In earlier articles, we have discussed the basics of Pennsylvania Bluestone, its shapes, textures and colors. If you have read these articles or even simply reviewed a portfolio of pictures, you have a good feel for this wonderful natural stone. These factors, plus the process of installation are both important to its final appearance. Thickness is a key factor in installation options.
Two primary installation options exist for flagstone: dry and wet setting. Dry setting means installing over a compacted stone sub-base and a screeded stone dust setting bed with a flexible finishing joint material. Wet setting means installing over a concrete base on a mortar bed with a solid finishing joint material. Examples of flexible finishing joint materials include stone dust (aka screenings), polymeric stone dust, decorative stone, and topsoil. The most typical solid finishing material is mortar.
Natural clef pattern and irregular flagstone are available in two major thicknesses: 1” and 1 ½”. Because the thickness of these products results from natural separation along fault lines, the thickness is neither consistent nor exact. The thickness of one piece will vary across the length and breadth of the piece. If the thickness is generally between ½” and 1 ¼”, the flagstone is considered 1” flagstone. If the thickness is over 1 ¼”, it is generally considered 1 ½”. When flagstone is particularly thin, i.e., ½” or less, it is separated out at ½” flagstone. Some quarries will separate flagstone that is 3” or over into “heavy” flagstone. The extra thin and extra thick material are frequently included in 1” and 1 ½” categories.
For dry setting application, you will need 1 ½” material. This thickness is required to provide enough strength to handle minor shifts in the base and setting bed caused by geological and weather conditions. The wet setting application, you will want 1” material. This thickness has less variation within and between pieces making it easier to compensate for those differences in the mortar base.
Thermal flagstone is cut to a nearly exact thickness, usually 1” or 1 ½”. For dry setting, use 1 ½”. For wet setting, either thickness is acceptable as there is no variation in thickness to be absorbed in the mortar bed.
If you use 1” flagstone in a dry set project, you risk having the material crack or break over time. If you are doing the project yourself and wish to avoid the weight of 1 ½” material, you can accept this risk and replace broken pieces later as necessary. If you have a professional contractor installing your project, make sure 1 ½” material is used.