With today's lighter weight construction practices, new products have been developed to eliminate some of the problems associated with the use of cement backerboards. Uncoupling membranes such as Ditra and ProVa-Flex take the place of cement backerboard, completely doing away with the possiblilty of tile cracked along the backerboard joints. We highly recommend the use of ProVa-Flex Tile Underlayment when installing PortStone or other tile products over a wooden subfloor or over concrete when there are cracks in the concrete substrate.
ProVa-Flex can be easily cut to size using a utility knife or heavy duty scissors. To install, spread thin-set mortar onto the substrate using a 1/4" X 1/4" X 1/4" notched trowel, roll the ProVa-Flex out onto the fresh thin-set mortar. Use a flat trowel or short 2X4 board to smooth out any potential air bubbles underneath the ProVa-Flex. The tile or PortStone can be laid immediately.
Why does PortStone Manufacturing Corporation not recommend the use of cement backerboard under PortStone?We have been manufacturing and selling PortStone since 1998, and over the years we have observed certain patterns, tendencies and issues related to various PortStone installations. When we spot something that could be a a potential problem for our customers, we try to take steps to help them avoid those issues.One of the patterns and issues that we saw was related to PortStone floors installed over cement backerboard vs PortStone floors installed over isolation membranes such as Ditra® or Prova-Flex®. In some cases, albeit a very few cases, where PortStone was installed over cement backerboard, movement in the subfloor would cause cracks to appear directly above the joints between the individual sheets of backerboard. This certainly did not happen in every case, nor was it in most cases, in fact, it was only in a very small number of cases. However, we never saw any issues of the sort when an isolation membrane (such as Ditra® or Prova-Flex®) was used in place of cement backerboard.We have had some professional tile installers say that they have installed ceramic tile over backerboard for many years and have seen very few if any such issues with ceramic tile installed over cement backerboard. That may very well be true. One reason for this is that while PortStone is a very durable product and our tests have shown it to have more impact resistance that ceramic tile, PortStone is a cementitious product, not a clay fired ceramic product and as such, PortStone does not have as much tensile strength (strength against being pulled apart) as ceramic tile.The benefit of using an isolation membrane such as Prova-Flex® or Ditra® is that it acts as a buffer between subfloor movement and the flooring material installed over it. The isolation membrane absorbs most common minor movements associated with expansion and contraction of wooden subfloors. It is also very useful as a crack isolation membrane over minor cracks in concrete slab subfloors.Just to be clear, the movement we are talking about here is horizontal movement. Nothing can prevent damage to any flooring material caused by vertical movement.Having never seen a single issue to date of cracks in a properly installed PortStone floor using Prova-Flex® or Ditra® caused by horizontal subfloor movement, we made the decision that it is better to be safe than sorry. Therefore, PortStone Manufacturing Corporation recommends that when installing over a wooden subfloor, PortStone only be installed over a properly installed quality isolation membrane such as Prova-Flex® or Ditra® In full disclosure, we do carry Prova-Flex® for our customers' convenience. It can also be found commercially available in some areas of the US. Ditra® can be purchased at most any Lowes® or Home Depot® stores among others.