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Installing a Green Roof, Images 1-6

Installing a Green Roof, Images 1-6

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    Interlocking pre-grown panels add little dead load to the roof.

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    The base layer of Green Living Technology's modular roof panels — a dimpled HDPE membrane — acts as a water reservoir, but has perforations so that excess water can drain out. The root-stabilization layer consists of a nonwoven polypropylene matrix that allows water to flow freely through it, with a gel-like backing that can absorb moisture. The third layer, the planting medium, is a lightweight blend of local gravels and soils with perlite.

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    When shipped, the vegetated panels weigh about 100 pounds apiece and are stacked about 12 to a pallet.

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    GLT roof panels are available for virtually any roof pitch. On this green roof, L-shaped aluminum brackets taped to the waterproofing layer finish the panel edges and help prevent wind uplift.

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    The existing rooftop deck (top) didn't need any structural reinforcement, but an additional EPDM waterproofing membrane was installed over the existing one (bottom).

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    Self-adhering EPDM tape was used at the seams and to seal the membrane to the sidewalls and deck posts.

Last fall, I was in a client’s second-floor office hashing out the details of an ongoing landscaping project when I noticed a door to the outside. I asked where it led. “Nowhere, really,” my client said. After checking for myself, I understood what she meant. The door opened onto an unattractive deck that — despite a great view of Lake Ontario — was clearly unused. An EPDM membrane covered the floor, and I could imagine how hot that black surface got in the summertime.

I suggested adding a roof garden. Though my client knew about the environmental benefits of green roofs, she was surprised to learn about the lightweight planting system my company had recently been trained to install. While these vegetative panels from Green Living Technologies (800/631-8001, agreenroof.com) are primarily used for commercial construction, they’re reasonably priced and easy to work with, and they make green roofs feasible even for small residential projects, where aesthetics are more important than LEED points. The maker coordinates distribution with regional growers and uses trained, certified contractors for installation.

A Modular Approach

GLT’s modular panels — an alternative to the more common complex layered green-roof systems (see sidebar) — combine a thin growing medium with a root stabilizer and water reservoir. Designed so that they interlock, the 39-by-39-inch (or 10.72-square-foot) panels are about an inch thick before planting and 3 inches thick when they’re vegetated. Each can retain approximately 8 gallons of water, but even when they’re installed on the roof and fully saturated, they weigh only about 15 pounds per square foot.

The panels we used on this project are designed for roofs with pitches up to 4/12. GLT also makes panels for roof pitches between 5/12 and 12/12 and has a bracket system that can be used to adapt the panels for pitches up to 90 degrees. In addition, the company offers self-contained trays for flat roofs. We preferred to use panels for this project, because they can be cut on site to fit irregular shapes. Our plan was to mix preplanted panels with site-built ipe deck sections to transform the 10-foot by 20-foot deck into an attractive living space. 

Framing and Waterproofing

Because of their thin profile and light weight, the panels can often be installed with no major modifications to the existing structure. Here, the deck was a slightly pitched porch roof above an entryway and was conventionally framed with pressure-treated lumber supported by 6x6 PT posts. Though ugly, the single-ply 45-mil EPDM membrane covering the 3/4-inch plywood was in good shape and showed no signs of leakage. After consulting with our engineer, we decided that the deck wouldn’t require additional reinforcement to support the new loads from the roof panels, but we did want to beef up the waterproofing layer.

First, we thoroughly cleaned the old membrane by sweeping and vacuuming it; then we laid down another 45-mil EPDM membrane on top of it. We lapped the edges up the walls, tucking them behind the existing synthetic stucco siding as best we could, and cut in around the existing 4x4 deck posts, then primed the edges and seams before sealing them with self-adhesive EPDM seam tape. Because the roof deck framing was enclosed by the porch ceiling underneath, we used the same basic waterproofing details we would have used for a deck above living space.

Next, to protect the EPDM against punctures but still allow for drainage, we laid down a heavy-duty landscape fabric — DeWitt 20-year weed barrier (800/888-9669, dewittcompany.com), the same 4.1-ounce fabric we use for landscaping projects on the ground. Wind uplift can be a problem on rooftop gardens. On larger projects, GeoEdge (800/356-9660, permaloc.com), an L-shaped aluminum edge restraint, is normally used to prevent uplift at the eaves and at the transitions between planting areas and hardscape or drainage areas. On this small deck, however, we finished the outside edge with 5/4 ipe deck boards laid on edge and fastened to the outer face of the deck posts. A slight gap between the ipe and the landscape fabric allows water to drain into the porch roof’s gutter.