Slate

By Tim Carter
©1993-2012 Tim Carter

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Summary: Slate is great for flooring. Slate tile appears to be random in size and placement, but the slate flooring pattern does repeat. Natural slate is set in a mix of silica sand and Portland cement.

DEAR TIM: Slate was used in the house I grew up in. The slate tile seemed to be a random mix of squares and rectangles. I'd like to install this material in my own home. Is slate flooring a job that can be tackled successfully by a beginner? What are the top things you need to watch for when you choose and install a slate floor? Steve R., Sarasota, FL

DEAR STEVE: The slate tiles you saw as a child did a great job of fooling you. Although they appeared random in size and placement, they were not. If you were to go back to your childhood home and study that slate flooring closely, you'd discover quickly that there is a pattern that repeats going both side to side in the room and front to back. This pattern oddly enough is called random Ashlar, because the intention is to make it appear the slate tile are placed willy nilly.

It's my feeling you can install slate tile if you are a person that pays attention to detail. There are several mission-critical steps that done wrong, will doom the job to failure. You need the same tools you would use to install ceramic-tile flooring, but in addition, you absolutely need a diamond wet saw or an angle grinder to cut the slate. Ceramic tile can be scored and snapped producing amazingly straight cuts. Try this with slate and you end up with shattered pieces with irregular edges.

Slate is an attractive and durable flooring material. There are many other uses for it as well around the house. PHOTO CREDIT: Tim Carter

Natural slate is set in a mixture of silica sand and Portland cement. This dry material is called thinset, and is mixed with water to the consistency of cake or pancake batter. It's important the thinset is applied evenly to the floor, and the tool that professionals use is a notched trowel. This tool creates a corduroy texture so that as you push the slate tiles into the adhesive it doesn't ooze out along the edges.

To prevent cracking after the slate floor is complete, it's important to ensure the subfloor has no flex. You can install slate on a wood subfloor system, but the floor must have no bounce to it and be as stiff as whiskey served at a Western saloon. It's also a great idea to install a crack-isolation fabric between the slate and the subfloor. This membrane prevents cracks that might transfer to the slate and grout in the event the subfloor moves due to seasonal humidity changes or minor settlement.

Slate floors also require great skills when you first lay out the pattern. It's really imperative that you maintain a consistent spacing between the pieces of slate. The joints between individual pieces is often 3/8 inch, but in some instances it may be slightly less. If you don't pay attention to this spacing, the different pieces in your pattern will not interlock well.

It's actually easy to maintain the spacing if you take the time and chalk lines that the slate follow during the installation. The primary line you start with should be parallel with the most visible wall in the room. If you make a mistake, the random pattern is very forgiving.

The slate tile flooring needs to cure in the thinset for at least 24, and preferably 48, hours. Without the grout between the slate, you can easily dislodge slate as you walk or kneel on them. Once the slate is ready to grout, be sure to apply a grout-release liquid on the top of the slate tiles. This material interferes with the bond between the slate and the grout, so make sure it doesn't drip onto the edges of the slate.

Grouting the slate floor tile is perhaps the hardest part of the job. By this time, you may have grown impatient to get the work finished. If you cut corners here, you'll regret it. The uneven cleft texture that imparts much of the beauty of slate is a demon when it comes to grouting. The sanded grout and grout paste sticks to these micro ridges in the slate like bubble gum in your hair.

Sanded grout is used because the joints between the slate is larger than 1/8 inch. The silica sand in the grout creates a miniature concrete mix. You can ruin the strength of the grout if you add too much water when you blend it, and if you use too much water as you sponge the grout off the top of the slate.

Before you decide on which slate to use, look at all the colors. Black slate is stunningly beautiful, as is green slate. My personal favorite happens to be red slate. The red hue is typically a deep earthy red that transforms a room with its rich character.

Selecting a color for a project like this can be stressful. After all, once the floor is down it's permanent. One method that has worked well for me for years is to make test panels. You can buy small amounts of slate of different colors and quickly attach them to thin pieces of plywood. You only need to make up panels that are slightly larger that 30-inches square to get a feel for the color.

Set these panels in the room where you intend to use the slate, and look at the color over a period of weeks in sunny and cloudy weather as well as at night. The slate will look different depending upon the light. If you desire the wet look, apply a wet-look sealer to the panels so you really see what it looks like.

How To Polish Marble

By Tim Carter
©1993-2012 Tim Carter

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Summary: Marble floor polishing can remove small scratches. You can polish the flooring either by dry sanding or wet sanding. Use special silicone-carbide sandpaper in range of grits to slowly work out those imperfections.

DEAR TIM: I need to discover how to polish marble. In my house there is some marble flooring that has seen better days. The scratches aren’t too deep, but they look bad. I also had to cut a piece of marble and now have an ugly dull stone edge that needs to look like the high-gloss polished finish on the top. What’s the secret? Do I need all sorts of expensive equipment? Do I need to use water? Paul H., Santa Barbara, CA

DEAR PAUL: You’re in luck. The great news is that you’ll be able to get very nearly professional results in short order. It may take a little practice, but I really think you’re going to be amazed at how easy it is to take a dull stone edge and transform it to a highly polished surface that broadcasts the natural beauty of the stone.

PHOTO CAPTION: A drill outfitted with a flexible pad equipped with special silicone-carbide sandpaper allows you to dry polish marble. PHOTO CREDIT: Tim Carter

Guess what? You don’t need thousands of dollars of expensive equipment to polish marble. It’s quite possible that you have a simple tool in your garage or workshop that will allow you to achieve very good results. At the bare minimum, you need a regular drill that has a variable-speed motor. If you have access to a stone grinder, that’s all the better.

Understand there are at least two ways to polish marble and other stones. You can use the dry method or the wet method. It’s hard to believe, but dry polishing works very well. This method creates a fine dust, so you need to work in a place where dust won’t ruin things. When dry polishing, you need to wear both eye and breathing protection. It’s not a good idea to ingest stone dust into your lungs, especially stone dust that contains silica.

Using special silicone-carbide sandpaper that’s made for stone polishing, you can cut away successive ultra-fine layers of the stone until you have a high-gloss finish. Sandpaper can be purchased that attaches with adhesive or hook and loop technology to a flexible round sanding pad. These pads attach to a regular drill or to a special mounting base that connects to a stone grinder.

The polishing process starts with a coarse-grit sandpaper that has a low number like 24 or maybe 60 grit. As the grit number gets larger, the size of the silicone-carbide particles gets smaller and smaller. Grit sizes march through the double digits, triple digits and beyond. I guarantee you that you’ll be using 120 grit, 220, 400, 500, 600 and over 1,000 grit.

The process starts by using a coarse-grit paper and cutting away a fine layer of the stone to remove the scratches or saw-blade marks. It’s mission critical that you cut away any and all grooves leaving a surface that may be rough, but all in the same plane. If you leave small grooves, tinier than the thickness of a piece of hair, you’ll see these as you start to achieve a high polish.

The high-gloss finish works to your disadvantage as you polish. The higher the gloss, the more light reflects off at a different angle from imperfections. This is why you now see the scratches in your marble floor as light that hits the scratches bounces away from you at a different angle than the light that reflects off the polished areas that are free of scratches.

When you use the tools, you need to keep them moving. By that I mean that the drill or grinder will be spinning as it’s on, but don’t keep it stationary in one spot. You must slide it constantly across the surface of the stone so the sandpaper doesn’t cut a circular groove into the stone. Moderate pressure is all you need. Let the sandpaper do the work for you.

It’s best to use many different grits of sandpaper as you start to achieve the polish. You may start with 24 grit, then use 60, 120, 220, 320, 400, 500, 600 and then 1,000. If you skip a grit, you may end up with tiny scratch lines that show through the polish.

You can also use dry diamond polishing pads in the same manner. They also come in different grits with some grits that are as high as 3,000.

It’s best to test your skills on a scrap piece of marble first. Try to get a piece that matches your floor marble now so that you can see what it’s like. Use some regular sand to scratch the scrap piece and then start to see if you can polish these scratches out like a pro. Once you have achieved success with the scrap piece, advance to your floor and to the cut edge.

Wet polishing or sanding has some advantages as the water helps keep dust down and it reduces the friction of the abrasive materials allowing them to last longer.

But the downside to wet polishing is the water on the stone causes the color of the stone to naturally deepen tricking you into believing you’ve achieved success. You may feel you have a gorgeous polished surface as you’re working because the color of the edge or surface matches that of the polished top. But when the stone dries, which can take a while depending upon the porosity of the stone, a dull or scratched surface shows up. It takes quite a bit of practice to achieve professional results using wet-polishing methods and tools.