November 6, 2019
Ballasted Roofs – A New Look at an Old System
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While ballasted roof systems aren't as popular today as they used to be, they are still being installed successfully across the country.
Stone-ballasted roof systems began appearing sometime in the early 1970s. While they appear superficially similar to built-up roofing (BUR), there are major differences between the two systems. Both are topped with rocks, but BUR uses a thin layer of pea gravel or crushed stone no larger than a quarter-inch diameter partially embedded into the asphalt topcoat to protect it from the sun's UV rays. In a ballasted roof, the stones are much larger - at least an inch in diameter - and applied much more heavily. In fact, the weight of the stone ballast is what holds the roof components in place. The weight can vary from 10 pounds per square foot (the minimum allowed by code) to 25 pounds or more. The most common ballasted assembly was a loose-laid EPDM membrane over a rigid insulation board.
By the 1980s, designers were integrating concrete pavers into ballasted roof designs, creating access paths, pedestrian walkways, and even rooftop plazas. When the green building movement came along in the late 1990s, it was natural to transition the ballast from stone to soil, creating vegetated or "green" roofs.
Ballasted roofs are loose-laid; this means the contractor can assemble all the components, including the roofing membrane and insulation, without fastening them to each other or the roof deck. Membrane seams are sealed, of course, and the waterproofing layer is secured to the parapet and at roof penetrations, but it isn't adhered to the roof deck or the layers beneath it.
By eliminating nearly all the adhesives and fasteners other assemblies require, ballasted roofs typically cost less and are quicker to install than other systems. EPDM is popular because it can be ordered in large sheet sizes, which minimizes seaming. TPO and PVC are also popular as single-ply roofing membranes under ballast.
For designers, ballasted roofs provide a natural-looking surface that blends well with a range of architectural styles. With paver-ballasted designs, the roof can become a plaza, patio, or other usable outdoor space suitable for recreation, walking, or relaxation.
For the building owner, ballasted roofs are durable and long lasting. Stones or concrete pavers protect the waterproofing layer from UV rays, hail, and foot traffic. If repairs are needed, the loose-laid layers are easily taken up. And at the end of the roof's designed lifespan, the lack of adhesives ensures the membrane will be fully recyclable.
Advantages Economy: Ballasted roofs use economical materials and are among the fastest to install. In fact, they have one of the lowest lifecycle costs of any roofing system on the market today.
Scheduling: These roofs can be installed in a wide range of weather and temperature conditions, and close in the building envelope faster than most other systems. For occupied buildings, there are no offensive smells associated with the install.
Aesthetics: Ballast can vary from large round cobblestones to pavers. This natural look is appealing to many building owners and architects. Rock can be combined with pavers to provide a variety of textures and utilitarian purposes.
Amenity Space: With proper planning, ballasted roofs are suitable for plaza decks, walking paths, recreation areas, and other uses.
Energy Efficiency: Ballasted roofs reduce heating and cooling loads. A system with a weight of 17 pounds per square foot saves as much energy as an ENERGY STAR-rated reflective roof.
Fireproof: Stone and concrete are virtually fireproof, so ballasted roofs provide the highest fire rating available. Class-A fire resistance can be achieved without gypsum board underlayments or expensive fire-retardant chemicals.
Durable: The stone or concrete pavers also provide protection from UV rays, hail, foot traffic, and extreme temperature fluctuations.
Ease of Repair: Removal and re-installation of the ballast and insulation is easy, and both can be reused. No adhesives or fasteners are used, so it's easy to separate the components. Even in a complete replacement of the waterproofing membrane, the ballast stone or concrete pavers can be reinstalled.
Recyclable: Most of the components are reusable and/or recyclable. Rocks, pavers, and rigid insulation board can be reused. The unadhered membrane is easy to remove and recycle.
Stormwater Management: Green roofs and other options like Carlisle's Stormwater Retention Option can retain as much as 65% of the rainwater that falls during a storm. This can help owners and developers reduce fees.
Consult the EPDM Specification on the Carlisle SynTec Website here or consult the Ballasted Stormwater Retention Brochure here or contact Craig Tyler at [email protected] for further questions.