Tile/Stone Talk: Installing Sound Rated Floors in Multi-Story Residences

By Gregory Mowat, Forensic Tile Consultants

Mortar shrinkage causing indent fracturing
Mortar shrinkage causing indent fracturing

In urban areas, where multi-use and multi-family dwellings are abundant, sound control and sound insulation is a prominent concern to all involved. Let’s review the regulations, guidelines and manufacturer recommendations to ensure a successful installation when it comes to sound transmissions and sound control.

Sound transmission in the International Residential Code (IRC), listed in Section AK101, is described as:

“General, Wall and floor-ceiling assemblies separating dwelling units including those separating adjacent townhouse units shall provide air-borne sound insulation for walls and both air-borne and impact sound insulation for floor-ceiling assemblies.

Section AK102.1 General, Air-borne sound insulation for wall and floor/ceiling assemblies shall meet a Sound Transmission Class (STC) rating of 45 when tested in accordance with ASTM E 90. Penetrations or openings in construction assemblies for piping, electrical devices, recessed cabinets, bathtubs, soffits, or heating, ventilating or exhaust ducts shall be sealed, lined, insulated or otherwise treated to maintain the required ratings. Dwelling unit entrance doors, which share a common space, shall be tight fitting to the frame and sill.

Section AK103.1 General, Floor/ceiling assemblies between dwelling units or between dwelling unit and a public or service area within a structure shall have an Impact Insulation Class (IIC) rating of not less than 45 when tested in accordance with ASTM E 492.”

Note the STC rating of 45 and IIC rating or 45 are for field testing. Laboratory test has STC rating of not less than 50 and IIC rating of not less than 50. The United States Department of Housing and Urban Development (HUD) in 1963 has Grade 1 (Luxury) buildings with an STC rating of not less than 55 and an IIC rating of not less than 55 to 60 depending on adjacency. HUD also provides guidelines for Grade 2 and Grade 3 construction, which are less stringent.

Tips for Installers

Sound reduction membranes may be trowel applied sheet or composite membranes that are bonded to a suitable substrate so that tile can be directly bonded to the sound control membrane.

The sound control membrane standard for Bonded Sound Reduction Membranes for Thin-Set Ceramic Tile Installation is ANSI A118.13. Verify that the sound control membrane being installed meets this standard.

As always, follow the sound control membrane manufacturer’s installation instructions explicitly.

Typical Symptom of Failure with Improper Sound Rated Floor
Typical Symptom of Failure with Improper Sound Rated Floor

Verify the floor is level/flat and acceptable prior to the installation of sound control membrane.

The finish flatness requirement for the floor prior to installation of a sound control membrane is ¼” in 10 feet for tile and 1/8” in 10 feet for stone tile installations.

If the floor is not level, then level the floor prior to installation of the sound control membrane.

Do not install thin, self-leveling underlayment over the top of sound control membranes.

Always use perimeter isolation when abutting restricting surfaces.

Installation of movement/expansion joints is mandatory for areas exceeding 24 feet in length or 12 feet in length when exposed to sunlight, in wet areas, and directly over any cold joints in the underlying concrete slab control joints.

Further, allow additional cure time for setting mortar before opening to foot traffic or use fast-setting type mortars which use special cements that bind residual moisture in the mortar. Latex- and polymer-modified mortar have increased cure times when installed over a sound control membrane.

Also, verify the moisture sensitivity of natural stone tiles or clay bodied tiles prior to installation over sound control membranes. Make sure the tile or stone tile is suitable and recommended over sound control membrane.

To accommodate shrinkage in mortar and prevent indent fracturing of the finish tile or stone tile, the maximum thickness of thinset mortar is 3/32” after the tile is embedded following ANSI A108.5.

Maximum thickness of dry set mortar for large and heavy tile (LHT) mortar (previously called medium bed mortars) installation as a bond coat over a sound control membrane is 3/32” to ½” after the tile is embedded.

For mortar beds thicker than 1/2“, use a wire-reinforced mortar bed that meets ANSI A108.1A, 108.1B, or A108.1C. The properly installed wire mesh is needed to control shrinkage of the thicker mortar.

Do not install tile or stone tile without grout over a sound control membrane. The minimum width grout joint for narrow joints is 1/16 to 1/8“.

Condominium or apartment conversion owners can hire an acoustical consultant to have sound tests performed following the tile or stone installation. If the sound test fails, the entire sound control floor will be torn out and a new installation performed to meet the IIC and STC sound control requirements. If you installed the floor and did not install the required perimeter isolation, then you may be held financially responsible for the failure.

A review made of the many sound control membrane failures include:

Not following sound control membrane manufacturer’s installation instructions explicitly.

Not leveling or flattening the floor prior to installation of the sound control membrane.

Using too thick of a mortar without wire reinforcing, and resultant indent fracturing of the tile or stone tile.

Not installing the required perimeter isolation where abutting restricting surfaces.

Penetrations installed through the sound rated floor voiding the movement of the floating floor. Anchoring door stops through the sound rated floor assembly may cause or contribute to a failure.

Installation of stone tiles with modified backs and using the wrong setting mortar.

Peel-and-stick sound control membrane losing bond due to tension caused by too thick of a setting mortar creating tension, and contributing to indent fracturing as well as a loss of bond between the sound control membrane and the primer. Sound control membranes are tested in compression, but not in tension.

The sound control membrane should abut cabinets and bathtubs with perimeter isolation. The sound control membrane does not need to be installed underneath cabinets or bathtubs. Should the floor assembly fail, then the bathtubs and cabinets would not need to be removed. Avoid point loading in small areas, i.e. using metal legs supporting a stone slab countertop in small bathroom, instead of using cabinets.

Not removing contaminants from the concrete slab prior to installation of the sound control membrane.

Finally and most importantly, remember to always work toward successful tile and stone tile installations.

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