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Scratchcoat - For Floor Tile Installation


By: Harry Dunbar
The system shown below is what is know as scratch coat. It consists of wire lath and a skim coat of mortar (usually unmodified). It adds nothing of value to a ceramic tile or stone (floor) installation and in many cases ... will fail miserably.
If you are having your floor tiled with this system ... you're getting the short end of the stick from either the builder, the retail tile outlet or the tilesetter!
(I was invited to take these photos by the tilesetter doing the work.)
 
scratch coat This is a scratch coat application by one of the largest builders in Ontario. Shown here going over Oriented Strand Board (OSB)

The method of preparing a floor surface for a ceramic tile or stone installation is the most important factor for a successful installation, .... and the "Scratchcoat / Lath " method is NOT one of the recommended procedures. It is in fact the most ineffective base for ceramic tile. It's no wonder that in every respectable ceramic tile handbook, guide, or workshop this method has never been promoted or defended.

The system you see here is meant for walls, NOT floors.

In other words ... IT'S WRONG!
scratch coat So why are they using it for floors? Beats me. Maybe because the materials are cheap, or maybe because they just don't know better or maybe this is how they've learned to do it and because they've been doing it this way for so long ... they believe it's correct.

Ever question someone's method and they respond defensively with .... "well I've been doing it this way for 20 years."
To me that just seems like a ridiculous statement to make. It's almost like saying that ever since day one of their apprenticeship ... they've learned nothing?
The Terrazzo Tile and Marble Association of Canada (TTMAC) tested a scratchcoat system in their lab using all the required materials and procedures ... in many ways, probably even superior to what we'd find in the field. With their permission I've posted their conclusion to the tests given to the system similar to the one shown above.

In the words of the Terrazzo Tile and Marble Association of Canada (TTMAC):

"This installation failed to meet the lowest level of performance criterion defined by ASTM C627 for residential applications."


American Society for Testing and Materials (ASTM)
This system when used for floors has failed every test given by the Tile Council of America (TCA) and the Terrazzo Tile and Marble Association of Canada (TTMAC).

If anyone has questions regarding this method, or would like to understand how to determine the requirements for a successful ceramic tile installation over any type of substrate ..... the forum is a perfect place to find out the facts.
Go To The Forum



Failed Ceramic Tile Installation (over scratchcoat) ... Only 7 Months Old

scratch coat
At the request of the homeowners (Mr and Mrs. Manuel in Barrie Ontario) I inspected this job and took these pictures. Unfortunately the photos don't show the extent of the damage as well as I had hoped.

Harry Dunbar


The entire kitchen is like this. The grout is cracked and broken everywhere.
scratch coat

Grout has cracked to the point where it ends up inside the vacuum cleaner during regular maintenance on the floor.

He used the "scratchcoat" method which consists of wire lath stapled to the subfloor and then a skim-coat of mortar coating the wire.

... this job is only 7 months old.

After I scraped out the grout, the tile was simply laying loose on the floor. The tile had completely lost its bond due to all the stress caused by deflection (up and down movement of the floor).




Here's how it appeared beneath the tile on another shabby installation.

The same problem with this floor except a self-leveling-compound was used over heating cables. $800 worth of self-leveling has done absolutely nothing to make this floor free from deflection.

These tiles were completely loose of their bond and held in place by gravity and grout.

By adding solid blocking between the floor joists and a crack isolation membrane, this job MAY have lasted indefinately.

There are other factors which probably contributed to the failure of this job .... and a poor installation allowed those factors to present themselves. In this case (photos to left) it seems the mortar was exposed too long before the tile was set. The tiles may not have been back-buttered or have been pushed into the mortar properly.

There are so many cracks in this floor I can't honestly say they have all been caused by deflection. It's possible that expansion and contraction may have played a big part in the destruction of this ceramic tile system.

THOUSANDS of dollars worth of damage and no one to take responsibility.

Self Leveling Compound - incorrect.

The bond coat (thinset mortar), has even lost its bond to the self-leveling-compound in many areas.

Basically, the only real advantage (if any) to using scratchcoat (as shown in top 2 photos) is that it has no bond to the floor and will eventually become a crack isolation mebrane. A very FRAGILE isolation membrane. There are various other methods which resemble the above procedure which DO work. Mud beds are usually 1-¼" thick ... but in certain applications can be slightly thinner ... maybe ¾" thick.


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