10 2018 | TECH TECHNICAL FEATURE Hollow-sounding tiles and spot bonding Inspections can be revealing or misleading By Stephanie Samulski, NTCA Director of Technical Services Delving into the deep, dark corners of the internet yields some useful intelligence – no, not the dark web, but rather the back side of NTCA-managed websites. Here, using analytics software, plugs-ins, and other vir- tual gadgets, it’s evident there’s considerable interest in information about hollow-sounding tile as well as the sometimes related topic of spot bonding. It’s no surprise; the contributing factors for both have been on the rise, prompting articles, white papers, and revisions to indus- try standards over the last several years. This article is being added to the collection to give an overview and to generate member discussion online about these not- always-black-and-white topics. A hollow-sounding floor is probably fine When a floor sounds hollow, it may seem as though something is amiss, but usually it’s nothing more than the natural sound transmission of the installation. Central to the question of whether there is a tile installation prob- lem is whether the tile just sounds hollow or if there are visible issues occurring. Because of the various reasons a proper installation can have a hollow sound, language to clarify that a hollow sound alone does not indicate an installation problem was added to the TCNA Handbook in the Membrane Selection Guide. This is a clue that use of a membrane is one of the most common reasons a perfectly-good installation might sound hollow. Other reasons a good-quality installation might have a hollow sound include: • a less-dense substrate material, for example, wood versus concrete • a substrate or configuration that has open (hollow) spaces, for example, a wood-framed floor or steel- framed wall • an installation substrate or component with intentional voids – for example, some uncoupling membranes and hollow-core concrete slabs. As flooring materials go, ceramic is more of a transmit- ter, so hollowness will typically be more obvious, what- ever the cause. A real issue will probably be obvious Unfortunately, sometimes a hollow sound is because of a tile problem or is worsened by a tile problem. But when this is the case there will generally be more appar- ent evidence such as cracked, loose, or missing grout or cracked or loose tile, for which there is no other explana- tion. It might be possible to remove whole or large pieces of tile – without force – to check for any easily identifiable causes or contributing causes – such as “spot bonding” – a means of adhering and leveling tiles with “spots” or blobs of mortar instead of properly troweling out mortar to arrive at a more uni- form, consistent bond coat that minimizes the amount of empty space between tile and substrate. (Note: this article applies only to tile intended to be installed by a thin-bed mortar instal- lation method, not to the spot-bonding style installation meth- ods that are used and acceptable only on walls and only in specific, limited applications.) Although “5-spotting” has been widely observed and documented, any pattern of mortar “blobs” can be considered spot bonding.