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,
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
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
- Quickly locate the source of any leaks
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:
Source: ZinCo International
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.
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.
Source: ZinCo International 3/98 Brochure
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