re-entrant corner. A folded piece of Self Adhered flashing (SAF) is used, applied shingle style over the lower course of WRB. Then the ell flashing, stucco stop, edge flashing and traffic topping system is applied.
SUMMARY:
The Type A balcony, with traffic topping and simple sheet flow drainage is simple and economical. However it is limited, and there are many requirements which may preclude its use.
One such is that it is often not allowed or desired to sheet flow water off to the area below; in these cases, an internal drain with traffic topping may be used which is the focus of the next post.