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Unread 10-24-2020, 03:09 PM   #18
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 14,225
It is VERY rare in a residential building to have L/720 unless the original design planned for natural stone as it adds potentially, significant costs for no benefit. Now, how easy it is to retrofit, depends.

There are two components of deflection that must be taken care of...along the joists, and in between them. Subflooring is tasked with in between, the joists take care of along them. A tiled floor is a distributed load, whereas point loads can be problematic as well. In a well constructed structure, as long as you don't overload it, it should work.

Another thing to consider is that long-term, wood is more fluid than you'd think. Picture a bookcase with a simple board as the shelf. Install it, put some weight on it and it bends, but over time, the warp continues to increase, and if it sits long enough, becomes bent into a semi permanent curve (flip it over, and it will bend back, eventually). So, what might work initially, can fail eventually because of this creep. Picture an old barn or other building. Over the years, it's not uncommon to see the ridge line no longer straight. Now, put a rigid tile on that surface and over the years, you'll either break the bond, or break the tile or both. Ever been to Salisbury Cathedral? The granite columns around the bell tower have a very distinct bow to them...they were straight when installed. As they decided they wanted a taller tower, the structure was overloaded and over hundreds of years, the stone literally bent and was reinforced. So, as is very apparent visiting that place, the rocks in the crust of the earth are also bent over time. Your house is not in the same time scale, and movement will not be tolerated. Testing has found you need a certain level of strength for a tiled installation to survive. Ignore that at your own risk.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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