A vineyard and bee hives on the roof of City Hall in Paris.
Introduction
A roof’s primary purpose has been to shelter its inhabitants from the elements, and now the underutilized potential of roof surfaces is being realized. Roofs with overburden can make a significant impact on the sustainability goals of a building including: increased energy efficiency, stormwater retention, energy generation, biohabitat restoration, food production, reduced urban heat-island effect, and outdoor space.
However, selection of the overburden system is only part of the design. Selection and design of the roof assembly is critical for a long-lasting installation and performance and paramount to the overall success and longevity of the overburden system.
Overburden Assemblies
Overburden is material or equipment that is located above the roof assembly. While each property is unique, there are many overburden options such as vegetative roofs, blue roofs, blue-green roofs, purple roofs, and rooftop solar.
Membrane Selection
Both single-ply membranes and modified bitumen roof systems can be an excellent choice for overburden installations. The type of overburden will determine whether the membrane should be installed at the deck level or at the top of the assembly. Membranes that will be used at the deck level, such as for vegetative assemblies, should be evaluated for durability and longevity since access for repairs will be limited and costly. Membranes where overburden systems, such as solar, are installed directly on the membrane should be able to withstand abrasion and puncture from roof elements above it. For blue roofs and water catchment systems, single-ply membranes are commonly used.
Membrane Color
When the membrane is exposed, the color can have a significant impact on the performance of the system. Reflective roof membranes can lower ambient temperatures, decrease urban heat-island effect, and may decrease the heat load into a building’s interior. Vegetative roof systems can take advantage of reflective membranes in areas where vegetation is not installed.
Membrane Attachment
Mechanically attached single-ply systems are subject to billowing in high wind events, which can cause stress and fatigue over time. Overburden installed over mechanically attached systems may billow with the roof membrane during high wind events, causing abrasion of the membrane surface. The overburden may also experience stress and fatigue, potentially decreasing overall service lives.
Insulation
Insulation is critical for the energy efficiency of the building. The higher the R-value, the better the thermal performance of the insulation and its effectiveness at maintaining interior temperatures. Higher R-value per inch means less material is required to achieve the desired insulating value.
Coverboards
Inclusion of a coverboard must be considered after selection of the overburden. Rooftops with overburden assemblies generally have increased foot traffic on the roof. Coverboards provide added protection against foot traffic, penetrations, service contractors, wind-borne debris, and hail.
Summary
While the roof is the primary defense to prevent water entering into a building, this previously underutilized asset is being recognized by owners for the sustainability opportunity and the additional space that the roof offers. Once the overburden type is selected, the roof assembly needs to be determined and installed to match or exceed the service life of the overburden systems. Special consideration should be given to roof assembly design decisions as the overburden success is dependent on the success of the roof beneath them. To learn more view the article here.
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