The use of stone in both residential and commercial flooring applications continues to expand, far outpacing the growth rate of ceramic tile and other hard surface coverings. This incredible growth can be attributable to the beauty and variety that stone offers, plus when coupled with a steady decline in the cost per square foot of stone, it’s easy to see why designers are opting to upgrade their flooring needs to stone. However, for all the beauty that stone provides, its surface is still prone to cold temperatures and therefore a level of discomfort to homeowners. However, the use of radiant heating systems in conjunction with stone flooring is exploding, as more and more designers and homeowners become aware of its affordability and ease of installation. But while relatively simple, the installation of electric radiant heat under stone floors requires some knowledge of basic procedures to ensure a successful installation.

Before any work starts, we need to make sure that the surface is rigid enough to accept the stone flooring. The Ceramic Tile Handbook published by the Tile Council of North America (TCNA) explains that it is the responsibility of the project architect or engineer to determine if the substrate meets the proper stiffness and deflection criteria in accordance with the governing building code and the use criterion of the space. The substrate needs to meet the maximum allowable deflection for the finish and the installation system that will be installed. “Deflection” is the potential movement that the installation can experience when subjected to load and use. The suitable substrates for most electric heat flooring systems are:

a. Concrete

b. Mortar beds

c. Backerboard

d. Exterior Glue Plywood (EGP)

These surfaces should be prepared in accordance with standard industry installation practices and the electric heating system manufacturer’s installation instructions.

The electric heating system

Now it is time to address the electric heating system. Let’s start with laying out the space to receive the heating elements. It is essential to properly layout the room prior to installation of the radiant flooring, so that the correct size mat or system is ordered and installed. Improper measurements may result in hot or cold spots on the floor, costly returns or call-backs and ultimately an improper installation resulting in a failure. To aid in correct room layout, it can be helpful to select a manufacturer that offers electronic room layout software. This software will allow you or your customer to quickly and easily size a room, add whatever fixtures may be relevant, and it then automatically calculates the exact amount of square footage of floor warming needed.

Once the correct size mat is selected, installation of the system is fast and easy. It is helpful to select a product that incorporates a self-adhesive backing on the radiant mat, as this will speed up the installation. By using a mat with self-adhesive backing, the need to staple the mat down is eliminated, and the mat will lay flat, thus making it easier to trowel over because there are no waves or curves sticking up and interfering with the stone installation.

Even though drawings may clearly indicate the start and stop locations of the mat, it is advisable to do a “dry run” first to make sure the system lies properly. Due to the wide variety of mats available on the market today, follow the specific instructions of each manufacturer in terms of adhering the mat to the substrate prior to installing the stone tiles.

Installing the stone flooring

Now that the electric heating mat is in place, we can begin with encapsulating the mat and then installing the stone flooring. The electric heating system should never be connected or on during the installation of the encapsulating mortar, stone and grout.

Three basic methods can be used to encapsulate the mat. Be sure to follow the electric radiant heat manufacturer’s guidelines for this process. The methods and means of installation may vary from manufacturer to manufacturer.

1. The first one uses a self-leveling portland cement underlayment and primer. This method follows TCNA Method RH-116-05 and RH140-05. The floors must be properly cleaned prior to installing the primer and underlayment. Once the floor is clean, the primer can be applied and allowed to tack up. Then the self-leveling underlayment is poured to the required thickness. Most self-leveling manufacturers will prescribe a minimum thickness pour of ½ inch (12 mm). Then, allow the self-leveling underlayment to harden prior to installing the floor. This method is beneficial if the area is very large, and if a perfectly flat floor is required. It is ideal when using large-format stone. The flatter the floor, the easier it is to install large format finishes. It helps to reduce lippage in the stones.

2. The second and third methods follow TCNA Method RH-130-05 and RH-135-05. The first option is to encapsulate the radiant mat system using a latex-fortified portland cement thinset mortar and allowing it to harden. This method is beneficial if an adjustment in the thickness and height of the mortar is required. The mortar is used to fill in and around the wire mat and cabling - in effect encapsulating the entire radiant mat system. The trick is to get the leveling and encapsulating mortar as flat and level as possible, and the encapsulating mortar is then allowed to harden. Then the stones are set with the same mortar.

3. The other method that falls under this category uses the same latex-fortified portland cement thinset mortar in a one-step method of encapsulating the mat and installing the stones at the same time. Make sure that the mat is down flat. Additional staples may be required to hold the mat flat. The mortar is flat troweled to encapsulate the mat. Then, additional mortar is carefully applied with a notched trowel over the mat to install the stones. This is a beneficial method to use when there is a small area to be finished. It can be taken care of in one step.

Under all circumstances, installers need to be careful that their trowels and other tools that they may be using will not cut or nick the electric radiant system. The last method of installing the stones in the one-step method can be more prone to nick the wires. Most radiant heat manufacturers will provide a circuit-monitoring device that continually checks the electric mat circuit during the installation to ensure that no breaks or shorts occur.

If the installation is in an area where water can get on the floor, a waterproofing membrane should be used. The third method that was described would not be used if a membrane were used. Many waterproofing membranes also act as anti-fracture membranes. Thermal expansion and contraction of the stone caused by the cycling of the heating system can create stresses on the floor that may develop into hairline cracks in the stone finish. The use of these membranes can help to prevent this from occurring.

Always honor expansion joints that may exist in the substrate and bring them through the stone finish. Movement joints should also be placed at the perimeter of the installation, as well as in the field of large installations. As a rule of thumb, follow the guidelines outlined in the “Handbook for Tile Installations (NATC)” Expansion Joint Detail EJ171. Although this is a Ceramic Tile Industry publication, the Marble Institute of America also points back to this detail in its installation manual. Use a suitable flexible sealant that is non-staining and will hold up to the traffic and maintenance that the area will experience.

As the finishing touch, the grout used should really complement the stone installation. If a portland cement-based grout is chosen, the use of a high quality latex-fortified grout is always a good idea. For even better performance, cross-linking technology epoxy grout can be used.

Natural stone when combined with radiant floor warming offers one of the most durable, beautiful and comfortable floor systems available for residential or commercial applications. As consumers increase their demand for higher end stone floors, the use of radiant heat will mirror that growth. It is important that proper installation techniques are followed to ensure a successful installation.