Installation Guide

Discussion of natural stone and single-layer wood subfloors
NATURAL STONE
22 NATURAL STONE
Natural stone is a product of nature with a wide variety of colors, patterns, and textures that come together to distinguish it as one of the
premiere surface coverings available on the market. Some of stone’s characteristics, which add to its beauty and uniqueness, are veins,
fissures, starts, and dry-seams. While these characteristics enhance its aesthetic appeal, they’re also indicators that point to the inherent
variability of the flexural strength of natural stone, which can have detrimental effects on serviceability. This variability is underscored by
examining the range of typical flexural (bending) strengths of ceramic tile compared to the range of strength for some common natural stones.
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. Five samples
of each tile and stone were tested, with the minimum recorded values displayed in the figure below. We have chosen to show only minimum
values since these represent the weakest samples, which would be most prone to cracking in service over a bending substrate.
It is clear from the figure that the minimum recorded flexural strengths of these dimension stones tend to be significantly less than those
of the ceramic tiles. In some cases, the differences are dramatic. For example, the minimum recorded flexural strength of the weakest
travertine sample (337 psi) was only 14% of the minimum flexural strength of the weakest ceramic sample (2438 psi). In other words, the
weakest ceramic sample was more than 7 times as strong as the weakest travertine sample. As another example, the minimum recorded
flexural strength of the weakest porcelain sample is more than 12 times as strong as the minimum recorded flexural strength of the weakest
travertine sample.
Q.
Why does Schluter
®
-Systems recommend a double-layer wood floor for installing natural stone over DITRA-HEAT?
A. There are three principle reasons: 1) As illustrated above, the fact that most stone products have a minimum flexural strength that
is substantially lower than what is typical for ceramic tile; 2) Stones are products of nature and complex heterogeneous materials with
naturally occurring regions of discontinuity, such as veins and fissures. Such features can be weaker than the surrounding stone fabric and
act as “stress risers,” concentrating bending stresses within the region of discontinuity; and 3) When wood floor assemblies are subjected
to forces such as loading – both live and dead loads – they produce flexural stresses in the surface covering which can cause weak and
brittle materials to break or crack.
Engineering mechanics as well as field observations show that the location of maximum flexural stresses in the floor assembly is directly
over the floor joists and at seams in the subfloor panels. Therefore, we recommend double-layer wood floors when installing natural stone
in order to increase the stiffness of the sheathing assembly and position underlayment seams away from the joists to minimize flexural
stresses in the stone covering directly above the joists and at seams. Refer to page 14 for underlayment installation guidelines. For more
information on the development of these guidelines, please refer to the article titled “Position of Underlayment to Prevent Cracked Tile and
Grout” on our website at www.schluter.com/5138.aspx.
Porcelain A
5236
Ceramic B
4485
Porcelain B
4241
Granite
2490
Ceramic A
2438
White Carrara
1856
Slate
1488
Crema Marfil
539
Travertine
337
0
1000
2000
3000
4000
5000
6000
Minimum Recorded Flexural Strength (psi)