Flagstone is nothing more than sedimentary rock. Flagstone colours vary, as do the content of the rock's silica, sandstone and the other natural materials it contains. Flagstone -- such as sandstone, limestone, etc. -- can be cut or split into slab layers and used as pavers for walkways and patios. But these flagstone slabs can produce a hollow sound. Problems with hollow flagstones include base creation, bonding and water impacts.
The base for your flagstone tiles or pavers must be level in order to prevent the problem of hollow-sounding stones. It must also be clear of any debris that could cause the flagstone to be unlevel compared to the neighbouring stones, or to not bond properly. The flagstone pieces that are placed on a poorly constructed base -- especially if small -- are more likely to become loose and to sound hollow when used.
Once your base is level and your bonding agent has been chosen, check to ensure that your flagstone does not have any residual debris on it. Dirt or other matter remaining on the flagstone can prohibit bonding from occurring, too, which can also result in the hollow sound being produced when it's walked on. Bonding time and agents vary based upon the flagstone used, but should be followed according to recommendations on the label or according to the installation specialist your consult.
When flagstone does not bond to the ground below, it moves. Bonding agent attaching it to nearby stones can work loose as well, making it not only an irritation in regards to sound, but a potential danger for those who walk upon it.
Some geographical locations are more prone to freezing temps than others, which can impact the flagstone you install. If a poor base was constructed due to bad weather conditions in the first place, or the weather prohibited adequate drying of the bond agent, your flagstone can sound hollow. Any time the flagstone has moisture left beneath it, or allowed to seep in around it, the hollow sound can occur, indicating the stone is not adequately secure and grounded.
The larger and heavier the flagstone, the least likely it is to not properly adhere to the bonding agent or make a hollow sound when walked upon. In addition, if the bonding process is begun, but the installer does not get all of the flagstone installed before the bonding agent dries, this can cause the hollow sound as well.