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December 2014: Up on the Roof Top

It’s that time of year again when eight flying reindeer are due to prance upon your rooftop. Building owners and designers should be aware that modern membrane roof design doesn’t provide enough sound isolation to keep the children resting in their beds when Santa comes to town. Read on for an overview of roof construction for sound isolation.

There are three key factors to a well-performing roof/ceiling assembly for sound isolation: Mass, absorption, and resiliency. Mass is essential to attenuate low frequency sound. Absorption is needed in cavities to absorb resonances. In some constructions, adding fibrous insulation such as glass fiber or mineral wool can double the perceived sound isolation. Resiliency is critical to break the structural connection from one side of the assembly to the other. This physical break greatly reduces impact and airborne sound transmission across the assembly.

Membrane roof construction typically consists of 18 to 22 gauge corrugated steel deck under several inches of rigid polyisocyanurate insulation topped with an ethylene propylene diene monomer (EPDM) membrane for waterproofing.

Roofing Picture

Typical membrane roof shown with ballast. Photo credit: EPDM Roofing Association

The steel deck provides more mass than insulation and roofing membrane, but very little mass relative to concrete or other roofing materials. Polyiso insulation is great for thermal insulation, but contrary to popular belief, it is a very poor sound absorber. Since it is rigid, it also provides a strong physical connection from the EPDM membrane to the steel deck. In building spaces with exposed ceilings, rainfall on membrane roofs can be a significant source of noise.

Even with the addition of a lay-in ceiling, airborne sound isolation may be lacking. Buildings within proximity to airports may be required to achieve specific levels of exterior-to-interior sound isolation. Schools attempting to achieve LEED Green Building certification may need to achieve a minimum Sound Transmission Class (STC) rating of 50 between top-floor core learning spaces and the outdoors. Commercial lease agreements may spell out the required sound and vibration requirements for top-floor tenants. Typical membrane roofs achieve STC ratings in the mid 30s. Raising this performance to STC 50 or higher commonly requires additional mass, absorption, and in extreme cases, resiliency. Unfortunately, adding mass alone may not make a roof construction immune to noise and vibration issues.

Georgia Pacific DensDeck

Lightweight roof with added mass layer. Photo credit: Georgia-Pacific DensDeck

Heliports on top of hospitals, emergency generators above occupied spaces, and parking structures above noise-sensitive spaces are prime examples of the need for resiliency. In these heavy-duty applications, the roof structure may be composed of multiple layers of materials including layers to provide mass and others to provide resiliency. A real-world example of this concept is a cinema currently in design in New York that is to have exposed parking on the roof of the second floor auditoriums. Concrete is required to support the vehicular loads; however, this structural concrete slab will need to be accompanied by a floated concrete slab above it to control the anticipated airborne and structure-borne sound.

January 2015: Our New Year’s Resolutions
November 2014: Steering You in the Right Direction