Stormwater treatment facilities located within building code setbacks are required to be waterproof and cannot allow leakage of collected rain water along foundations, property lines, or if on slopes greater than 10%. It is common practice when faced with limited space on a development site to utilize flow-through planter boxes to meet stormwater requirements. Planter boxes can be built as part of building foundations and if constructed to be “water tight”, can be positioned within setbacks.
Traditionally, waterproof stormwater planters have been designed with 30mil PVC liners. These liners are commonly used in landscaping ponds and are purchased in sheets or rolls. Proper installation of the liner can be a challenging task. As the photo above shows, it is very difficult to manually position and manipulate the thick plastic liner within the planter box. Devising an anchoring system that will stand the test of time and our Pacific Northwest weather is also a challenge. Some companies who specialize in liner installation may be able to use a process similar to heat shrinking that can result in a much more efficient installation, however this can be costly.
A much more effective approach is to build stormwater planter boxes via monolithic concrete pour. The term “monolithic pour” refers to a single-stage process where the walls of the box are framed and constructed/poured at the same time. The bottom slab can also be set during this procedure as well. Once the concrete has been “set”, a waterproof sealant is applied. By constructing stormwater planter boxes in this manner, developers can also build these systems as part of building foundations for a more aesthetically pleasing, and often times, more functional system. This practice is especially advisable when there is limited room on development sites and conflicts with building code setbacks.
For technical assistance on stormwater facility construction, contact the Bureau of Environmental Services at 503-823-2059.