GreenSage E-zine Fall 2009

GreenSage Guide to Slate Tile
by the GreenSage Team

Slate is a natural rock morphed over thousands — or even millions of years — from sediment that has been compressed under tons of the earth's pressure and heat. This natural process causes such profound physical and/or chemical change that slate is considered a metamorphic rock, "changed in form" from another type of rock. Slate is just one example of metamorphic rock. Others include gneiss, marble, schist, and quartzite. Metamorphic rocks make up a large part of the Earth's crust.

Slate is made of thin laminated layers with elements of quartz, mica, calcite, chlorite and other types of rock in it. It mostly comes from Brazil, Vermont, China and India, but can also be found in Wales, Germany, France or Italy. Generally its formed in riverbeds from geographically varied sediment so that slate from these different parts of the world look very different. The tiles come in multiple colors and vary in thickness, even among the same batch.

Characteristics
Slate tile is naturally beautiful, its colors varying from browns, rusts, grays, greens, golds, pinks and black — all in varying hues. Its widely desireable as a surface for interior and exterior floors and walls, not only because of such a wide color compatibility, but its a highly durable material, which we love for the sustainability of any durable material.

Other sustainable qualities include its inate fireproof and non-slippery characteristics. Although not a rapidly renewable material, there are extremely high grade slate deposits all over the world making this an abundant material. Its cost effective and gaining in popularity as our culture turns toward a more natural and 'green' aesthetic.

Slate is available in different levels of waterproofing, hardness and compressed strength. It can be classified in four main ways. First, by color. Then by size. Commonly, slate is produced in all sizes of squares and rectangles, including very small mosaic tiles to tiles that are over 2 feet wide. There's bullnose, pencil and other trims and recently a Corinth mesh of varying narrow width rectangles. They're also available in irregular flagstone style. Thickness is a third classification varying by manufacturer, but generally they're under one inch thick. Lastly, slate can be classified by type of finish. Some slate tiles are tumbled for a smooth, more naturally edged look, others are cut very straight. They can be glazed matte or glossy, or darkened with a stain finish.

Considerations
Slate can be susceptible to breaking along the layers if not installed properly and cutting it can sometimes be tricky because of this layering. So if you're a do-it-yourself homeowner, be prepared to experiment. Its important that slate is installed on a solid subfloor like backerboard. Flexible subfloors, like sand outside or plywood inside, could cause the stone to crack.

Consider maintenance and upkeep. Slate needs to be sealed. Some say it needs to be sealed repeatedly over the years to prevent wear and stains but, I personally have slate throughout my home, installed 5 years ago and still have no need to reseal. However, although it has been installed on the bathroom floor, I chose a faux slate for the shower floor simply to avoid this issue where soaps, oil products and the pressure of water would wear down the sealant.

Tiles are irregular and can pose a tripping hazard where (or if) the edges stick up above one another on the finished surface. For us, this seems to only be problematic when we wear flip-flops or sandals that can catch on these edges.

VOC's, Radon and other IAQ Hazards
Slate emits NO VOC's, and according to both Bion Howard, Building Environmental Scientist, and radon expert, Dave Saum, neither has run across any situations in 10+ years of radon mitigation research where large hunks of rock, like slate, has been found to be a significant source of radon in houses.

Sealing the Slate
While not absolutely necessary, most homeowners prefer to seal a new slate floor to help protect it. First, clean the slate thoroughly. If needed, a vinegar/water mixture can get off the toughest crud. Wipe completely with large sponge and plain water. Allow to dry completely. Be sure to use a water based, oil- and stain-resistant penetrating sealant specifically made for slate. Its not only low in VOCs (volatile organic compounds), water-based polyurethane is non-yellowing and non-cracking, giving you years of good looking low-maintenance slate floor tiles.

Avoid applying it over wet or waxed slate. Apply with a paint brush, pad applicator or large, clean cloth in a thin coat, following the grain of the slate. Let it dry according to sealant directions or for two hours before applying a second coat. Stay off the floor for 24 hours while the sealer cures to avoid foot marks.

If you prefer the natural unsealed beauty of slate, it is still a good idea to seal the grout lines with tile sealant. Work the sealant over the grout lines with a small, 1/2-inch paintbrush and remove any excess. Allow it to dry for at least four hours.

If you prefer to use a premixed, waterbased, urethane grout, it doesn't need sealing. The colors are made to remain fixed (i.e., not fugitive) and it requires much less maintenance. You may want to clean and seal the slate prior to installation when using this type of grout.

Don't begin grouting too soon. Let the mortar dry and cure before grouting the tiles. It’s a good idea to wait at least a day before grouting your slate tile in order to allow the mortar to properly set. Some tile installers recommend mixing penetrating sealant with grout instead of water to increase the bonding strength of the mortar and grout and improve water resistance.

Maintaining a slate floor is easy. Just dry mop or vacuum regularly and clean it with mild soap and water, or just water alone, when needed. Waxing isn't necessary. It won't hurt, but it will fade the natural beauty of the slate. It can turn the tiles a little darker and may yellow the grout, unless its polyurethane grout.