Reducing Rooftop Noise in San Francisco, California
by Marcus Dodson, editor & publisher
Rooftop equipment screens are usually installed to hide mechanical equipment or add an architectural feature to a building. However, sometimes they are used for noise abatement, which was the case with a recent installation in San Francisco, California. Like many large cities, San Francisco has limits on the amount of noise that can be produced by mechanical rooftop equipment. One such enforcement recently came into effect at a private school in the Haight-Ashbury district after a tenant in an adjacent building complained to the city about the amount of noise the school’s equipment was generating.
San Francisco code states that rooftop equipment cannot exceed 6 decibels over the ambient sound level of the city. To measure this, an inspector from the health department used a decibel meter to measure the sound level on the roof without any equipment powered on, showing the ambient noise level. Next, the equipment is turned on and another measurement is taken. The difference between the two measurements determines the decibels over the city’s ambient noise level.
When responding to the complaint, the inspector took the sound measurements and found it to be 7.5 decibels above the ambient sound level, which was 1.5 decibels too loud. However, 1.5 decibels is inaudible to the average person. In fact, you need a 3-decibel gain in order to audibly perceive a difference in sound. This triggered a violation for the school, which was then expected to reduce the noise level emanating from the rooftop.
To decrease the noise level, the general contractor suggested a pre-fabricated equipment screen from Cascade Rooftop Screens, Inc., located in Santa Clara, California. Mike Kaupert from Cascade advised installing a simple V rib corrugated metal panel for the facing material. Kaupert said, “There are various noise treatments that can be used to absorb the sound coming from the mechanical units, including rock wool insulation. This would be installed on the back of the panel to help reduce the sound.”
Kaupert proposed starting with Cascade’s standard frame and a V-rib panel, then take a reading with the decibel meter. If this did not reduce the sound enough, then rock wool would be installed to the back side of the panel. It was decided that this was an acceptable course of action and steps were taken to begin the installation. Pacific Weathershield, Inc., Santa Clara, was then contracted to install the equipment screen.
Cascade’s Simple-Frame screen system typically ships fully assembled. For this project, however, Pacific ordered the materials unassembled so they could be loaded through the roof hatch, as obtaining a crane and the necessary permitting in the congested neighborhood of Haight-Ashbury would have delayed the project. The frames shipped in three components that easily bolted together and the facing panel was installed to the front.
For any screen project, there is a possibility that the wind load that is transferred from the screen to the roof framing can sometimes be too much for the existing roof framing to handle. Kaupert stated, “In the case of Cascade’s standard screen system, the Simple-Frames will almost always be able to handle a great deal more loading than the building’s roof framing can.” He went on to explain that if the roof framing needs to be reinforced, it is important to choose a reinforcement method that will allow for an easy and sufficient waterproofing detail of the roof penetrations. This usually excludes details such as rooftop sleepers that require an awkward and expensive roofing method.
When the project engineer decided that the existing roof framing would not be able to handle the additional wind force from a 7’-tall screen, it was determined that the load would need to be distributed across multiple roof framing members. Fortunately, there were 6” of rigid insulation with a multi-ply, cap-sheet roof. This allowed for easy installation of a below-roof, 4”x6” sleeper on its side across the top of the roof structure. Cascade’s Simple-Frames were installed directly on top of these. The thick insulation was then filled back in, creating an even roof surface and a clean, weatherproof roofing detail.
Pacific Weathershield completed the screen installation and preliminary readings were taken with a decibel meter by a field technician. The reading was only 1.5 decibels over ambient sound level, signifying a drop of 6 decibels. The sound was, therefore, well within the limits and the expense of the rock wool insulation was not necessary. An inspection by the health department obtained the same results, determining that no fine would be given to the school. While more cities are now requiring screens to hide rooftop equipment, they can also be used for noise abatement, backdrops for signage, or simply to provide architectural features when updating a building. Often, metal panels of various shapes, colors, or textures are used for the facing material, although simulated brick, stucco, composition roofing shingles, and many other materials can be chosen. This story is just one illustration on how equipment screens can often be aesthetically enhancing as well as functional.