Several ‘Sandwich’ Roof Assemblies Mitigate Sound Transfer

We all want a roof over our heads to protect us from the cold winter months, hot summer months and precipitation year round. How much thought goes into the sound-control construction of a roof, though? Have you considered the acoustic properties of your roofing system? Admittedly, acoustics is not a topic that many roofing contractors think about. The construction of a roof, however, can have a significant impact on the sound quality of the building interior. While this may not seem important in every project, it can be a critical element of the design for concert halls, theaters, auditoria and even school classrooms.

Sound Isolation

The acoustics of a space depend on many criteria, including sound isolation, sound reflection, impact noise and sound transfer. In many cases, particularly in noisy, urban environments, there is a need to prevent loud outside noises, such as traffic, sirens and airplane noise, from entering quiet spaces. Sound isolation depends on the entire envelope of a space, including external walls, windows and roofs.

Green roofs, particularly the “intensive” version, which includes several inches of heavier-weight soil, can provide effective sound control.

Green roofs, particularly the “intensive” version, which includes several inches of heavier-weight soil, can provide effective
sound control.

Historically, roofs over sound-sensitive spaces have been built with fairly dense materials, such as concrete, which by themselves are relatively effective in blocking sound transfer. As construction methods have developed, however, more lightweight construction is being used. If thought and care are not given to the assembly, these lightweight construction methods can cause serious issues with acoustics. Rain noise, mechanical noise and other exterior sounds can all transfer readily through a thin, lightweight roofing system.

In an effort to use lighter-weight construction, a “sandwich” assembly may be used to mitigate sound transfer. Similar to an Oreo cookie, a sandwich assembly’s outer layers are comprised of a heavy, dense material, and the inner filling consists of insulation and/or airspace. The materials of this assembly can differ from concrete to roofing board, rigid insulation to fibrous insulation, gypsum board to acoustic ceiling tiles. The components can be combined in a variety of ways, each with varying levels of sound isolation.

One of the principle phrases often heard when discussing sound isolation is “mass air mass”, which refers to the separation of two bodies of mass by an air space. The greater the mass and the deeper the air space, the more sound isolation will result. For this reason, a heavy mass, such as 5-inch concrete, followed by a deep air space, such as an 18- to 24-inch ceiling cavity in which ducts are run, followed by a continuous layer of drywall ceiling will provide a high level of sound isolation. Additional steps, like adding sound-absorptive material to the air space and/or using resilient connections when supporting drywall, further improves the sound isolation of the assembly.

Sandwich Roof Assemblies

Several sandwich roof assembly approaches are possible, including:

Good: Multiple layers of dense roofing board (at 2.5 psf per board, a final density of 10 psf or four-ply is often recommended) on either side of insulation, which ideally would be a sound-absorptive fibrous fill, like mineral wool, can reduce sound transmission. This approach is similar to a “floating floor”, often used in interior spaces to isolate sound transfer from one room to another. (Equivalent Sound Transmission Class, or STC, ratings can range from low 50s to low 60s, depending on whether a ceiling is included below the deck.)

Drywall ceilings hung on resilient hangers in conjunction with a lightweight roofing system provide even greater sound isolation by virtue of the resilient connection or “decoupling” of the drywall layer from the rest of the building structure.

Drywall ceilings hung on resilient hangers in conjunction with a lightweight roofing system provide even greater sound isolation by virtue of the resilient connection or “decoupling” of the drywall layer from the rest of the building structure.

Good: Green roofs, particularly the “intensive” version, which includes several inches of heavier-weight soil, can provide effective sound control. These can be part of a sandwich approach with airspace or rigid insulation between soil and a more-dense roofing material, similar to the roofing board described in the previous example. The mass-air-mass combination is similar to the approach just mentioned, and the benefits of green roofs appeal to many building owners for a multitude of reasons, including minimizing urban heat islands and storm-water management.

Good: A 5-inch slab of normal-weight concrete (150 pcf) has a density of 62 psf. This tried-and-true method is still used regularly and often proves to be the most cost-effective method of enclosing a space. The best sound isolation will occur if this is used in conjunction with a ceiling below, but on its own it still provides a reasonable level of isolation in many environments. This isn’t technically a sandwich system unless paired with a ceiling below or a green roof above. (Equivalent STC ratings can range from low 50s to low 80s. The highest ratings require pairing a resiliently hung ceiling with the concrete, as described under “Multi-function Roof Assemblies”.) IMAGES: Threshold Acoustics LLC

Multi-function Roof Assemblies

Some roofing assemblies are designed to be multifunctional, providing a sound-absorptive finish material in addition to the roofing structure. These are often used in large spaces, such as gyms or cafeterias, where there is a need to tame the sounds generated within the space.

Concrete over acoustic metal deck allows for sound absorption on the interior space, which is particularly helpful in large rooms, such as gyms or cafeterias, where there is a need to tame the sounds generated within the space.

Concrete over acoustic metal deck allows for
sound absorption on the interior space, which
is particularly helpful in large rooms, such as
gyms or cafeterias, where there is a need to tame the sounds generated within the space.

Good: A proprietary roof deck assembly that consists of a lightweight sandwich of sound-absorptive shredded-wood panel, rigid insulation and roof board provides sound absorption on the interior space. By itself, this system provides a relatively low level of sound isolation (STC 31) and does not fully prevent rain noise from being audible in the interior space. When used in conjunction with an additional sandwich layer of insulation and multiple layers of dense roofing boards, a noticeable improvement in sound isolation can be expected. During heavy rains, however, even this may not fully protect rain noise getting through the system.

Better: Concrete over acoustic metal deck allows for sound absorption on the interior space and sound isolation to or from the exterior. This is useful when a gym or cafeteria functions as an all-school gathering space and where there is a need to prevent exterior sounds from getting into the assembly space. In such cases, the concrete plays a crucial part in sound isolation.

Double-sandwich Roof Assemblies

These systems include a ceiling below the roof assembly, creating a second sandwich layer where the finish ceiling and roof assembly behave as the outer cookies in the Oreo. The addition of a continuous ceiling improves the level of sound isolation provided to the space below.

Good: Continuous acoustic ceiling tiles/panels, which can be perforated metal ceiling tiles, in conjunction with a lightweight roofing system can be effective in preventing some sound transfer from exterior to interior. Commonly used because of its ability to hide ductwork, plumbing etc., the addition of a lay-in ceiling also improves the sound isolation of the floor/roof assembly above.

Sound Transmission Class ratings and corresponding perceived levels of speech privacy

Click to view a larger version of Sound Transmission Class ratings and corresponding perceived levels of speech privacy.

Better: A drywall ceiling in conjunction with a lightweight roofing system provides a greater level of isolation because of its mass of drywall, compared to the mass of acoustic ceiling tiles.

Best: A drywall ceiling (in conjunction with a lightweight roofing system) hung on resilient hangers provides even greater sound isolation, by virtue of the resilient connection, or “decoupling”, of the drywall layer from the rest of the building structure.

The Future of STC-rated Assemblies

There has been a recent push by organizations, such as the Washington, D.C.-based U.S. Green Building Council in its LEED for Schools program, to provide STC-rated assemblies for all building components that enclose a teaching space, including the roof itself. For a quiet site, the target is STC 50, though that number is just a guideline. The LEED for Schools recommendations were based on ANSI S12.60, which defines recommended noise levels and reverberation times for classrooms, as well as STC recommendations for building components that enclose a classroom.

While specifics could be discussed at length, the overarching criterion is simple: Keep excessive noise from entering into spaces that require quiet, like classrooms, hospital rooms, auditoria and theaters. If those rooms are located adjacent to loud mechanical equipment, airports, highways or even the general din of urban living, the building components that separate the indoor spaces from the noise sources—walls, windows and even roofs—will need to be constructed in a way that keeps the noise out.

IMAGES: Threshold Acoustics LLC

Suggested Roof Components for Sound Isolation

Dense roofing board: DensDeck
Mineral-wool insulation: Roxul
Shredded-wood panel, rigid insulation and roof board: Tectum III

About Laurie R. Kamper, AIA

Laurie R. Kamper, AIA, is a consultant with Threshold Acoustics LLC, Chicago. Her musical background complements her architecture focus on acoustics.

Comments

  1. Very interesting article. I have recently been asked by a homebuilder if our concrete and clay roof tiles provide a STC rating for sound isolation. This article was one of my earlier findings after learning what STC was all about. We have long promoted tile roofs as being sound isolating because of the nature of the materials and construction of a tile roof. The varied, curved shapes of the concrete and clay tiles disperse much of the sound. The mass of the tiles absorbs as does the air mass beneath the tiles due to the lapping and the thickness of the tiles. We encourage enhancing the performance of the tile roof system in a multiple ways with our elevated battens to reduce the heat in the roof system by allowing ventilation, increasing air space for greater insulation as well as sound isolation, and allowing drainage of moisture out of the roof system.

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