Case Study: How the StormCap System is rehabilitating an ailing green roof pioneer and improving functionality for years to come.
By Rodney Chaney
The Severn Bank green roof in Annapolis Maryland (MD) was ahead of its time. Like many building owners who needed an onsite stormwater solution, this green roof was intended to be a workhorse. With historic rainfall in recent years, it was starting to burn out in sections. Stormwater Capture Co was invited to inspect the problem and offer a solution to improve its functionality and restore its beauty. Below is the full story about this traditional green roof, now transformed with the StormCap system enhanced by Urbanscape.
Green Roof Pioneer: The beginning
The 13,000 square foot Severn green roof sits atop of a four story parking garage connected to a five story office building. Its primary purpose was to address stormwater management needs near the historic downtown area of Annapolis – likely one of the first in Maryland built for the purpose of managing contributing stormwater. Tecta America a pioneer in North American green roofs was consulted and contracted to install the roofing and green roof. Tecta America has diligently maintained the roof since.
For years, the green roof had functioned as designed. This workshorse managed well over seven million gallons of stormwater and maintained a healthy plant palette that the owners in the adjacent offices would look upon with pride. The amenity space within the green roof area has provided a place to enjoy lunch or take a call alfresco.
Fast Forward A Decade Later: “Houston, we have a problem”
The Severn green roof had been designed to accept the contributing runoff from nearly half of the adjacent 20,000 square foot building roof managing the anticipated rain event. It retained both precipitation and runoff. This workhorse managed twice the amount of water that fell directly on it. It was a great setup and worked beautifully for over a decade. Then, something happened…
Dealing with Historic Rain Events
Two exceptionally wet summers appeared to have caused the green roof to die in several significant areas. This was confusing as two wet summers should not have been enough to make the roof fail. After all, it had been designed to withstand a good bit of runoff.
In the last several years the region has seen historic rain events. Many site stormwater management systems and municipal storm sewers have
What went wrong and how to fix it?
As symptoms became apparent the owner and Tecta America were quick to act. Stormwater Capture Co was asked to provide 3rd party consultation. At the initial inspection, we were surprised to discover that although many areas of the green roof were lush with excellent coverage other areas had almost completely burned out. What had gone wrong and how could we fix it? Upon closer inspection, we found several contributing factors.
Green roof with wet feet
The overflows designed to manage the onslaught of heavy flow from the contributing roof area hadn’t provided the fail-stop to keep the roof from “bogging out” for extended periods. Even with a record of 7”+ rain events there was no evidence that the excess flow was reaching the height of the collar to allow for bypass. Compounding the problem, the drainage assembly at sumps had been built at a time when drain perimeters weren’t typically installed. In addition, the biomass of over a decade had accumulated to a point where the additional mechanism of evaporation was slow to dry. While biomass is good, with these other factors this green roof was left with prolonged periods of “wet feet”.
If the green roof needed to manage only the water falling on it, this system would likely have continued to thrive. However with this pioneer in contributing drainage, it was necessary to move the stormwater throughout the system more efficiently.
StormCap improves roof functionality and restores beauty
To remedy the problem, six hundred linear feet of perforated drain tile was trenched throughout. The indirect pattern allow for improved movement of water to all areas of the green roof. Then eight expanded drain sumps were opened and framed with new SCC .063 perforated metal edge, SCC drain board, and ballast. Following this, the overflow on the “high side” of the green roof was adjusted by lowering the bypass collar height. It was factored that 99% of rain events will be treated by the green roof. So, even in heavy rain events the overflow won’t be tripped too often.
Upon our inspection, we found that only 3,000 square feet out of the total green roof footprint needed replacement. This was the center of the green roof that received the majority of contributing drainage. We decided to solve this problem using several methods.
Re-using Native Plants
After removing 3,000 square feet, we were able to reuse all of the nutrient rich biomass that had been built up over the past 12 years to build semi-intensive berms to harbor some deeper rooted natives and adaptives such as rudbeckia, asters, beards tongue, russian sage, lavender, phlox, babtisia, etc.
Not only did this reuse the existing material and help manage the excess of stormwater from the impervious deck area, it also created a great sightline from the amenity area that didn’t exist before.
The removed area was replaced with a high efficiency StormCap 1+3 profile by Stormwater Capture Co. Our proven assembly, consisting of UrbanScape’s® high-performance needled mineral hydro blanket topped by our pre-vegetated mat, provides a healthy and instantly green roof . This innovative systems now provides the retention needed and stormwater flow necessary to avoid “the bog” that was causing parts to die out in recent years.
After a few days of prep and a Saturday installation, the result amazed the owners Monday morning when they arrived to work. Their green roof was in fact GREEN again! The instant coverage of the sedum mat and the addition of the semi-intensive berms surrounding the amenity space were striking. Less noticeable but more important than the “instant” coverage was the fact that the green roof will now manage stormwater more effectively and be ready for the sunny days well as the storms ahead.
Proud Green Roof Owners
The functional upgrade, and the addition of new visual interest, gave the owners a greater appreciation of what a healthy green roof can provide environmentally and emotionally. In this scenario, we were able to solve a problem, add functionality, as well as provide a great new aesthetic to this existing “workhorse” green roof.
Your Green Roof Specialist / Design Consultant / Maintenance Support
Stormwater Capture Co offers a single source solution for a complete green roof system package such as the StormCap system. We are a one-stop-shop. Supplier of vegetated mats, high performing base layers, pavers and accessories. We customize the system to local requirements and provide warranty and support.