How to Lay a Floating Porcelain or Ceramic Tile Floor Over a Concrete Slab That Has Cracks, Contraction Joints, or Expansion Joints

They said laying tile over a cracked concrete slab was impossible, 
so I designed a new method that produces a beautiful floating tile floor.

Concrete Basement Bathroom Floor With Cracks and Contraction Joint.

Floating Porcelain Tile Floor With Permanent Mortar and Standard Hard Grout Laid on a Bed of Sand and a Fiberglass Mat.

Concrete slab with cracks and contraction joint.

Awesome floating porcelain tile floor over cracked concrete.

Click to see an enlargement.

Enlargement Picture

Zoom Crack Picture

Click to see an enlargement.

Enlargement Picture

Zoom Tile Picture

Laying porcelain or ceramic tile over a concrete slab in the past was a risky proposition when using standard tile-laying methods. New concrete may look great when the tile is first laid, but the concrete is most likely to develop a contraction crack that also cracks the tile. Laying tile over an existing cracked concrete slab has previously been discouraged, because the two separate concrete pieces will eventually move and crack the tile. A previously recommended method of using an expansion grout near the concrete crack has negative ramifications, because of poor appearance and future maintenance. The expansion grout does not match the hard grout, and the expansion grout disintegrates over time.

General Installation Steps for the New Floating Tile Floor

The following is a general list of the major steps taken to lay a floating tile floor over a concrete slab that has both cracks and contraction joints.

Porcelain tile floor over cracked concrete slab.

Porcelain tile floor over cracked concrete slab.

Click the pictures to see an enlargement.

Detailed Installation Steps for the New Floating Tile Floor

The construction materials shown below were used. Click the picture to see an enlargement..

Medium sand.

Sanded Thin-Set mortar.

Color sanded tile grout.

V-notch trowel.

Large square notch trowel

Grouting strike-off tool.

Fiberglass mat.

Tile mortar mix.

Floor Tile Cutter.

Lay tile on the bare concrete floor to determine the beginning pattern. Either the tile center or tile grout line should be centered with a toilet. This is most important. Tile slightly off center will be very distracting. The tile at the walls should be no smaller than a half piece if at all possible. A narrow sliver of tile at the wall is very distracting and should be avoided.

Cut and lay one section of fiberglass mat prior to spreading the sand. Roll the mat 3/4 of the way back. Place weight or bricks on the end to keep the fiberglass from moving out of place. Spread the medium sand with the small 3/16" x 1/4" V-notch trowel as seen in the lower left picture. The goal is to spread a thin, even layer. The sand breaks the bond between the tile-mortar-fiberglass structure and the concrete slab to allow slab movement without cracking the tile. This creates a floating tile floor.

Unroll the first section of fiberglass mat carefully over the sand. The fiberglass mat should be heavy, with a tight weave resembling the texture of heavy canvas. The fiberglass mat used was Type 1708 Knitted Fabric, 25.3oz. X 38", +/-45 17 oz w/ 3/4oz. Mat. On hindsight this may not have been the best choice, because the double bias weave was fairly loose and the underside mat was not woven. However, other tighter square weaves were not available in small quantities as needed. A better material would be the Style #7781, 8.9oz. X 60" Cloth. Avoid weaves where the fibers are heavy in one direction but light in the other direction. A double bias weave (diagonal to the mat) appears to be best. Lightweight cloth is not recommended, because it could wrinkle easily while spreading the mortar and may not have enough strength. The mat used worked perfectly well. The woven side must be up in order to bond with the mortar.

Medium sand on cracked concrete slab.

Sand, fiberglass mat and tile over cracked concrete slab. Sand, fiberglass mat and tile over cracked concrete slab.

Overlap the sections of fiberglass 6" as shown in the picture just below the V-notch trowel picture above. Spread the mortar between the sections with the small V-notch trowel and above the overlap area with the smaller V-notch trowel to prevent a thicker section at the lap joint. Use the larger square-notch trowel in all other areas.

Lay a row of tile with the edge of the next fiberglass section about midway under the tile. This row overlaps both sections of fiberglass, creating a structural bond between the two sections of fiberglass. Overlap the fiberglass more than 6" if necessary to place the edge of the second piece midway under a row of tile.

Mix a small quantity of mortar as shown in the above picture below the square notch trowel. Spread mortar on the fiberglass with the larger square notch trowel. Ignore the instructions on the mortar which say it should not be laid on fiberglass. Those instruction are probably for epoxy resin fiberglass as used in building a boat. The mortar adheres very strongly to the fiberglass mat. Place the tile on the mortar. Do not press down hard, but press enough to make each adjacent tile level with each other. Do not place your weight on the tile. Place "X" spacers at the corner intersections of each tile.


Upon reaching the far end of the bathroom it became obvious that the layout at the vanity opening was not good. The decision was made to slide the entire floor 4" to the right as seen below in the picture on the left. A board with 10 nails protruding 3/8" was placed at the far end. Ropes were run to small pulleys placed at the base of the wall and extending out of the doorway. Pulling the ropes moved the floor. The pictures below show the procedure. The picture on the left shows the initial poor layout toward the wall near the doorway. The picture in the center shows the board in place. The picture on the right shows the results. This maneuver is not recommended, because the tiles in the new location may not lay perfectly flat on the floor, but it does prove the floor floats on the concrete slab.

Floor layout is not centered on in the vanity opening.

Floor is pulled 4" toward the camer.

Floor in new position centered in the vanity opening.

Cutting and setting the edge pieces is the next step. A new tile cutter was used for the straight cuts. An old diamond blade rock saw was used to make the special cuts. The opening at the toilet was made by cutting a hex-shaped hole in the tile. Both sides of the tile were cut to the six corners. Diagonal cuts were made from corner to corner on both sides to form pie-shaped pieces. The tile was placed on a common bathroom towel. The pie shapes were individually pressed lightly at the center until it snapped at the hex cutout. A good tile was made in two tries. The first one cracked in half because this procedure was not used.

Floor tile cutter.

Tile with hexagon cutout.

Tile with delicate cutout.

Grouting was done differently from the standard recommendation. The "X" spacers are removed. The color grout was spread in the grooves using a rubber kitchen spatula. The excess grout was scraped off with a rubber grouting trowel by moving in a diagonal direction. A sponge was not used at this stage as commonly recommended. The 3/4" end of a bricklayer's mortar tool was run along the grout grooves to quickly make a perfectly uniform rounded depression. The excess grout must be frequently wiped from the bricklayer's strike-off tool. The excess grout was wiped from the tile using a grout sponge. Rinse the sponge often and squeeze out excess water. The tile can still have a light film that will be removed later. Re-strike the grout grooves where necessary.


This recommendation is not standard practice for professional tile setters; however, this is not a standard tile floor. Walking on the floor before the grout is hard and strong will cause small grout chips to pop out. This fiberglass, tile and grout structure depends on the grout to resist compressive stresses. The fiberglass resists tension stresses. Standard tile floors do not function in this manner. The grout requires two weeks to achieved the compressive strength needed in order to resist the load.

Porcelain tile floor over cracked concrete slab.


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