What’s Under That Stuff?

I recently stayed at a historic hotel a few states away. It’s a very historic hotel – over 130 years old, and a designated historic landmark. The framing of the grand old building is all wood. I don’t know exactly what is under the finished floors in the guest rooms, but I do know that wood framing and wood subfloor are among the components in there.

Our bathroom had large-format natural stone tile – about 12 inches by 24 inches – probably not “period” – and also not very old. Based on a few facts that I know from staying there several times over the last half century, and a long-held interest in the building, my best guess is that the large-format stone tile on our bathroom floor had been installed just over 15 years ago.

The stuff is thick – 3/8-inch to 1/2-inch thick, if the base and wall tile are the same as the floor tile – and it appears to be natural marble.

And it’s cracked, in multiple places. And multiple bathrooms on the same floor also have cracked floor tile.

Cracked large-format marble floor tile, hotel bathroom. Photo by Liz O’Sullivan

The stuff under this tile is probably the reason for the cracking. I’m not exactly sure what’s under there besides the wood framing and the wood subfloor, but one thing I do know for sure is that there is barely a single level or flat hallway floor or guest room floor in the hotel. More importantly, some of the floors feel a bit… flexible.

Levelness isn’t a big issue – tile can be installed on ramps, after all. (Our bathroom floor was a bit like a ramp, by the way, noticeably sloping from the door to the back wall. Ah, the charm of an old hotel.)

But my perception of flatness… that is an issue for the substrate under large-format tile. And the flexing in the floor that I felt when I walked down the hall or across my guest room? That is definitely not good under tile. This flexing is probably at the root of the cracking issue. Deflection is an important thing to address for floors which are to receive tile.

Now, I bet that some measures had been taken with the preparation of the substrates for tile so that they weren’t as flexible or unlevel as they are elsewhere in the building. The tile floors certainly didn’t feel flexible or unflat. But… those tiles cracked, after installation. So my guess is that the measures weren’t quite enough.

Obviously humans have been walking on marble for ages – even on thin marble such as tile. The astonishing roof terraces of the Duomo in Milan, Italy, are paved in marble, and any ambulatory person who can get up there (via elevator or stairs) can walk around the roof terraces, on the marble pavers, and even on the sloped marble roof tiles.

I don’t know what’s under the Duomo rooftop pavers and tiles. (Wouldn’t that be a fabulous tour – the attic of the Duomo?) But I do know that a different substrate installation than what is existing might have helped prevent the cracking of the hotel bathroom floor tile. And my opinion is that if the existing conditions were such that not enough could have been done to create an appropriate substrate for large-format natural stone tile, perhaps a different finish should have been selected for those hotel bathroom floors.

The construction industry has installation guidelines for so many parts of buildings. Manufacturers of specific products and assemblies have their own published installation instructions, and proper installation is often tied to warranty validity. For example, in many cases, an EPDM roofing installation must be done under certain weather conditions, must use specific products approved but not necessarily made by the EPDM manufacturer, and must be inspected by the EPDM manufacturer’s technical representative in order to get the specified warranty.

Building codes incorporate some standards into their requirements – in order to meet code, certain building products and assemblies must be installed according to certain published standards. For example, some building codes require that suspended acoustical panel (“acoustical tile”) ceilings are installed in accordance with the provisions of ASTM C636, Standard Practice for Installation of Metal Ceiling Suspension Systems for Acoustical Tile and Lay-In Panels.

Other building materials, such as lumber, plywood, brick, glass, and natural stone tile, don’t necessarily come with manufacturers’ installation instructions, and since there are many different ways that these materials are used in construction, building codes don’t necessarily govern their installation, either. But industry organizations have developed guidelines for the installation of these materials and so many more. There are at least two separate industry organizations who have developed some guidelines for the installation of natural stone floor tile. The Natural Stone Institute (formerly the Marble Institute of America) has some important guidelines in its Dimension Stone Design Manual.1 A publication by the Tile Council of North America, the TCNA Handbook for Ceramic, Glass, and Stone Tile Installation2 is referenced by the Dimension Stone Design Manual, and is also a very important stand-alone document. The TCNA Handbook has tile installations called out by alphanumeric designations that many people are familiar with – many tile setting and grout manufacturers refer to specific TCNA installations in their product info, and many specifiers use the TCNA designations in Tile Installation Schedules in the Tiling spec sections. Some architects and interior designers carefully refer to the TCNA Handbook when they’re figuring out the designs of tile installations.

But sometimes design professionals just don’t realize that there are industry-standard ways to install things. And then there’s the special condition of an existing, historic building – historic buildings certainly can be tricky. And sometimes the approach that makes the most sense is to work with what you have, and not do any invasive explorations to verify suitability of substrates for new installations. One problem with a project that involves nothing but updating interior finishes is that sometimes the design team is made up completely of people who are considering nothing but the surfaces, and the person selecting the floor tile may not realize that one floor tile is not necessarily interchangeable with another floor tile. Natural stone isn’t as strong as most ceramic tile. So a proper installation of natural stone floor tile requires a stiffer substrate (a substrate with less deflection) than an installation of ceramic floor tile requires.

Both the TCNA and the Natural Stone Institute address stiffness of subfloor for natural stone tile in their publications. For stone tile on wood subfloor without room for a thick mortar bed, TCNA calls for the joists to be no more than 16 inches on center, supporting the plywood subfloor, over which should be installed a plywood underlayment, then a backer board such as a cementitious backer board, then the stone tile. The Natural Stone Institute and the TCNA both call for stone tile subfloor areas in frame construction to have a deflection not exceeding L/720 of the span. There may be some wiggle room with some of the TCNA guidelines (joist spacing, backer board) when using certain uncoupling mats which have specific manufacturers’ installation instructions for the mat and the tile, but the 2 layers of plywood (subfloor plus underlayment) seem to be the best practice in all natural stone floor tile installations.

I’m not an authority on tile, or on the Milan Duomo – the point I’m trying to make with this blog post is that there are ways to design, detail, and specify, in our construction documents, the proper installation of most building materials, and this is not where a design professional’s creativity should take the lead. This is where the design professional’s technical side needs to be guiding the documentation. There are manufacturer requirements in some cases, building code requirements in some cases, and industry best practices in so many cases, including the case of natural stone tile. The designer should become familiar with these. Not all of this technical stuff can be taken care of with the project specifications – some of it needs to be shown in the drawings. Not all finishes can be applied or installed the same way as other finishes, even in a remodel, even when replacing (what-was-probably-small-ceramic) tile with (large-format-natural-stone) tile. When materials are designed and detailed properly in the drawings and specs to explicitly describe a correct installation, they’ll look as good decades later as they did immediately upon completion. (Maybe they’ll even still look good centuries later.)

Duomo Roof Terraces, Milan, Italy, September 2019. Photo by Liz O’Sullivan

Marble roof paver on the left, marble roof tile on the right, Duomo Roof Terraces, Milan, Italy, September 2019. Photo by Liz O’Sullivan

Notes:

  1. The Natural Stone Institute’s Dimension Stone Design Manual is on their website for free. You access one chapter at a time.
  2. The TCNA Handbook for Ceramic, Glass, and Stone Tile Installation is updated every few years. It’s available for $50 on the TCNA website.

6 thoughts on “What’s Under That Stuff?

  1. Hi Liz:

    Good post.

    Another consideration is, what is the elevation of the bathroom flooring relative to the adjoining bedroom? If the same wood subflooring is under both rooms, bathroom flooring of plywood underlayment (minimum 3/8 inch) plus cementitious backer board (1/2 inch) plus setting bed plus tile (minimum 3/8 inch) gives at least 1-3/8 inch, minus whatever the bedroom flooring is (say wood flooring, 3/4 inch thick)—so a minimum difference of 5/8 inch that needs to be accommodated with a sloped threshold. There’d be little if any height differential if the stone tile was installed directly on the wood subfloor—but then you’d end up with cracked tile.

    When I’ve had to specify large-format tile (and as you know, tiles are available a lot larger than 12 by 24 inches), I’ve always pushed for a thick-set setting bed. Easier to accomplish in new work than when constrained by existing conditions.

  2. Liz –
    As usual, you have done a good job of illuminating a relevant and important topic – to me, at least.
    Your self-restraint in leaving open the possibility of reasons other than deflection for the hotel’s tile cracks exceeds mine. Applying large pieces of heavy, brittle flooring on an old, undulating wood-framed floor sounds like a sure recipe for cracking to me. While old lumber is apt to be stiffer than today’s lumber, whatever design load standards existed in 1891, they are sure to have increased several times since then. In 1891, framing design was very likely to have been left to the carpenter anyway, and was based more on experience than calculation.
    Assuming that the floors were built level, undulations in them are a heavy hint that the structure is not stable, having deflected, shrunken, or settled, or all three. Whether the undulations indicate over-stress or not, leveling with poured or troweled underlayment adds the most new load where the problem is the worst already, thereby exacerbating the problem.
    Clients may not like it when told that their existing floors should be re-built in order to take heavy, large tile units with an appropriate underlayment. Unfortunately, if the right choices are made, clients will note the cost of those choices, at least for a little while. After some years, if the floor performs well, the cost is forgotten. The floor will have met expectations. No complaints, but no kudos either.
    Only when cracks appear will the owner be reminded daily that bad choices were made.
    It probably isn’t necessary to go further and fully expose my cynicism.
    But hey, the cracks add to the character of the place, right? Maybe they were intended all along.

    • Brian, thank you so much for reading and commenting. Good point about noting/forgetting the monetary cost of doing things right! And good point about being reminded about having made bad choices based on cost.
      Sometimes, owner decisions about cost are the reason the wrong things are done. Sometimes, ignorance is a bigger factor. I have no way to know on the example in question. It’s kind of fun to speculate, though, and then try to avoid all those possible situations.

  3. Liz, Great article, I love the perspective and in depth review of the issue. I do agree industry standards if followed, may have prevented this from cracking. Unfortunately, as you pointed out so well, those who specified that marble probably had no clue about the standards. I agree that more than likely the deflection and the soft nature of some stones is the the issue. I believe the tile and stone industries have some of the best and more complete standards. Thanks

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