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I was recently confronted with a challenging installation involving 400 square feet of 18X18 travertine on a second floor exterior porch. The porch was framed well and the substrate material was two layers of 3/4″ PT plywood, glued, screwed and sloped to drain away from the house. Flashing against the second story brick exterior had been installed directly to the wood substrate. What would be the best system to ensure an absolutely waterproof, stable setting bed for the marble?
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I have used Nobleseal (Dalseal) TS over plywood for exterior tile installation, without any problems. Follow the manufacturers instructions to the letter. Use flexible thinset. If epoxy is required for for your marble, you will have to check the literature to see if this is acceptable.
*Frank,Ive had great luck with quick flex from TEC for floors subject to a lot of movement.
*Mike, I considered going over the plywood with only Noble-Seal rolled into v-notch thinset. However my concern against using only Noble-Seal was with movement in the wooden structure supporting the porch. The liability was extraordinarily high with this job. I wanted something light, yet rigid enough to support the marble without deflection. What I decided to do, and I'm anxious for your opinion, was to roll out several overlapped layers of GAF Weather-Guard (Ice & Rain Shield) directly over the PT plywood. Weather-Guard is an adhesive underlayment for roofing (90 lb) and the advantage is that you can nail through it, screw through it - whatever - and it's still watertight. I tied the Weatherguard into the existing flashing. Over the Weather-Guard I screwed 1/2" DUROCK and flashed the porch perimeter. I then thinset Noble-Seal TS to the DUROCK and pressed it with a 100 lb roller. Then (finally) I got to set my marble. Did I over-kill it on the waterproofing?
*In spite of my opinions against over building, I don't believe that you can ever achieve over-kill when it comes to waterproofing. Using Nobleseal TS over a wood deck on an exterior application is problematic. When I first did this it was okayed by the rep. The literature no longer recommends it. Nobleseal is not an approved roofing material. Ideally one would pour at least 1.5" of concrete or lightweight concrete on top of the deck with a primary roofing membrane and then use the Nobleseal. Your method sounds like the next best thing. Often in situations like this, thickness of the entire assembly is of some concern.
*Mike, have you used the GAF Weather-Guard product? Good stuff. Thanks for the advice!