Porous Pave: Permeable Pavement Solution for a Rooftop Patio

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West Michigan Symphony Completes its Green Roof with a Unique Porous Pave Patio

The West Michigan Symphony, the professional orchestra in Muskegon, Mich., has been part of the area’s cultural landscape since 1939. In 2013, they moved their administrative and ticketing offices into the newly renovated Russell Block Building. Situated in downtown Muskegon a block away from the Frauenthal Theater where the symphony performs, the historic Russell Block Building dates back to 1890.

“The move gave us the opportunity to fulfill a long-deferred dream: having a flexible space for smaller fine arts performances and education programs,” said Carla Hill, the symphony’s president and chief executive officer.

Named The Block, the 1,800-square-foot space includes seating on two levels for up to 150. In addition to providing an intimate venue for the symphony, The Block is available for meetings and special events. Its west-facing windows look out toward Muskegon Lake. However, an unsightly old tar roof right outside the windows spoiled the view.

Port City Construction & Development Services, LLC, Muskegon, Mich., the company which planned and managed the building renovation, Fleis & VandenBrink, Inc., Grand Rapids, Mich., the project’s landscape architecture firm, and officials from the symphony  discussed what might be done. They envisioned transforming the unadorned roof into a rooftop patio and garden.

“The goal was to create an accessible and appealing outdoor space for The Block,” said Harry Wierenga, landscape architect, Fleis & VandenBrink, Inc. “We designed a 900-square-foot green roof with 380 square feet of vegetation and a 520-square-foot patio.”

Fixing the Roof

“The existing roof was tar over a concrete deck. Several holes had been boarded up and patched with tar,” said Gary Post, manager, Port City Construction & Development Services. “To add the green roof, we first had to re-roof.”

The Port City crew removed the existing roof down to the concrete deck, which they repaired. The installed two new roof drains. The crew fully adhered a new single-ply membrane to the deck. Next came a geotextile fabric to protect the membrane and a geotextile drain sheet atop the protection fabric. The drain sheet provides drainage to the existing and two added roof drains.

A new 40-inch-high wall around the roof shelters the outdoor space and makes it safer. The project also included widening the opening out to the rooftop from the interior of The Block and glass double doors for a generous, transparent transition from indoors to outdoors.

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Finding the Right Surface

The pavement material for the patio had to meet a rigorous set of requirements:

  1. To eliminate standing water and allow excess stormwater to flow to the drains, the paving material had to be permeable.
  2. The plan called for installing both the permeable patio and green roof on top of the geotextile drain sheet.
  3. The maximum static plus live load for the roof is 100 pounds per square foot, so weight was a factor.
  4. For easy access and safety, it had to result in a low profile threshold at the entry into The Block.
  5. To create visual interest within the rectangular shape of the roof, the plan emphasized irregular shapes with angles to break up the space. The paving material would have to be flexible to adapt to the design.
  6. The musical note motif Wierenga devised for the patio is the signature element of the rooftop. The paving material had to offer the versatility to express the design.
  7. Finally, in keeping with the green roof, a green paving material was preferred.

The project team assessed composite decking and concrete pavers. These linear materials were not flexible enough to conform to the shape of the patio or versatile enough to convey the musical note design.

“Pavers could be cut to fit the patio’s shape and express the design, but not seamlessly,” said Hill. “We did not want it to end up looking like a jigsaw puzzle.”

Pavers presented another problem. With a permeable paver system, the threshold from the patio to the new doors would have been four inches, a tripping hazard.

So what paving material would work?

Wierenga had used Porous Pave in projects at grade. “I knew that it is lightweight, permeable, and pourable within forms to conform to the angles of the patio and give us the musical note pattern,” he said.

Porous Pave XL consists of 50 percent recycled rubber chips and 50 percent stone aggregate with a moisture-cured liquid binder. Porous Pave XLS is 100 percent recycled rubber chips with a more elastic binder. One inch of the XLS all-rubber formulation, weighing only three pounds per square foot, was installed for the patio.

“We engineered Porous Pave for permeability with more void space than other permeable pavement options. Water goes right through to the drain sheet, leaving no puddles of rainwater,” said Dave Ouwinga, president, Porous Pave, Inc., Grant, Mich. “The threshold ended up at only an inch and a half. The recycled rubber makes Porous Pave a green material and slip-resistant as well.”

Offsite, wood forms were constructed in the shapes of the four musical notes. Before the Porous Pave XLS was poured into the forms, the sides were coated with vegetable oil so the material would not adhere to them as it cured. On the rooftop, the completed note pieces were positioned in place, and the rest of Porous Pave XLS was mixed and poured around the notes. The three lines through the musical notes that contribute to the page of music design were poured into forms on the rooftop.

The Results

The installation achieved a smooth and seamless expression of the design. The gray and black custom color mix harmonizes with the color of the new wall and complements the gray concrete elements of the Russell Block Building.

“We had not previously used this pour-in-place, permeable paving material on a rooftop,” commented Post. “Based on the results and the material’s performance since installation, we would recommend Porous Pave and specify it again for rooftop applications, particularly where its light weight would be an advantage because of load considerations.”

~Written by the USGBC Chapter Partner, Porous Pave