Today’s blog post is based on a brief installation Q & A with Matt Smith of Smithcraft Tile and Stone (Portland, OR)
Q: Is there a type of backer that should always be used for a tile project? What factors need to be taken into consideration when evaluating an install? How do different types of structural substrates and floor height considerations impact which approach is most appropriate?
A: There is not a specific type of backer required for different types of tile or stone. From a merely structural standpoint, the building construction of a floor system including joists/post & beam and the type of sub-floor decking or concrete slab will determine the necessary tile backer system to be used. Other factors come into play with design specs such as wet areas, exterior applications, floor heat etc. which all impact final floor height.
For applications over floor joists 16"o/c with a plywood subfloor or marine grade osb I recommend securing the subfloor with corrosion resistant ring-shank nails, evaluating for minimal deflection 1/8" in 10', and reducing high areas or floating leveler in low areas (this should be done by an experienced technician as failure at this level could mean product delamitation and grout fracturing due to flexion). An uncoupling membrane can be adhered with thinset directly over the wood subfloor which provides a solid backer and isolates the tile from the structural floor system. This product is made by Ditra or Laticrete. Ditra's product is waterproof and can accommodate floor heat at a total impact of 1/4" elevation.
However, Laticrete's Stratamat can be applied with fortified thinset being perforated for ventilation in the drying process and impacts floor elevation by only 1/8".
In the case of t&g subfloor, a layer of 1/2" plywood must be applied over the surface or the t&g can be removed and replaced with a minimum of 3/4 plywood decking. If either of these methods is undesirable then a 1/2" concrete backer can be applied over the t&g with thinset and corrosion resistant screws in combination with the uncoupling membrane atop the backer.
For slab concrete, confirmation of proper drain tile is critical since rising water tables in some areas will cause severe floor damage raising and fracturing the slab during rainy seasons and consequently the tile floor.
The uncoupler technology is ideal for concrete slabs since it isolates the tile from the slab and allows for independent lateral movement and fracturing of the slab without impacting the tiled surface.
Always account for approx 1/16" elevation for each thinset layer with the exception of the layer under an uncoupling membrane which must be rolled out flat. Remember that your most important factor in calculating a finished tile floor height is to first determine the type of floor system construction. Ultimately, the goal in producing a long lasting tile floor which correlates well with adjacent flooring is to create an inflexible surface which allows for the building and the tile to freely expand and contract at their individual coefficients.
Q: Let’s look at a specific example that puts some of these guidelines into practice. Let’s say I have 1/4 inch oak flooring (old, but still very much nailed down firmly in place) covering about 75% of my house. The other 25% is newly installed 3/4" OSB. The top surface of the OSB is at the same level as the subfloor beneath the hardwood, that is the old and new subfloors are co-planar. I could remove the hardwood before installing tile; but it is tedious to remove and it sounds like it may not be absolutely necessary.
The hardwood is varnished and has a membrane beneath it. It was installed in 1961.
One installation method I've considered it to use 1/2" backer on the new subfloor and 1/4" backer over the hardwood. Given that my crawlspace is dry and well ventilated and the oak sits atop a membrane, I'm not too concerned about moisture coming from below and warping the hardwood (main worry with leaving it in place). Should I be concerned, or are there other considerations?
Several other things/variations I wonder about:
1. Could I simply build up the new subfloor 1/4" (either with backer-board or plywood underlayment) and then directly apply a decoupling layer across both the existing hardwood and the backer-board or underlayment?
2. Whether applying backer-board or decoupling layer on top of the hardwood, do I need to prepare or treat the surface in any way? For example, I don't think that thinset will stick well to a varnished surface and I would be concerned that the decoupling layer might not stick well. I'd be less concerned about backer-board because of the many screws that hold it down; but if used I still wonder if any surface prep or treatment would be needed.
A: Super simple really, you're almost there. First, if there is the possibility of any compression movement at the transition between the existing hardwood and osb you will need to fortify from underneath with blocking, liquid nails, and screws. Apply 1/4" Hardie on the 25% portion of osb (be sure to use a modified thinset between osb and Hardie backer), clean wax off of the hardwoods with acetone (only if necessary) and rough the surface of the hardwood with large swirls using a block and 40grit paper (minimal is good), then roll on Hydroban over the hardwood to create a moisture barrier and prevent swelling (this is recommended for the osb as well prior to applying Hardie unless it is gold-edge marine grade). Finally, apply Laticrete Stratamat uncoupling membrane over entire surface with modified thinset. Now you are ready to tile!
Thanks Matt! Contact Rock Bottom Tile and Stone today and see how we can assist with your project!