Slate is a shale type sedimentary rock that is often made of clay and volcanic ash. This rock splits easily into sheets which is why it has been historically used for roofing and outdoor applications. The color range of this rock has made it enormously popular for both indoor and outdoor use. For an outdoor patio or step, slate is often a good choice since it has natural texture which reduces slip, it has varied color which enhances its natural appeal and it is reasonably durable if installed properly. Slate tiles can be challenging because tiles can vary in dimension with one side of a tile thin and the other end thick. Slate tile, as in this project, should be laid over an existing surface. We laid ours over concrete.

Finished slate step over concrete installation

Step 1

Sort out your tile for color, thickness and unfortunately for breakage. Slate is a bit more problematic than manufactured tiles. When choosing where to start a tile project it is helpful to keep in mind where it shows and where it hides. This means that lead or prominent edges are more visible or show more so you will want to use whole tiles at show edges and cut at less prominent edges. Lay out a few tiles (with joint allowance) to see how they fit on your concrete. If you are using a pattern, dry fit a few rows as well. Many people prefer to lay their whole tiles first and cut tiles last. Once you have your fit and your start point, be sure to clean your concrete thoroughly. (Power wash if necessary.)

Step 2

Mix your mortar using the five-gallon bucket, mixer rod and drill, or using the mortar hoe and wheelbarrow/tray. Wear a face mask, gloves and safety glasses. Your mortar should be moist but not runny. Only mix one-fourth of the bag at a time until you know how fast you will use it as it will dry out. Use a mortar tray or small plastic basin that is longer than your notched trowel. Use your paintbrush to wet down the area of concrete you are working on (keep the surface slightly moist). One small bucket should be kept with clean water while the other is used to moisten mortar or clean up. One sponge should be kept clean as well for the same reasons.

Mortared slate before grouting.

Apply the mortar using the ¾-inch notched trowel. Spread it evenly across the space of one or two tiles at a time. Select your tile and moisten it with a damp sponge. Slate tiles can be very uneven so you may need more mortar on one end. Check the relative level of the stone. Wiggle the tiles to settle them into the mortar and make slight height/level adjustments.

Step 4

Add spacers between tiles. Install your whole tiles being sure to vary your colors to achieve the proper slate effect. Try to make sure you don't have very uneven tiles as these look bad and may cause tripping.

Step 5

Measure and cut tiles using your wet saw. It is best to do all of your cuts at once to reduce costs if you are renting a machine. Lay all of your tile. On jobs too large for a single day you may want to cut the second day. At the end of each day of laying tile, mist down your tile and concrete with water and cover with plastic sheeting to raise the humidity and improve adhesion of the mortar and grout.

After grouting, the stone is wet from cleaning.

Use a sanded grout for outside applications to improve anti-slip properties. We found we needed to resift some of our grout because it had grout stones (small bits that got moist somewhere in handling). A kitchen-type flour sifter worked well. We opted for using a tile bag rather than a float because of the high texture of our stone and the depth of our tiles. Wearing a mask, mix your grout until it resembles cake frosting. This tends to be more moist than mortar. Mix enough to fill your bag three-quarters full.

Close-up of corner work. Cut edges are kept at the back of the tread.

Moisten the tiles and edges you are working with a wet sponge. Remove spacers. Squeeze grout between your tiles doing one to two tiles before tamping the grout in, running your finger along for a nice shallow dip and cleaning off the surface with your cleaning sponge and clean water (it will get dirty fast). You can also use a putty knife to press grout into deeper areas, such as where steps meet walls. Complete all sections of one area before moving to the next, cleaning as you go. When you are finished, lightly mist your finished project, cover with plastic sheeting for 24 hours to allow for a slow and more even curing of the grout.