{"id":2035,"date":"2014-02-11T08:30:38","date_gmt":"2014-02-11T13:30:38","guid":{"rendered":"http:\/\/www.constructionspecifier.com\/?p=2035"},"modified":"2017-01-17T15:33:48","modified_gmt":"2017-01-17T20:33:48","slug":"vegetative-roof-waterproofing-for-long-term-performance","status":"publish","type":"post","link":"https:\/\/www.constructionspecifier.com\/vegetative-roof-waterproofing-for-long-term-performance\/","title":{"rendered":"Vegetative roof waterproofing for long-term performance"},"content":{"rendered":"
\"Photos<\/a>
Photos courtesy Baystate Medical Center<\/em><\/figcaption><\/figure>\n

by Brian Kelly
\n<\/b>Every major storm reminds architects, urban planners, and specifiers of the need for more vegetative roofing\u2014since adequately waterproofed green roofs can fulfill their fundamental purpose of buffering stormwater runoff and improving the environment.<\/p>\n

<\/a>Green Roofs for Healthy Cities (GRHC) reported a 24 percent increase in installed vegetative roofs in its latest annual industry survey. Last October, the U.S. Environmental Protection Agency (EPA) announced a strategic agenda that continues to support community involvement and adds a new focus to make green infrastructure \u201cbusiness as usual\u201d across government.1<\/sup><\/a><\/p>\n

This article reviews the general advantages and challenges of vegetated roofs, including how and where they are most likely to fail. It also reviews strategies for overcoming those challenges and provides advice for achieving long-term performance.<\/p>\n

Advantages to cities and urban planners<\/b>
\nVegetated roofing assemblies provide the following advantages to city development and planning.<\/p>\n

Stormwater control<\/i>
\n<\/a>Minimizing flows into storm sewers and protecting local water resources is a prime consideration of governing bodies and a motivating factor in developing incentives and environmental policies. While no amount of green roofing can protect urban centers from severe storms or flooding that overwhelm sewers and waste treatment plants, every vegetated assembly can absorb and retain some rainwater before reaching a saturation point. It has been estimated a green roof can cut a building\u2019s peak stormwater runoff by as much as 75 percent while improving the quality of water flowing into waterways due to the natural filtering effect of planted soils. 2<\/sup><\/a><\/p>\n

<\/a>There are many variables to soil drainage rates, but the saturation point is essentially a measure of the voids in a total volume of soil. Often used to calculate soil porosity, Darcy\u2019s Law expresses a simple proportional relationship between the instantaneous discharge rate through a porous medium, the viscosity of the fluid, and the pressure drop over a given distance. Porosity is not a constant, however, due to soil compaction and existing moisture levels. The rate of infiltration through a soil bed can also be empirically measured with an infiltrometer.3<\/sup><\/a><\/p>\n

\"A<\/a>
A non-disruptive, odor-free, cold, fluid-applied reinforced resin membrane was specified for this 2229-m2 (24,000-sf) healing garden at the Baystate Medical Center in Springfield, Massachusetts.<\/span><\/figcaption><\/figure>\n

Free moisture at the surface will evaporate, with the rate dependent on ambient temperature and humidity. However, the bulk of rainwater will seep into the soil and be available for absorption and transpiration by plants. When soil is saturated, excess water must be efficiently channeled to drains. Green roofs then act like giant sponges with the absorptive capacity directly related to overall volume of the planted area, the depth of overburden, and the density and type of plantings. Deciduous trees can generally retain more water during the growing season than conifers, but evergreens are active year round. Leafier trees generally hold more than pines, but there are many evergreens, such as hollies, that also bear leaves.<\/p>\n

Drainage rates are also related to the types of growing media, and engineered soils or loamier growing media are generally preferred over sandier ones. Drainage mats and retention trays are typically used to further regulate the process, and channel drains must be kept clear of foliage and other debris to handle their maximum intended volume.<\/p>\n

Air purification and cooling<\/i>
\nPlants act as natural air purifiers, taking in carbon dioxide (CO2<\/sub>) while enriching the air with oxygen. To the extent they sequester pollutants and freshen the air, green roofs can contribute to the overall well-being of area inhabitants, eliminating many airborne contaminants that can aggravate those with asthma or other upper respiratory ailments. Contributions to air quality are incremental, but offer real benefits. Of course, pollen from trees and grasses can aggravate those with allergies; landscape architects may want to take this into account in selecting plant species, particularly for areas such as healthcare facilities.<\/p>\n

Green roofs can also act as natural air-conditioning for cities during the summer months. Asphalt and buildings absorb and then radiate heat, causing a dome of warmer air, known as the urban heat island effect. It is estimated the annual mean air temperature of a city with one million inhabitants can be 1 to 3 C (1.8 to 5.4 F) warmer than surrounding areas.<\/p>\n

Plants do not radiate significant levels of heat. Further, because they retain moisture and capture morning dew, they can also provide evaporative cooling that is intensified by just a light breeze. Again, the cooling impact is incremental, but it is cumulative and can begin to be felt as the ratio of \u2018green\u2019 to \u2018black\u2019 landscape increases.<\/p>\n

City planners can begin to accumulate these shared benefits by incorporating green roof designs into public buildings and sustainable development projects, as well as by encouraging private-sector investment through local environmental policies and commercial tax incentives.<\/p>\n

Advantages to commercial and public property owners<\/b>
\nVegetated roofs represent a significant building improvement contributing to resale value of commercial properties, and translate into higher rates for rentals, leased properties, and co-operative housing developments. Of course, valuations will vary based on location and the real estate market.<\/p>\n

Sustainability is growing in importance to many corporations as well as cities, and vegetated roofs are a tangible way to project those values. Green roofs can qualify for environmental design credits under the U.S. Green Building Council\u2019s (USGBC\u2019s) Leadership in Energy and Environmental Design (LEED) program. USGBC offers one point each in the following categories for inclusion of a green roof:<\/p>\n