Having looked at a simple balcony of sheet flow and traffic topping in Part 1, let’s look at a balcony which uses a center drain.
The immediate difference is that the geometry of the balcony is constrained by several factors. Most traffic topping manufacturers recommend a minimum slope of 1% to achieve positive drainage on the surface. However, when the balcony is required to be accessible, the maximum slope allowed is 2%. Therefore, the proportions of the balcony will not want to exceed 2:1, otherwise th longer slope will fall below 1% and risk having too little slope. This is particularly perilous in wood construction since wood will shrink and sag over time, and any slope less than 1% risks actually becoming negative resulting in ponding. No matter how well made the topping material, long enough exposure to ponding will result in failure of the system.
The use of a center drain necessitates a change in framing. In order to make four panels slope to the center, the most common method is to create a lower, flat structural deck with rip strips cut to the slope, topped by a secondary level of plywood.
Traffic topping is different than most waterproofing membranes. In most waterproofing systems, the primary theory is to have all membranes shingle lap over each other to shed water. However, according to Bill England, a representative at Westcoat, a leading traffic topping manufacturer, the topping compounds bond to the edge flashing and create a watertight seal which can function even under a negative lap.
Their suggested edge detail is as shown here.
When this is applied to a center drain balcony system with rip strips and a secondary plywood deck, the detail looks like this:
Now lets look at the drain itself. Traffic topping systems are normally very thin, about 1/4″ total, which requires the use of a special drain. Drains intended for use in topping slab systems have a thin sheet metal flange designed to receive the topping system. Unlike conventional two stage drains, the waterproofing system does not turn down into the drain body itself; the topping compound adheres to the sheet metal flange and terminates at the grate.
Several plumbing manufacturers offer drains for topping system applications- Watts is shown for example, but another is Thunderbird which has a larger flange allowing more bonding area for the topping material.
The connection of the topping system to wall, threshold, and the re-entrant corners are as described previously in Part 1.
Both styles of traffic topping systems are an economical and efficient way to waterproof a balcony or deck. However they have maintenance requirements since the traffic coatings are the wearing surface and require periodic re-application of the topcoat. It is important to inform the owner about the maintenance requirements of the coating system provided by the manufacture. This will help to extend the service life of the waterproof coating system.
For balconies and decks which are subject to heavy use such as plazas and common areas, or for projects where the owner doesn’t want the periodic maintenance costs associated with traffic topping, the architect should suggest the use of a more rigorous system where the waterproofing is protected by a concrete topping slab. This will be discussed in the next part if this series.