Why is by Exterior Covered Deck Tile Backer Board Installation Causing Tile to Crack?


Hello.... We had travertine tile installed over durrock both indoors and outdoors. Inside is fine but outside developed cracks after a few years. Cracks are linear and appear to follow the durrock lines or the treated plywood lines.... Hard to tell.... But definitely cracking along parallel and perpendicular angles.
Treated wood was 23/32 tongue and groove.

What was done improperly to cause the cracking?
Tile is about 3/8 in thick. I'd like to send a picture but don't see how to do that.

The builder says he did everything properly and provided me with the receipts for the treated wood.

We have this issue on a covered screen porch and an open 2 story porch.
Thank you!


ANSWER - Normally when you get cracking over a suspended floor following the joints of the tile backer board that the tile was bonded to, it is due to a combination of excessive deflection in the floor and improper installation.

Per the Tile Council of America Tile Handbook detail F144-20 the subfloor can't have any deflection greater than L/360 both along the joists and between joists.  The floor joists should be 16" on center.  The exterior grade plywood should be Tongue and Grove (T&G) and they should be set off-set from each other and have a 1/8" gap between sheets.  The cementitious backer board should be off-set perpendicular to the plywood joints and off-set from each other.  The backer board should be fully bonded and supported to the plywood with an thin-set mortar meeting ANSI A118.11.  The backer board should have 1/8" gaps between sheets and be filled and taped with 2" alkali-resistant glass fiber mesh tape using a thin-set mortar adhesive.  The tile should achieve 80% to 95% contact between tile and backer board with full support at corners and edges of the tile.  The perimeter joints and every 8-12 feet should be filled with a resilient caulking meeting ASTM C920 (silicone or polyurethane).

Chances one or more of those steps were not done.

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