QUESTION

Exterior Deck Installation - We are going to set porcelain tile on an exterior wood deck next week. We will install cementitious backer board first. Question is, since weather will likely be in 30's and 40's in the mornings, do we need to use a special thinset? We use a very good quality polymer fortified thinset for most every job. Will this be OK in the colder temps? We're talking high desert in AZ.

ANSWER

ANSWER - Beware of exterior decks and particularly when they are over a wood substrate.  There are many exterior deck failures that become very expensive to replace.

The TCNA handbook does not show an exterior deck over wood framing, but only over a concrete slab.  There are notes in the TCNA handbook that says with appropriate precautions you can install over a wood substrate for exterior applications that are not subject to freeze thaw conditions by using either a wire reinforced mortar bed per TCNA F141 or with cementitious backer board (CBU) per TCNA F144.

Keep in mind that only exterior grade plywood is recommended for the subfloor and the bottom of the plywood deck needs to be covered so it isn't subject to getting damp.  If the plywood gets wet it will expand and warp.

The substrate can not have excessive deflection and must be in conformance with IRC and other applicable building codes.  Deflection can not exceed 1/360 of the span when measured under a 300 lb concentrated load.

You need to slope the deck to a drain or to the edge of the deck into a drainage trough so the water can escape and not cause staining at the edges.  There needs to be a waterproof membrane over the sloped surface with a slope to drain of 1/4" per foot within the membrane surface.

I'm not sure how you are going to create your slope with CBU, but you could try to apply a patch material that is designed to be applied at 1" thick and down to a feather edge. It would be better to put a roofing membrane on the wood subfloor and then apply a wire reinforced mortar bed sloped to drain and then apply another waterproof membrane on its surface.

Schluter (www.schluter.com) has a system called Bekotec that is used with their Detra membrane system that they might recommend for this application.

Of course you need to have expansion joints at the perimeters and throughout every 8' to 12' in each direction.  The expansion joint sealant must meet ASTM C920 per TCNA EJ171 and installed correctly (this is not latex caulking, but rather a structural silicone or a urethane sealant).

You will need to put a tent up and use electrical heaters to keep the surface at least over 40 degrees F.  Don't use propane heaters because they can cause problems.  Check out the data sheet for the modified thin-set you are using and follow their instructions.

It is not easy or cheap to do it the right way, but on the other hand it gets extremely expensive if it fails.