The Filter, Drain, and Water Holding Components of Green Roof Design
By Allan Wingfield, AIA
Photos Courtesy Allan Wingfield and Colbond, Inc.
The successful initial growth of vegetation on any green roof or roof garden and its continued health is dependent on three important functions. Each of these functions take place unseen beneath the plant material, but each one is critical and care must be taken in the design of the roof to provide these: Every green roof system must have the ability to drain excess water while at the same time filtering the water so that the soil medium stays in place and the drainage function is not impeded by sediment. A third and important function is for the green roof to be able to store water for use by the plants. Each of these functions may be accomplished in different ways but they must all work together to provide a green roof that will thrive over time.
The purpose of the filter course is to prevent fine soil medium particles from washing into the drainage layer causing clogging or reduction of the drainage capacity of the drainage layer. The filter may be a separate layer or part of a geocomposite drain mat or board. The filter layer may be independent of the drainage layer or an integral part of a geocomposite drain. The filter layer typically should be installed parallel with the drainage layer.
An entangled filament geocomposite drain mat (Enkadrain) being installed over the roof membrane of a green roof.
Geosynthetic fabrics, commonly known as geotextiles, are typically used and are available as woven and non-woven materials. These filters need to be tough to withstand abuse while other layers are installed, while being open enough to provide good flow for water into the drainage layer. In a granular or what is sometimes known as a single layer green roof the filter fabric is used to separate larger granular material for drainage from finer material in the growing media. The ability of the geotextile to allow water permeability is referred to as hydraulic conductivity by permittivity. The filter must allow good water flow in the normal direction (vertical for green roofs) while inhibiting the movement of soil medium particles into the drain layer. The apparent opening size, or the AOS, is a common physical property that is tested on geosynthetic fabrics used for filtering. This determines the size of the majority of the openings in a particular filter fabric and is determined by allowing very small glass beads of a known diameter (measured in millimeters) to pass through the fabric. The size is increased until a point when the fabric is able to hold back 95% of the beads. This is called the O95 or the AOS. It is desirable that some of the plant roots are able to pass through the filter. This is most critical for extensive green roofs where soil media depths are less.
For thin extensive roofs where there is minimal total volume to hold water, access to water held on the surface area of a drainage composite and the water that accumulates at the bottom of the drainage composite can be very useful to the plants. Very fine roots can reach this water by passing through the filter. A ten year study of roots and and their affect on drainage layers done by the University of Hanover (Germany) has shown that the amount of roots penetrating the filter diminishes over time as plants become established so there is not a problem with roots significantly reducing drainage capacity.
There is not a test currently for this although the Apparent Opening Size may help in evaluating different fabrics. It is important that the geotextile have some puncture resistance so that the filtering capability is not compromised from damage during construction or by aggregates used for drainage or water holding. The tensile strength and tear resistance of the material is also a consideration for long term performance of the roof. Geotextile filters should have sufficient resistance to anticipated soil borne chemicals or microbial growth both during construction and after completion since it will be frequently in contact with moisture and fertilizer compounds. Geotextile filter fabrics are not permanently weather proof and the design should take that in to account. Some considerations are the effect of ultraviolet light before the material is covered and also the effect of high and low temperature swings as the seasons change.
The purpose of the drainage layer is to remove excess water from the roof. The drainage system must consider surface water, sub-surface water and transporting water from the roof. A drainage course is to provide sufficient void space and slope to allow excess sub-surface water to be transported to roof drains or gutters where it can be removed from the roof. Surface water may be removed by proper sloping of pervious and impervious surfaces to roof drains or gutters. Roof drains, gutters, scuppers and associated piping then conduct water away from the roof.
The primary functions of drainage may include: maintaining the overlying growth-supporting media in a drained condition; preventing chronic anaerobic conditions; providing the principal mechanism for discharging storm-event runoff including eliminating surface flow and minimizing seepage flow in the growth-supporting media. Secondary functions may include providing a suitable horizon for introduction of irrigation and increasing root-volume available for the plants.
Drainage divides into two basic classes, aggregate drains and geocomposite drains, which may be combined or used separately in conjunction with the drain outlets. Aggregate type drain layers less than 4 inches in depth should be freely drained. With deeper layers drainage restriction can be used to provide water holding capacity. A number of granular materials may be considered including gravel & fines, lava & pumice, expanded clay & slate, and different recycled materials such as crushed roofing tiles or brick.
Geocomposite drains are any drains composed of two or more materials, one of which is a geosynthetic. Drains may also be a combination of drain types. "Entangled Polymeric Filament Mats" are mats composed of entangled filaments of different polymers which allow water flow from any direction. Typically mats have a filter geosynthetic attached to one or both sides. Another type is "Shaped Plastic Membranes." These drains are made of continuous sheets of different polymers deformed to form dimples, studs or pegs. Some varieties allow drainage to occur on one side of the membrane. Other varieties provide drainage capacity on both sides of the membrane as well as water holding. The drain capacity on the bottom is only accessed by joints between sheets or by perforations in the membrane. Typically these drains have a filter geosynthetic attached to one side. "Porous Synthetic Mats" are drains made from fabrics or non-rigid foam compressed together and often from recycled materials. These drains are available with or without a filter geosynthetic. The last category of geocomposite drains are "Rigid Grooved or Deformed Foam Boards." These drains are formed from rigid foam boards with grooves or deformations on one side to allow drainage. If drainage deformations are placed on the bottom access is by joints between sheets.
A geocomposite drain (Enkadrain) with the entangled filaments of the drainage core with a non-woven fabric filter on one side.
A geocomposite drain with a core of recycled polypropylene and a needle punched non-woven polypropylene filter fabric.
Drain outlets consist of any drain structure that removes water from within the vegetated area and outside of the vegetated area. These would include roof drains, gutters, scuppers and associated piping. These structures remove both surface water and water from the drainage layers. An overflow drainage method should be provided for a secondary means of water removal in event of blockage of the primary system. Drain outlets should be located so they remain accessible after anticipated vegetation growth and are protected so as to prevent vegetative growth from impairing their function. An additional internal drainage networks of perforated piping that resides in the aggregate/granular drainage layer or on top of a geocomposite drain layer may be necessary for some designs. These function to carry excess water directly to drain outlets.
There are a number of properties that should be considered in order to evaluate drainage layers for green roofs. Thermal stability is important and materials selected should take into account the anticipated climate and be able to withstand hot and cold temperatures without compromising drainage. The structural stability of drain materials selected should have sufficient strength in situ to retain shape and support weight of layers above as well as the anticipated live loads so that long term settlement is minimized. In aggregate materials granular shape should be considered. Salt content is another consideration. Soluble salt content should not exceed .46 oz/gal (3.5 g/liter) for extensive greening and .33 oz/gal (2.5 g/liter) for intensive greening. This is based on the recommendations of the “Guidelines for the Planning, Execution and Upkeep of Green-Roof Sites” as published by the FLL - Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau.
This intensive green roof installation photo shows the real abuse geocomposite drain layers (Enkadrain) can sometimes get.
The weight of a green roof system is something to consider, especially in retrofit applications. A measurement of total dry weight of the drainage layer compared to the wet weight can be used to determine the water holding capacity of only the drainage layer or it can be used to evaluate the complete system including the soil medium. The wet weight is a measurement of the free draining, water saturated weight of the drainage layer. This would include weight of water adhering to components of the system and water absorbed by any porous material.
One of the most important properties of the drain layer is, of course, the amount of water that can flow through to the drain outlets. This is known as transmissivity. Both aggregate type drains and geocomposite drains must be evaluated for their flow rates. For granular layers this can be computed from hydraulic conductivity. Although there are no widely accepted tests for measuring this within course media, in geocomposite drains transmissivity - or the flow of water in a drainage layer after passing through the filter layer - is measured in the in-plane direction under the anticipated hydraulic gradient under the anticipated loads of the green roof components above. There is a standard test (ASTM D 4716) used for this although the slopes and the pressure on the drains are so low in a typical green roof application that the test results are very inaccurate and underestimate drain performance.
There are several other properties to consider that are particular to aggregate or granular drain layers. Granulometric distribution should be considered. Aggregate material of less than .002 inches (0.063 mm) should be limited to 2% of total mass of the aggregate layer. The pH of the aggregate should be evaluated based on the needs of the anticipated vegetation. The pH at the time of installation should be between 6.0 & 8.5 up to 10.5 for extensive greening. Special types of vegetation may require other consideration. The carbonate content in aggregate materials should be evaluated to prevent forming of calcium hydroxide and corrosion of roof drain outlets & piping. Carbonate content should not exceed .8 oz/gal (6g/liter).
Every green roof needs to provide a way for the plants to have access to water. T he primary function of water holding is to retain water following irrigation or rainfall events for use by plants and to benefit storm water infrastructure by reduction of runoff. Water holding capacity can be separated into two forms: transient water holding capacity for water held for a certain number of hours or days before it begins dissipating and long term water holding capacity for water that is only removed through plant transpiration or evaporation. Water holding may occur in multiple layers simultaneously, which is the method that provides reliable long term performance. Having redundant or alternate methods of water holding provides a higher probability that plants will survive during stressful climate fluctuations. Water can be stored both in the soil medium and in the drainage layer. The soil medium may be designed to contain granular media with high porosity or water absorption and also has pore space between particles that retain water where it is accessible to plant roots. The drainage layer can store water in porous aggregate, in containers or reservoirs formed by the geocomposite drain, through the use of thickened mats or fabrics, or by partially blocking the drain layer to allow water to be stored in a portion of the bottom of the drain layer. The measure of wet and dry weights discussed earlier can be used to evaluate water holding capacity. Currently there is not a standard established test to evaluate water holding capacity.
Some of the typical layers on a green roof
The water filtering, water drainage and water holding ability of every green roof project is critical to its long term success and consideration of each of these should be a part of every green roof design process. Simply put, green roofs need to drain well so that the growing media does not become over saturated with water, while at the same time they need to retain water so plants can survive and thrive during dry conditions. A green roof also needs to retain water to give the added value of slowing down and reducing storm water runoff to our cities. By carefully considering provisions for good drainage and retaining water, we can maintain healthy living roofs that provide so many wonderful benefits to our communities and the people who live there.
Allan Wingfield is a registered architect and a member of the American Institute for Architects. For the last three years Allan has been a member of the ASTM E06.71 Green Roof Task Group. He is responsible for technical marketing of building products in North America for Colbond Inc. Colbond is an international corporation, headquartered in the Netherlands, that has been involved in the green roof industry for more than 25 years. Colbond manufactures geocomposite drains, mats for pre-vegetating green roofs and mats used for root reinforcement of green roof plants in sloped and high wind conditions.
For information about Colbond and their green roof products visit our website www.colbond-usa.com.