Ask any stone or tile inspector what the number one test they use to determine if a tile is bonding and most will say, the sounding test. For those not familiar with sounding, the test is simple. A hammer, chain, golf ball, chisel, or any other hard apparatus is used to tap the tile. If the tile sounds solid it will produce a pinging sound. If it is not bonded or there is a void in the setting mortar, the tile will sound hollow and produce a thud-type sound. The sounding test is a very worthwhile test. However, with that said, it needs to be performed by a qualified expert who has a good ear for sounding since the sounding test can give erroneous results and can lead to incorrect conclusions. A sounding test should always be followed by a destructive test to verify the results of your sounding. A definitive conclusion should never be made based only on the results of sounding.
The following are some of the reasons that a stone or marble tile can sound hollow and detached when in fact it is not.
* Membranes- Many installations today use soundproofing, crack isolation, and other membranes. This is especially true for slab-on-grade installations. Sounding over these membranes can produce a different sound than the same tile or stone installed without one. The sound is difficult to describe, but it sounds muffled. The overall sound should be consistent. If not, a destructive test should be conducted in those areas where the sound varies.
* Voids in stone- If you are sounding stone, you must have a good understanding of the type of stone and its characteristics. Many stones, especially limestone, will contain natural voids within the stone. These voids can produce a different sound and may be mistaken for improper installation. The only way to verify this is a destructive test.
* Acoustics- (Low Ceilings) You must pay attention to what is above the floor you are sounding. If you are sounding a floor and the sound suddenly changes, look up and see if you have moved under a low ceiling or if the ceiling changes in any way. Metal ceilings will sound different than acoustical tile or other ceiling types.
* Trapped Air- Trapped air in the setting bed or substrate can also result in false hollow sounds.
* Sand Bed Installation- Many patio and pool deck pavers are set in compacted sand. The tiles are not bonded and the entire installation will sound hollow.
* Grout Joints- Believe it or not the width of the grout joint and how it is placed can also cause the tile to sound hollow. This is especially true if the grout is not pressed deep in the joint.
* Foam Shower pans- Foam shower pans are now being installed and will produce a hollow sound.
Sounding Test Tips
1. Always use the same device on an installation. My favorite is a golf ball but a hammer, broom handle, metal pole, etc can also be used. Be careful with handles that are hollow since you may be hearing the sound of the handle rather than the floor.
2. Be sure to tap the tile in several locations. I like tapping a tile in a grid pattern across the tile making sure to tap the corners.
3. Listen for a consistent sound. If the entire tile installation sounds hollow it could be the substrate and not a failed installation.
4. A floor that floods does not cause hollow tiles. See my article on flooding to find out why.
5. Getting Training to make sure you are performing the proper technique.
There are numerous reasons a tile can sound hollow and it does not mean the installation will fail. The best way to determine if there is a bonding problem is to conduct a destructive test to examine the setting bed and the bonding, coverage, etc.
Frederick M Hueston is the founder of Stone Forensics and has over 30 years of experience in the stone business. He has written over 30 books and hundreds of articles. He currently is Chief Technical Direction for SurpHaces and Director of SurpHaces Learning Institute. He hosts a weekly radio show, The Stone and Tile Radio Show and Podcast.