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Unread 04-18-2021, 05:53 PM   #1
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Tile installation on SIPS flooring?

I just had a sunroom built and the floor is constructed of 8" SIPS (Structural Insulated Panels), the longest span is 16'. I am preparing to install 9"x48" wood plank looking porcelain tile, and was going to use 1/4" hardibacker. The surface is very flat and straight, one of the qualities of SIPS.

I was told by the sunroom company that tile could be installed directly down on the OSB surface, but there was no way I would install it without the backerboard.

I starting installing the backerboard with a million screws, and came to realize that I totally missed the step of putting thinset down first. Before I go to the trouble of removing the installed backer boards to add thinset, I also wanted to make sure there wasn't anything else I was missing.

After some research, the SIPS are made up of 7/16" OSB with EPS or XPS foam sandwiched in between. The 8" joists are 24" on center. Normally a 5/8" subfloor is required, but with the SIPS, I just don't know. I have reached out to the builder to try and get a spec sheet for the SIPS to see what the deflection is, but I haven't heard back yet.

As it is now, the backerboard and tile will be just about the right height for aligning up with the entrance to the house, so ideally I wouldn't want the floor to be higher, but I also don't want tiles cracking.

Does anyone have any experience installing on this type of surface? Any suggestions or guidance would be greatly appreciated.
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Unread 04-18-2021, 09:16 PM   #2
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Welcome, Aaron.

Only SIP construction I observed back when I was building houses was done with SIP walls, but the floors were engineered joists with traditional OSB subflooring. I've never seen SIPs used as the floor structure and don't know quite how they'd do that.

You indicated your SIPs are 8-inches thick. And they're spanning 16 feet as a single panel, yes? And you further say they have joists that are 8-inches deep. Now, I'm certainly aware that we can achieve some impressive results with engineered wood structures, but you're still dealing with 8-inch joists on 24-inch centers with an unsupported span of 16 feet. I'm just not seeing that provide a deflection of less than L/360 required for a ceramic tile installation. Can your builder provide some specifications for those panels?

And the subflooring is another problem to my thinking. While I can understand the foam providing some between-joist support, having only a 7/16th-inch (which may be only a nominal dimension) subfloor spanning joists on 24-inch centers is still a bit of a stretch to my thinking.

I'd need a lot more technical information on your particular structure, but I'm a bit skeptical. A rather large bit, truth be known.

My opinion; worth price charged.

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