Installing Slate Tile

A closeup image of slate tiles and grout
Filmwork / Getty Images

Materials Required

Beautiful slate tiling
 TrendTreasures Inc.

A slate floor can be a beautiful and unique feature that can add character and value to a variety of interior and exterior spaces. Born in the belly of a mountain, these distinct pieces are very individual with one of a kind features found in every single tile. This can lend a sense of strength, interest, and visual appeal to your application.

However slate can be relatively expensive to purchase, and the price will almost double when you have to pay for professional labor to do the installation work. On the other hand, undertaking the task yourself can be a physically daunting challenge that requires skill, strength, and back-breaking effort to move, set, and properly place each piece. The decision to undertake a project like this should not be hastily made. It will take 1-3 days to complete the task on average, and then at least 3-4 days for drying, making it an incredible effort. 

When purchasing slate tiles ​make sure that you get 10%-15% extra to account for breakage and cuts. You also want to hold on to at least one extra box after the work is complete, as that will give you a set of matched materials which can be used for repairs down the line.

Materials Required

  • Slate Tile
  • Cement Board
  • 1 1/4" cement board screws
  • Mortar
  • Tile Spacers
  • Stone Sealant
  • Grout

Required Tools

  • Line chalk
  • Notched trowel
  • Tape measure
  • T Square
  • 2X4 Wood Piece
  • Rubber Mallet
  • Grout Float
  • Foam Brush
  • Work Gloves
  • Wet Saw
  • Goggles
  • Drill
  • Needlenose Pliers
  • Utility Knife
  • Sponge

Preparing the Subfloor

A prepared subfloor
TrendTreasures Inc.

Before you begin you want to make sure that the subfloor surface is perfectly smooth and flat. Any defects, even small ones, can become weak points in the installation later on down the line and can lead to tiles chipping or cracking when walked across.
If you have a plywood subfloor, then the surface should be sanded thoroughly to create a smooth, even face. With concrete, you can use filler to fix any dips or depressions that may exist. Afterward, wait for any liquid agents to dry thoroughly, then sweep the area completely clean, so that dirt particles do not end up getting trapped down there.
Cement board sheets should then be installed over the subfloor to provide a layer of protection for lower surfaces. These waterproof materials will also prevent moisture from seeping down between tiles, causing damage to lower portions of the structure.
The cement board sheets should be placed side by side, running from one end of the room to the other. A shop knife can be used to cut smaller pieces to fill uneven spaces reaching to adjacent walls. These can be secured to the plywood using 1 1/4” screws spaced evenly down the lines between individual pieces. If installing over concrete, a thin-set mortar can be spread evenly to create adhesion.
Note: as you work leave 1/8” gaps in between the individual sheets, and 1/4” gaps between the cement board pieces and the walls, to account for expansion during warmer weather periods.

When dealing with concrete subfloors, you may want to opt for the application of a waterproofing membrane as an alternative to the cement board. That self-leveling substance will create a clear, impervious coat, and eliminates the need to adhere to the sheets using screws.

Establish Reference Lines

Hands establishing a reference line
TrendTreasures Inc.

Using a tape measure, find the exact center of the longest wall in the room. This can be marked near the floor using a shop pencil. Then go to the adjacent wall and do the same. Then, with the two marks as your guide, snap a chalk line between them creating a temporary path that should completely bisect the room.
Repeat this process with the other two adjacent walls in the room, measuring to the exact center of each, and then creating a line chalk snap between them. This should divide the space into 4 even quadrants with a cross in the exact center.
To make sure that the two lines are perfectly perpendicular, you can use a T Square to measure the angle that they make. Alternatively, you can use the 3, 4, 5 methods, measuring the two lines to 3” and 4” out from the middle, and then making sure that the third measures 5” in length.

Dry Lay Out The Tiles

Laid out dry slate tiles
TrendTreasures Inc.

Slate is a unique material, and it can sometimes be volatile. This is especially true when using multi-color options. Before you go any further, it will be useful to dry lay out the tiles in the room and arrange them so that the colors and patterns which emerge within their surfaces all work together to create an attractive whole. This gives you the chance to fit everything together like a master puzzle before committing to a look with mortar and adhesive.
When you are satisfied, stack the tiles up in distinct piles that will maintain the order in which they were laid. During this process place them face to face, and back to back, and do your best to avoid any scratches that can come from them rubbing against one another. 

Spread The Mortar

Spread mortar for slate tiles
TrendTreasures Inc.

When you’re ready to begin, mix the mortar in a plastic bucket using the water to material proportions as stated on the manufacturer’s packaging. A wooden paint stick can be used to stir the ingredients until they have a consistent feel. Be careful not to mix more than you will need for about 20 - 30 minutes worth of work or it can start to dry and harden before you get a chance to use it.

This can be a messy process. You will want to wear work gloves and may consider knee pads to protect yourself from long periods of bending over.
Using the flat end of the notched trowel, scoop some of the mortar up and begin spreading it onto the cement board, starting at the center of the cross point chalk line you created, and moving out across a single quadrant. Only apply enough to cover a little more than a single tile will need. You should then run the notched edge of the trowel through the mix to make grooves, which will create a more powerful hold when the slate tile is put in place. 

Setting the Tiles

Installing a slate floor tile
TrendTreasures Inc.

Once you have enough mortar spread, take the first tile and place it into the adhesive bed firmly. Use enough force that you collapse the notched lines you created earlier, but not so much that you drop down to the cement board below. Twist the tile slightly to give it a more firm grip. You can also lightly tap the piece with a rubber mallet if you feel that more pressure is needed. Be careful not to get any of the mortar on the surface of the piece, and wipe it away with a damp cloth if necessary.
Note: The corner of this first tile should line up with two of the chalk lines at the center of the first quadrant that you divided the room into.

Tile Spacers

Using tile spacers
TrendTreasures Inc.

Once the piece is in place, put tile spacers around all four corners. This will help to create consistent gaps between them, which will later turn into straight and even grout lines. You can then spread more mortar, trailing down the line of chalk, and place a second tile next to the fist. Surround the open edges of this piece with more tile spacers. This process can continue in a straight row until you reach the far wall. 

Level the Tiles

Leveling slate tiles with a hammer and board
TrendTreasures Inc.

Once you have three or more tiles placed, you can even them dimensionally by laying a piece of 2X4 wood, covered in carpet, across their surface. Tap this lightly with a rubber mallet to ensure that no piece is any higher than any others.
If you are using a particularly dimensional slate with lots of peaks and gaps, then you may want to avoid this step as the irregularities in the material may lead to inconsistencies in height. In those cases employ caution, and do your best to eyeball the relative depth of each piece.

Cut Tiles To Fit Along Walls

A tiling wet saw

Once the first row of slate tiles reaches the wall, you will often end up with a gap. Measure the distance from the tile spacer on the last piece to the end of the room. Then employ a wet saw to cut a tile down to the dimensions necessary to fill that area in. When ready, place the specially sized piece into the space to complete your first line.

Be very careful when using a wet saw. You always want to wear eye protection, and might want to wear earplugs to protect yourself from the loud sounds that these machines can make.

This entire process can then be repeated, starting at the first tile placed, with a second being installed into mortar right next to it. Continue using tile spacers to ensure even grout lines, and work your way down with the first row acting as a guide.

Note: Use strategy when laying tiles. You will want to walk on the finished floor as little as possible until it dries, so try to establish a plan that places you in a position to escape to the next quadrant, and eventually from the entire room when done.

Removing The Spacers and Excess Mortar

A hand removing excess tile mortar
TrendTreasures Inc.

Once you have the first quadrant complete, use a pair of needle-nose pliers to pull the spacers up out from between the tiles so that they don’t end up getting into the mortar as it dries. If any adhesive has seeped up into the spaces between individual pieces, employ a shop knife to remove it.
Then you can move on to the next quadrant, laying your first tile in the center space bordered by two lines of the chalk cross that you created. Continue to place spacers between individual pieces, and work in rows following the already installed slate as a guide to ensure that everything comes out straight, even, and consistent. As noted earlier always be aware of your position, and don’t back yourself into a corner that you will not be able to escape from without stepping on your finished but not yet dry work. The method for leveling pieces can also be employed periodically to keep the installation as even as possible.

Allow The Mortar To Dry

Slate mortar drying on new tiles
TrendTreasures Inc.

Once you’re completely done, give the mortar adhesive at least 40 hours to dry. Keeping the space well ventilated with fans and open windows can help to speed up this process. In some cases, the manufacturer’s packaging will indicate that it requires a longer period to fully set. Always follow their instructions, and do not allow anyone to step on the floor until it has hardened into a solid installation. 

Sealing The Tile

Sealing a slate floor with a brush
TrendTreasures Inc.

Slate is a strong, earth-born material but it is also naturally porous and can be susceptible to stains and dimensional discolorations. Once the mortar has dried, wipe away any excess that may have gotten on the surface. Then seal the entire installation to clog the pores and create an invisible protective barrier over it.
You will first want to apply a below surface sealing agent, which will go down into the tiny holes in the stone and clog them. This can be poured into an old coffee can, and then spread across the tiles with a small foam brush, using very thin, even strokes. Do not allow it to bubble up or accumulate into puddles. If this occurs, then use a dry foam brush to smooth the puddles out.
Allow this to dry for a few minutes, and then apply a second coat of above surface sealant to the slate. This will create an invisible but powerful barrier over the top of the stone, giving it further protection.
Give the sealant about 1 - 2 hours to fully dry and set into the stone before proceeding to the next step.

Grouting The Slate

Grouting a slate floor
TrendTreasures Inc.

Mix a small amount of grout in a plastic bucket, following the water to material proportions as stated in the manufacturer's packaging and instructions. You want to make sure that the mix has a nice consistency that isn’t too soupy, or too thick.
Scoop a small amount of the grout up using your grout float, and then apply it directly to the line gaps that you created with the spacers in between the tiles. Start at one of the far walls, and then work your way down each line, holding the float at a 60-degree angle, and being careful to push as much of the material into the gaps as possible. While you work, wipe away any excess that accumulates on the surface of the tiles.

Some grout will get on the tiles during this process, that is inevitable. Luckily the sealant that you applied will protect them from permanent damage, and you will be able to remove it during the next step.

Removing Excess Grout

Cleaning a slate floor
TrendTreasures Inc.

Use a large damp, but not saturated sponge, and run it along the surface of the slate floor to wipe away any excess grout that may have accumulated. Make sure not to get the installation too wet, as excess moisture can seep into the grout in the gaps, creating a muddy mess.
Once done, allow the grout a solid four hours to completely dry. Then repeat the process with a clean sponge, wiping any haze that might be left behind from your work. If necessary, soft cloth can help with the cleansing process, while also wiping up excess liquids that can accumulate.

Seal The Grout Lines

Sealing slate tiles with a brush
TrendTreasures Inc.

Once the grout has had a chance to fully dry and is hard to the touch, it needs to be sealed to protect it from stains and water penetration. A below surface sealer should be poured into a tin can and then spread along the lines using a clean foam brush. Make sure that you work methodically down each portion of the floor so that you don’t miss anything. You should also be careful not to apply too much sealant as that can cause the grout to become muddy again.
If desired, a second coat of sealer can also be applied to the full installation. This will better protect the tiles against future damage. Afterward, wait an hour two before walking on the surface. You will then have a brand new, fully installed slate floor, ready to add personality, character, and energy to the environment surrounding it.