On balconies and driveways false mud sets often have problems with efflorescence. The abundance of mineral deposits in stucco mortar often transmit to the top of an installation in the form of soluble salts. Rain, along with the minerals within the stucco combine and the white powder produced on top of the tile surface may never stop. An uncoupling would have prevented this from happening but unfortunately you cannot incorporate it with a false mud set.
This installation type has its roots in masonry and brick work but was adopted by tile installers to solve the problem with uneven substrates. While well intentioned, and capable of flattening or even leveling a tile floor during installation, it has some poor technical characteristics that can lead to failure at worst or an incomplete assembly due to its lack of accommodating necessary membranes in the least.
This article is based on this NTCA Technical Round Table-Region 4 in June of 2020. View this discussion to get an in depth view of what a false mud set is.
The application of stucco mortar to the substrate can make it very difficult to determine if complete coverage has been reached. Often around the perimeter of a tile when a false mud set is used there are voids. The first problem with the void is that the area in question does not have support to protect the area from impact or even load distribution. If a piano or refrigerator is rolled across a false mud set and a void under the tile is encountered, a break in the tile or stone is likely.
These voids can also trap moisture vapor. This can discolor marble and if the PH of the moisture is on the far ends of the spectrum, degradation of the stucco mortar is likely.
A false mud set, also referred to as the Miami Sandwich is a method of tile installation that is not recognized by the TCNA or ANSI. This article shows this methods limitations and encourages you to seek methods recognized in the tile industries handbooks.
So in closing I leave contractors and home owners with this. A mud set, false mud set, Miami sandwich, east coast how do you do, whatever you wanna call it, is not the worst in the world. They can be long lasting and beautiful installations. The only problem with them is they do require some good luck. In business if you rely on luck that means you prefer to gamble. Only you can decide how much risk is acceptable for your company or inside your home. Moving forward if you read this article you can no longer say you didn't know. Some day maybe with the right product development and additional techniques maybe a false mud set will no longer be known by such a name. But for now it's not in the industry handbooks, and that's a good enough reason to take warning and look into what is approved.
False mud sets are heavy. The installation could weigh over 16 pounds per square foot. An installation that uses self leveling underlayment is typically 11 pounds per square foot. This additional weight is significant. On a balcony or a wood sub floor this additional weight can be more than the structure can support. There is also the fact that all that mortar has to be brought on to the job, mixed and then placed. The amount of labor involved is impractical and totally unnecessary. A bucket of stucco mortar for a false mud set will usually on average only get an installer 9-14 square feet. Think about that, 14 square feet or three 24x24 inch tiles installed and then its time for another bucket. By contrast a typical modified thin-set mortar on top of self leveler can do between 45-55sq ft. And it can do it much, much faster.
SHOWER TILE INSTALLATION BUILT TO LAST
The proper thing to do when encountering a subfloor that does not meet the TCNA guidlines for flatness is to correct the floor before a single piece of tile is installed. View the video below on working with self-leveling materials.
This method is done by applying thin-set mortar to the substrate with a burn coat or keying in type of method. Some installers will use the notches of a glue trowel for this application. The next step is to apply mortar Stucco mix as a body or support for the assembly. It is usually piled in clumps or rows using a spade trowel and serves to increase the overall height of the floor, fill in low spots, or support a large heavy tile. The final component is a bond coat application of thin-set mortar to the back of the tile to be installed. It is also a scratch flat coat similar to the bond coat on the substrate.
After the three components are in place the installer will embed the tile into the stucco mortar while a level is used on top of the tile to affirm that the tile work is running level. A rubber mallet is used to beat the tile to the appropriate height. Installers using this method are often under the illusion that this is a method that requires a lot of skill. After all, they're using a level right? In actuality this is a very simple method that takes all of the technical strong points of a mud installation and throws them out the window. Don't get me wrong, A very flat floor can indeed be built like this. But this method can not provide a floor that can handle dynamic movement, provide maximum bond strength or accommodate membranes that would otherwise complete or re-enforce the tile assembly.
I have gone on several inspections in the last couple of years where a unit owner in a condo has said that their floor sounds like it has de-bonded. Their assumption is that because the floor has a hollow sound, the floor must be letting go. Upon inspection in these condos, there are usually two scenarios that lead to the hollow sound.
The first one being that cork was used as the sound control, whether or not a false mud set was used, the cork de laminated from the subfloor. This could happen because the cork was installed with the improper notch trowel, the subfloor was not clean and dry at the time of the install or the cork was a low quality variety. But more often this hollow sound is produced because a sheet isolation membrane was used, installed correctly, but had a marble or tile installed using a false mud set.
Sheet membranes used in sound control applications are intended to be close to the tile layer. This proximity helps disperse the sound laterally across the membrane reducing the transmission to the room below. A false mud set increases the distance from the tile surface to the membrane altering the characteristics of the intended sound transmission. A hollow sound is produced and I can tell you that while the floor has not de-bonded, a home owner that wanted a solid floor would deem the floor a failure when the floor starts to sound like a floating floor.
This can be even more problematic when the false mud set is particularly thick or the stucco mortar was a very wet mix. The membranes are impervious and when installed with an impervious porcelain above you have a recipe for de-bonding. The moisture vapor released during the stucco mortar curing weakens the bond coat on the back of the tile. Tiles often come off of the stucco mortar clean, as if thinset mortar was never applied to the tile. Here is a link to what real mud work on a floor is all about.
Uncoupling membranes and waterproofing
False mud sets eliminate the potential use ofuncoupling membranes. These membranes have several different features that make them very appropriate when a subfloor exhibits moisture vapor transmission, crack issues or if the new floor needs to meet a higher standard in terms of load distribution. An uncoupling membrane cannot be used below a false mud set. Uncoupling can be used on many substrates including a cured mud bed, but a false mud set is a fresh mud installation and has no proper way of including uncoupling.
While waterproofing can be applied to the substrate prior to the application of the stucco mortar, one cannot be used above it. A waterproofing membrane on the lowest level of an installation can be beneficial and at times required,but a false mud set on top of it can still allow efflorescence to occur. A waterproofing membrane in some instances is required on top of a cured mud bed to direct water to a clamping flange drain with weep holes. As stated earlier while a waterproofing membrane can be used in a pre-pitch scenario to the weep holes, this still does not help reduce the potential for efflorescence if a thick mud bed is on top of it.