As a Stone Consultant and Stone Contractor in Florida, and particularly South Florida over the past 30 years, I have observed many improper installations by local subcontractors. In particular, adhered stone installations for wall cladding using the spot bonding method (be it 4 spot, 6 spot, etc). Based on my experience any stone or tile adhered directly to the substrate should follow ANSI tile installation standards which require 80% coverage on the back of the tile for interior surfaces except showers which should be 95% and exteriors at 100% coverage.
In many cases even thick stone (3/4” or 1-1/4” thickness) has been installed using a variety of bonding mortars and also spot bonding methods but using no means of mechanical anchorage on interior walls as well as exterior walls. Many times the Owner, Architect or General Contractor are not aware of the proper installation methods and accept a low cost alternative to a mechanically anchored system.
Industry Installation Standards MIA – Marble Institute of America - Dimension Stone Design Manual
ANSI – American National Standards Institute – Standards for Installation of Ceramic Tile
TCNA – Tile Council of North America Handbook for Ceramic Tile Installation
The MIA Design Manual recommends using only ¼” to ½” thick stone up to heights not to exceed 15’-0”. The size of the stone panels should not exceed 36” in the longest dimension or be more than 720 square inches in total area. The weight of each piece should not be more than 15 lbs. per sq. ft.
The MIA endorses the use of some of the installation methods detailed in the TCNA Handbook for Ceramic Tile Installation for installation of stone tile. The methods endorsed for wall tile installations are W201, W202, W211, W221, W222, W231, W241, W243 and W244E. For stone tile walls directly adhered to concrete or stable masonry surfaces with thinset mortar W202 is the required method. Please note that a waterproof membrane is required for all exterior installations. TCNA method W215 Spot-Bonding Epoxy for ceramic tile, is as of the date of this writing not endorsed in the MIA Design Manual for adhered stone. The TCNA refers to the epoxy manufacturers specifications for ceramic tile installation instructions using this method. Also of note, movement joints are always mandatory per Method EJ171, as detailed in the TCNA Handbook (see below).
Movement Joints - Vertical and HorizontalEspecially important to a successful adhered wall cladding installation are the placement of movement joints. The TCNA Handbook establishes the guidelines for movement joints as follows. Interior movement joints should be placed 20’ to 25’ in each direction. Exterior movement joints should be placed 8’ to 12’ in each direction. Also, interior tile work exposed to direct sunlight or moisture should be placed at 8’ to 12’ intervals in each direction. Please note, in areas where the stone tile is spanning different materials such as concrete columns or slab lines to adjoining masonry walls or other changes in backing materials, such as gypsum board, then a movement joint is also necessary. Basically, a soft joint should be located where any dissimilar materials meet since they have different rates of expansion and contraction as well as differing movement as a result of differential movement of the building structure.
Substrate TolerancesWhen adhering directly to the backup substrate it is important to inspect for tolerances of the substrate and proper preparation by other trades. The MIA requires a more stringent tolerance for the backup substrate than the TCNA does for a ceramic tile installation. The MIA requires the backup to be within 1/8” in 10’-0” of the required plane for thinset stone veneer where the TCNA requires the backup to be within ¼” in 10’-0” for installation of ceramic tile in thinset mortar.
Joint SizeAnother important consideration for an adhered stone tile installation is joint size. Optimum joint size should be considered relative to the size of the stone tile and the stone tile fabrication tolerances in order to mitigate the appearance of lippage. Lippage, an installation tolerance, will be covered in an upcoming Blog Post.
Typically, and in accordance with the MIA Design Manual guidelines, exterior stone wall cladding should have joints that are ¼” width minimum but 3/8” preferably. For larger size exterior wall panels ½” wide joints are frequently required depending on inherent warpage and stone fabrication tolerances. Interior stone wall cladding should have joints that are 1/16” minimum but preferably 1/8” in width depending on stone fabrication tolerances and inherent warpage. For large panel size units ¼” or larger width joints are required frequently. Tight, hand-butted joints should not be used under any circumstances.
Mesh BackingDepending on the type of stone material used you may come across stone tile panels with fiberglass mesh reinforcement applied to the back side of the tile. This reinforcing mesh is embedded in an epoxy or polyester resin. This method of backing is generally used to reduce breakage and make handling safer when working with large slabs of fragile stone. However, please be aware that cementitious materials, such as thinset mortars, will not adhere properly to epoxy adhesives. Only compatible epoxy adhesives should be used that provide the required bonding strengths to the fiberglass and epoxy backing you are bonding to. It is interesting to note that a well known manufacturer of epoxy stone adhesive has specific requirements for removing any epoxy and mesh backing down to the bare stone where you will be using their adhesive to bond to the back of the tile. This should be done using a mechanical wheel grinder with a diamond blade before application of their epoxy stone adhesive.
Please let me know if you think this post was helpful and any other information you would like to share regarding your experiences.
David Ball-www.davidball.net" 
