Medical Office Building, Webster (Fall 2007)
- Design Team: Jacob White Construction Co., Houston, Texas, Joe Webb, Architect
- System Location: 251 East Medical Center, Webster, Texas
- Capacity: 175,000 gallons in 4' by 8' underground box culverts
- Catchment Area: 14,500 square foot roof area plus parking area
- Water Use: Irrigation and toilet flushing
The heart of the rainwater collection system is the 700 feet of 4' by 8' concrete box culverts that are concealed beneath the parking lot and are used for both storm water retention and irrigation. The culverts can collect excess runoff from the roof at a rate of 2,400 gallons per inch of rain, as well as storm water runoff from the parking lot at a rate of about 45,000 gallons, per inch of rain. They can hold about 175,000 gallons of water.
Another unique component of the collection system is the green roof on top of the building. The living roof includes 4 inches of spray foam insulation board placed on top of the concrete roof. The insulation board is covered by a 40 mil reinforced waterproofing membrane that mantles the roof, sidewalls, and drains. A water retention product consisting of a post-industrial, recycled polypropylene drainage core, fused and molded into a waffle pattern covers the reinforced waterproofing membrane. This water retention fabric is designed to hold 10 to 12 times its weight in water. Finally, a nine-inch-thick layer of special lightweight soil substitute with lava rock is placed on the retention fabric, and grassy, shallow-rooted plants planted in the soil.
The living roof can absorb and reuse up to 2 inches of rainfall. Seventy percent of all rain that falls on the living roof is retained to sustain the vegetation growth. The remainder (about 24,000 gallons a month) drains to the culvert collection system under the parking lots. The site retains and reuses 90 percent of its water.
The cisterns under the parking lot hold a three-month supply of water. They supply the facility's entire irrigation and gray water needs, reducing city water use (and the water bill) by 70 percent. There is no connection to city services for irrigation - filtered water from the cisterns is used. Water from the cisterns is also filtered, treated and dyed blue for use in toilets.
The building was completed in February 2007, and is now 75 percent occupied. Water and electrical savings are on target. The building is LEED Gold Registered.
The cistern system cost $225,000 but there was no need to build a retention pond which saved the owner $350,000. Net savings over 20 years are estimated to be about $349,000 including $125,000 on initial costs and $224,000 on recurring costs for city water, and taxes and maintenance for the pond.
Photographs courtesy of Jacob White Construction Co.