You will recall from Part 1 that Oaklyn Library has more than a green roof, it is eased into a wet hillside and it supports a mesic or water-loving meadow on its roof that blends seamlessly into the natural meadow on the seven-acre site. Technically, it is an earth-sheltered building. It has an 18-inch-deep overburden containing a lightweight planting mix, ponding elements, a subterranean irrigation system, and an additional horizontal network of horizontal drainage pipes. Even with all this exotica, the project came in below budget and below average cost for public libraries, due to the simple construction of the three buried walls.
This sort of economic optimization is the key to affordable sustainable design. Green roofs are also not typically this deep or this "intensive," the most common being an "extensive" green roof with about 4 inches of lightweight planting mix planted with low-maintenance species that can survive just about anything. Either type of system, if integrated early in design, can show an excellent return on investment and they allow for full utilization of the real estate. Design schools are now calling the roofscape the "fifth facade" and it is a terrible space to waste. We can ill afford to let this valuable resource remain an inaccessible, uninhabitable black tar desert that heats up the neighborhood and adds to storm water runoff problems. The technology has arrived to make these systems good, conservative long-term investments.
Overburden is a heavy word that weighs on one's mind, especially when it is your first green roof project and your seal will be found on the drawings when they drag them out of storage in anger. If the membrane leaks, everything above the leak, the overburden, has to come off to make the repair. This leads to several green roof design strategies I will outline in this post to make sure you don't have to do that prematurely and when you eventually do, it is a small portion, not the entire roof. Green roofs are typically designed to last at least twice as long as conventional premium roof systems. Part of the longevity of a green roof is due to the fact that an earth-sheltered membrane is not subjected to ultraviolet degradation or air pollution oxidation or extremes of temperature, expansion and contraction, foot traffic or other abuses that lessen the life of exposed membranes.
Modern earth-sheltered building design involves building something like a watertight upside-down inside-out swimming pool with the liner on the outside. In this case, after much research, we decided on an ultra heavy duty, fiber reinforced, heat-welded, 80-mil PVC waterproofing membrane that had been proven in Germany for thirty years in applications that included underwater tunnels.
We also added a second protection layer of similar material to protect the primary waterproofing membrane and separated the two with 1/4" thick felt. This second layer was also heat welded.
This system is designed to last for at least forty years, but Charlie Miller called it a "particularly robust system that will probably last for 50 years or more." Stay tuned. I hope to be retired by then and it is likely they will have added a second level to this building before the roof needs to be replaced.
In spite of that long life prediction, it is also a good idea to partition the roof membrane so an eventual leak can be easily located and repaired without digging up large areas. This is done with containment strips that divide the roof into distinct separate compartments and Oaklyn's roof has several. Another strategy is to place a leak detection system below the membrane. We also flood tested each section of the roof with 18 inches of water for 24 hours prior to covering (which is also a great test of the structural system).
Early in the design process it occured to me that the vast horizontal field of the membrane where welds were routinely machine welded was not likely to be an area of future concern. It was the edges and penetrations I would have to keep an eye on. If somebody screwed up a flashing detail on the parapet, the wall would get wet and the floor would get wet and somebody might assume the roof is leaking and start digging. So the roof system membranes were carried up over the parapets, penetrations were minimized and every typical detail was examined during design and construction to keep the contents dry for a very long time.
Ponding elements were formed in by encasing plastic pipe in protection membrane and a water retention fabric was laid over that. Then came a special lightweight water-retentive planting mix that was blown in place. Horizontal drainage pipe and drip irrigation systems were incorporated before the final depth of planting medium was reached. Sixteen varieties of native mesic meadow plants were planted by hand as plugs. We later found that the meadow grew better on the roof than on the surrounding earth. It requires mowing only twice a year with some additional maintenance to remove unwanted plants, such as tree seedlings, that might cause issues down the road. Like a natural meadow, the roof changes colors with the seasons and goes dormant during the winter. As the plants grow, they transpire water and cool the roof. Their photosynthesis absorbs carbon dioxide and pollutants and provides enough oxygen to keep 1100 neighbors alive. As a result of the earth-shelter construction and a high-performance HVAC system, this building uses about a third less energy compared to a typical library building its size.
To avoid the cave-like feel you can get in an earth sheltered building, this one opens up to the valley with 11-foot-tall windows across the facade and a "LightBridge" clerestory element that celebrates the entry and floods the main circulation area with daylight.
Because an earth-sheltered building has no back door, all utilities and uglies, like the chiller, are contained in a stealth service court hidden behind a roll-up screen at the far right of the facade.
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