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The greenroofing system may consist of a liquid-applied membrane, a specially designed singly-ply sheet membrane, or a built-up roof system consisting of 3+ layers.  Many of the oldest greenroofs are waterproofed with mastic asphalt, but bitumen sheets with polyester carriers and SBS modified coatings are becoming more common.  In the early 1990's, PVC and EPDM products were developed.  Root resistance can be achieved either by a laminated upper layer (usually copper) or by chemical additives in the coating.  To ensure drainage capacity, the support to the waterproofing layer should have a slope of at least 1.5% (Hendriks and Hooker, March, 1994).

Quality control in terms of diligent installation and maintenance procedures are essential for all roofs.  According to Oak Ridge National Laboratory (ORNL), most roofing systems fail from leaks, and the average roof needs to be replaced four times.  In addition to replacement cost savings and construction waste reduction, ORNL estimates that if low-slope roofing systems could be kept dry, more than three-fourths of a quadrillion BTUs of energy could be saved each year in the United States (Kennedy, PlantServices.com, August 15, 2006).

Depending upon the nature of the waterproofing membrane chosen (organic vs. synthetic), a root barrier layer may be needed to prevent the plant roots to penetrate and ultimately undermine the integrity of the waterproofing layer.  For example, asphaltic bitumen is organic and roots could naturally attempt to penetrate the surface while seeking nutrients.  A common practice on traditional roof gardens and intensive greenroofs is to pour a concrete protection slab over the membrane layer (Osmundson, 1999).

Design standards that are applicable to waterproofing systems include ASTM C981, ASTM C898, ASTM STP 1084, the Architectural Graphic Standard, and the NRCA Roofing and Waterproofing Manual (RCI's Greenroof Workshop, August 6, 2001).

Some believe the liquid-applied membrane provides a superior waterproofing and easier maintenance (McDonough + Partners, 1999).  Because it is applied as a liquid, it must be installed directly on the roofing deck, requiring that existing roofing be completely removed.  With certain limitations and particular code requirements, sheet membranes may be installed over existing roofing, although manufacturers prefer that existing roofing be removed.  Sheet membranes can also be more difficult to repair.  Most greenroof companies offer several choices of sheet membranes, providing a custom fit for each application.

Correct and meticulous application of the waterproof membrane is essential to the viability of the greenroof.  Quality control is assured through knowledgeable roofing procedures and a water impermeability test immediately following membrane application, with a minimum duration of 24 hours but 48 hours preferred.  Although the German technology, encompassing over thirty years of engineering standards and testing, has virtually eliminated the possibility of membrane leakage, there is at least one North American company offering a leak detention system.  Roofscapes, Inc. offers such a system as well as integrated detection systems engineered by AB Mess- und Trocknungstechnik (AB).  According to their website, Roofscapes can:

  • Conduct water-tightness tests of existing roof surfaces prior to installing a Roofmeadow™ cover system
  • Conduct annual surveys of green roofs to verify that the waterproofing system remains water-tight below the vegetated cover
  • Quickly locate the source of any leaks

AB Leak Detection System offered by Roofscapes, Inc.

Courtesy of Roofscapes, Inc.; www.roofmeadow.com

The AB system can locate pin-hole size defects with the same ease as large flaws.  Please see the Roofscapes, Inc. website here for additional information and consultation:  www.roofmeadow.com.

Membrane protection by ZinCo

Source: ZinCo International
3/98 Brochure

According to ZinCo literature, it is essential to mark the position of the roof outlets before installing the protection layer, so that they can be located easily and the root barrier and protection mat cut out accordingly.  Protection of the membrane from these components could include 10 mm or 1/3" of granular rubber (Hendriks and Hooker, 1994).  Reliable detailing at penetration and perimeter areas with durable protection is critical.  Any expansion joints which are not extended up through the waterproofing should remain free of plants.  They can, for example, be covered by gravel or paving slabs so that they can be easily located and accessible at any time.

The Green Roof Wall Flashing detail below from The Garland Company shows a wood, steel, or cement curb with a 6" minimum.

The Garland Company Construction Detail

Courtesy The Garland Company

Water features such as waterfalls, fountains and  ponds can be stunning greenroof architectural designs, but extra care is required.  Risk can be eliminated by isolating the lining of the water feature, and ensuring a drainage and protection layer underneath.  In the example of a pond detail from ZinCo shown below, the lining is laid onto a 30-50 mm or 1-2" bed of sand to distribute the load of water.  The pond bottom is covered by rounded pebbles to protect the lining, and the perimeter of the pond lining is buried to protect against ultra violet light.  If a leak does happen, the water simply seeps through the sand bed and is drained by the underlying drainage system.  This system was applied to the BMW building greenroof, shown below by ZinCo International.

Pond Greenroof Construction Detail by ZinCo  BMW Intensive Greenroof with Pond by ZinCo

Source: ZinCo International 3/98 Brochure

 

 

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