THE NOT SO SMALL FINE PRINT
Jun 5th, 2018 / A+ | a-
By Mark Violand
As consumers we buy stuff, we use stuff, we break stuff and it eventually wears out. When the stuff we own breaks we may then read the warranty to see if the part that broke is warranted. Sometimes we will read in the fine print that what broke on the stuff we have is not warranted, because it says so… in the “fine print”. We grumble because back in the day when mom or dad said “because I said so” it made us grumble then too.
What about the directions on how to use the stuff we have and we just can’t seem to make it work the same way we saw on YouTube, in the infomercial, at the Home and Garden show, or County Fair. We scoff and write a mental note to ourselves that “I’ll never buy that (favorite expletive) again.”
Grumble…grumble….grumble.
Over the past couple of months I inspected several natural stone installations that unfortunately failed, because in this inspector’s opinion, someone did not read the “fine print.” In all actuality the fine print was not so fine and/or hidden, it just needed to be read and understood.
In one home cracking was occurring in two large bathrooms. One bathroom had 18” x 18” polished marble and the other bathroom had 16” x 16” filled travertine. A total of approximately 800 feet of flooring that is trash. In another home the floor was 12” x 24” Vein Cut Limestone. It too is trashed.
As you can see in the photos below, cracks in the tiles are very long and run the entire length of the room or area and measure 4 feet apart, following the Oriented Strand Board (OSB) subfloor joints. And the cracks that run the opposite direction measure 8 feet apart, again the size of the subfloor panels. In many cases either the builder or the flooring contractor will sand the OSB making the floor and most especially the joints in the subfloor smooth and flat. In some cases cement board is now installed, except in these two cases. I ruled out deflection because I felt absolutely no movement in the floor when walked on, and I did not see cracked tiles in random areas or tiles that have lost their bond. I turned my attention to what were the layers under the natural stone supporting it.
Placing a steel ruler over the cracks revealed a gap between the top of the tile and bottom of the ruler indicating the tile was pulled down and as further investigation revealed, the cracks in the tiles were directly over the subfloor joints. The technical term for this phenomenon is called reflective cracking. The tile is reflecting whatever is occurring underneath the tile. In the previous paragraph I said the subfloors are sanded flat and in each of these cases the subfloors were OSB. During the build of the home the OSB may be rained on and absorb moisture, most especially at the edges and will become thicker compared to the field of the panel. Hence the need to sand the floor flat, most especially the edges. When the edges are sanded flat to the center of the panel and the OSB eventually dries out, the thickness at the edges will shrink and the thickness of the edge is now thinner and lower than the center of the panels. This movement is reflected through the layers above eventually cracking the tile.
The marble and travertine was installed in the two bathrooms by a tile setter where he installed FLEXSnap, mat floor heating system directly over a single layer of OSB.
The vein cut limestone was installed by a totally different tile setter where he installed DITRA-XL directly over a single layer of OSB. You may already be jumping ahead and saying, what additional subfloor or underlayment was used under these floors, the answer is none, which is the critical point here.
I performed destructive testing on the marble floor and as you can see in the photos, the cracks in the marble and travertine are directly over the OSB subfloor joints. According to FLEXSnap installation guidelines this system will work for ceramic tile but not stone.
According to Schluter®-Systems, Discussion of natural stone and single-layer wood subfloors, https://sccpublic.s3-external-1.amazonaws.com/sys-master/images/h71/h71/8858545389598/DITRA%20Installation%20Handbook.pdf
Schluter®-Systems contracted the Tile Council of North America (TCNA) to perform flexural strength testing on various commercially available ceramic tiles and dimension stones guided by the ASTM C880 Standard Test Method for Flexural Strength of Dimension Stone. The test revealed:
Minimum Recorded Flexural Strength (psi)
Porcelain A – 5236
Ceramic A – 2438
White Carrara – 1856
Slate – 1488
Travertine – 337
This testing shows how durable and crack resistant porcelain and ceramic tile floors are when compared to how easily White Carrara (marble) and most especially travertine will crack.
Regarding the installation guidelines for FLEXTHERM who makes the FLEXSnap, mat floor heating system, the installation guide stated:
The FLEXSnap+ installation system was designed to allow fast and efficient installation of FLEXTHERM’s Green CableTM Surface and Green CableTM Surface XL using the FLEXGlideTM tool. It can be installed:
• on a plywood or oriented strand board (OSB) subfloor sitting on joists spaced up to 24 inches, or on concrete;
• with polymer-modified mortar recommended for use with the FLEXSnap+system by manufacturers in conjunction with FLEXTHERM, (see list at www.flextherm.com) or a cement-based self-levelling underlayment adapted to the subfloor;
• with ceramic or porcelain tiles, engineered wood, floating floor, linoleum, carpet (without rubber backing or carpet padding), vinyl and parquet (not compatible with hardwood, cork and natural stone). I highlighted the key words, the font size is the same as all the other letters used.
At the other installation the same issue was occurring even though a familiar brand of underlayment was used that is purportedly not supposed to allow this type of issue to occur. I was shocked to see all the cracked tiles following the joints in the OSB subfloor. Then I read the installation specifications and I was truly saddened.
The specifications read:
According to Schluter® - DITRA Installation Handbook 2017
Floors, Interior - Natural Stone Tile
D—W—S—1 7
Double layer of OSB or Plywood subfloor
Areas of Application
• over any even and structurally sound double layer OSB or plywood floor
• interior dry or wet areas
Limitations
• requires double layer wood floor regardless of joist spacing
• minimum 2” x 2” (50 mm x 50 mm) tile
Requirements
• maximum spacing of joists, I-joists, or floor trusses is 24” (67 0 mm) o.c.
• double layer wood floor consisting of:
Once again I highlighted the key words, the font size is the same as all the other letters used.
Here again Schluter®-Systems says:
Q. Then why doesn’t Schluter®-Systems warranty DITRA-XL for stone applications over single-layer wood subfloors?
A. Schluter®-Systems acknowledges that, in many instances, the market is requesting stone applications over single-layer wood subfloors. This is due in large part because customers want even transitions to adjacent flooring surfaces and don’t want the additional cost of installing another layer of wood. Schluter®-Systems has responded to this request with DITRA-XL. In our opinion, based on laboratory testing and field experience, DITRA-XL is the most reliable system for installing natural stone over single-layer wood subfloors on the market today.
Still, the inherent variability in the flexural strength of natural stone in conjunction with the dynamics of single-layer wood subfloors can produce substantial uncertainty in predicting the in-service performance of the stone covering. As such, Schluter®-Systems cannot warranty stone coverings over single-layer wood subfloors.
When the risks associated with natural stone and a single-layer wood application are unacceptable to the owner, building design professional, general contractor, and/or installer, then detail D-W-S is recommended, which includes the use of a plywood/Oriented Strand Board (OSB) underlayment (double-layer wood system) in addition to the DITRA or DITRA-XL uncoupling membrane. Double-layer wood systems using DITRA or DITRA-XL for supporting stone are covered by the DITRA & DITRA-XL warranty
BOTTOM LINE
To be honest, I hate finding issues like this, because I know this is going to cost someone, most likely the installer, a lot of money. Mistakes like this could bankrupt a person, who wants to see that happen. However, my role in all this was to find the cause of the cracking and in each case specific rules were not followed. The use of the mat floor heating system under natural stone is not allowed according to FLEXTHERM. Ditra XL requires a double layer subfloor when natural stone is being installed over it, an adequate substrate was not provided.
The costly lessons learned here is to ask questions and research and find anything in the installation guidelines and rules that relates to natural stone and follow the not so small fine print.