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What is under a Central Ohio bathroom floor, by era, and what a tile floor asks of it

A tile floor is only as good as the floor under it, and what is under a Central Ohio bathroom was decided by the decade the house went up: board sheathing into the late 1930s, plywood from the early 1940s, oriented strand board later still. This page sets out what the national sources say about each of those, what a mortar bed is and why an old tile floor is thick, what the Residential Code of Ohio asks of a floor, and what the tile trade asks of one, which is a different test with a different answer. It also covers what a leak does to wood, what a heated floor adds, and what a shower with no step asks of the framing before it asks anything of the tile. Nobody publishes a count of what is under Central Ohio floors by decade, so the eras here are national and attributed, and nothing on this page is a statement about any particular house. The wall above that floor, era by era, is on the building page.

Last updated September 2026 · Federal rehabilitation guides, Ohio code rules, product standards and the tile trade's own pages were read September 2026 · No prices on this page; the Estimate Builder carries those · Part of the knowledge base

The floor is decided before the tile is

Tile is chosen last and the floor under it is settled first. A tile floor is rigid and it is bonded to something that is not: wood framing bends under load, and the tile has to be carried on an assembly stiff enough that it does not go with it. What that assembly is made of was decided by the decade the house went up, and the documents that date those materials put the changes earlier than the usual telling does.

What is under the floor, by era

Nobody publishes a count of what is under Central Ohio bathroom floors by decade. The era history below is national, from the federal housing department's rehabilitation guide of 1999, the federal forest service's wood laboratory and the product standards themselves. The nearest measured Central Ohio fact is how old the houses are, and the shares by year built sit on the lead paint and asbestos page, which also owns old floor tile and the adhesive under it.

The eraWhat the floor was sheathed withThe source that says so
Into the late 1930sBoards, 1x6 and larger, straight-edged or tongue-and-grooved, laid across the joists or diagonally to them, either way acting as a diaphragm that stiffened the floor.HUD's Rehab Guide, Volume 5
From the early 1940sPlywood begins to replace board sheathing, and the guide gives the cause: waterproof adhesives had arrived. That is earlier than the 1950s date in general circulation.HUD's Rehab Guide, Volume 5
November 1, 1966The federal plywood standard PS 1-66 takes effect, the first Voluntary Product Standard developed under the Commerce Department's new procedures.The Commerce Department's standard
The 1960sParticleboard and waferboard come into use as floor sheathing. Waferboard did not perform well in high moisture situations, and oriented strand board evolved from waferboard technology.HUD's guide; the Forest Products Laboratory
August 27, 1992, and afterPS 2 is approved, which APA calls the first consensus-based performance standard for these panels. Oriented strand board went on to replace softwood plywood in many construction applications; no source found dates that change.The Commerce standard; APA; the Forest Products Laboratory

That is what was ordinary nationally, not what is under any one bathroom. The oldest row is still lawful: the Residential Code of Ohio allows subflooring to be omitted where joist spacing does not exceed 16 inches and a one-inch nominal tongue-and-groove strip floor is applied across the joists, which is how a great many pre-war floors were built.

HUD's guide is blunt about age: many older houses built before 1920 have floor joists sized considerably below current requirements, it says, with 3x6 and 3x8 joists before 1900. It also separates two things a homeowner runs together: floors will rarely fail structurally, but they may have excessive bounce and feel unsafe. A springy floor is a tile problem long before it is a structural one.

The mortar bed, and why an old tile floor is thick

The National Park Service's brief on preserving historic ceramic tile floors records the nineteenth-century method, quoting an 1857 description: a two and a half inch bed of concrete of quicklime and gravel, or of Portland cement and clean sharp sand, laid first as the foundation the tile was set into. That is why an old tile floor is thick and sits low rather than proud of the boards around it.

Where tile went over an existing wood floor, the same brief records, the floorboards were taken up, sawn into short lengths and dropped between the joists to make room, and the Park Service says the practice has resulted in numerous cracked tiles and other failures.

A thick bed, a mortar bed and a thick-set installation all mean, in the Tile Council of North America's words, packing a mortar bed over a surface before installing the tile, either bonded to what is beneath or floating free of it on a cleavage membrane, a sheet whose stated job is to prevent the mortar bed from bonding to the substrate.

Then it thinned. The Park Service records setting beds shrinking from several inches to as thin as three thirty-seconds of an inch, and a tile mortar manufacturer's technical sheet gives the same low figure for the modern thin-bed method. That one change is behind the height mismatch in a redone bathroom floor: an old floor sat low because its bed was thick, a new floor on the same joists sits higher, and the threshold, the door swing and the toilet flange height move with it. What a new floor does to the flange is on the toilets page.

What the Residential Code of Ohio asks of a floor

Ohio prints its residential code sections without the model code's R prefix, so the numbers below are 301.7 and 502.8.1, not R301.7 and R502.8.1. The rules quoted carry a posted effective date of July 1, 2019. Which code governs a bathroom remodel and who inspects the work is the subject of the Ohio code page.

The figure everybody quotes is in Table 301.7: the allowable deflection of a floor is its span divided by 360. Two things about it go missing. The first is what it measures: section 301.7 sets the limit under the live load listed, not under everything the floor carries, so a mortar bed, a stone floor or a filled bathtub is dead load and never enters the test. The second is where it came from, and it was not tile. HUD's guide says the limit is derived from long-standing standards based on the deflection at which a plaster ceiling of the space below the floor would crack.

The live load is set by Table 301.5: 40 pounds per square foot for rooms other than sleeping rooms, 30 for sleeping rooms. A bathroom is one of the former, and Ohio's two floor joist span tables split the same way. Thickness sits in two tables organized differently. Table 503.1 gives lumber sheathing by joist spacing: 5/8 inch minimum net thickness at 16-inch spacing, and at 24-inch spacing 11/16 inch laid across the joists or 3/4 inch laid diagonally. Table 503.2.1.1(1) works from the span rating instead, the second number of which is the allowable span as subflooring: a panel rated 24/16 is 7/16 inch and spans 16 inches as a subfloor, one rated 48/24 is 23/32 or 3/4 inch and spans 24. The same rule requires panel sheathing to conform to CSA O325, CSA O437, DOC PS 1 or DOC PS 2, and says the Performance Category printed on a panel, a designation tied to its thickness range rather than a measured dimension, is what the code means by nominal panel thickness.

The section a bathroom collides with is 502.8, on cutting and notching, because a drain has to get somewhere. In sawn lumber, 502.8.1 says a notch may not exceed one sixth of the depth, may not be longer than one third of it, and may not fall in the middle third of the span; a bored hole is capped at one third of the depth and kept 2 inches clear of the top, the bottom and any other hole. In a house framed with trusses, wood I-joists or structural composite lumber, 502.8.2 prohibits cuts, notches and bored holes outright, except where the manufacturer's recommendations allow them or a registered design professional has accounted for them. That is the practical difference between moving a fixture in a 1950s floor and moving one in a 2005 floor.

What the tile trade asks, and why it is a different test

The Tile Council of North America's answer on deflection opens on the same number and then complicates it. Traditionally, it says, the accepted minimum requirement for floor rigidity is L/360, measured before the tile underlayment goes in. It warns that floors built to that limit at 24-inch joist spacing often have deflection between the joists exceeding L/360, which is exactly where the tile is. And it says something a homeowner would not guess: in fact, failures at L/600 have been observed. Stiffer is not automatically safer.

The trade's own account of that number is the surprising part. In an article published on its site in 2007, the body wrote that deflection language has been struck through in the 2007 Handbook, and that floor systems must instead conform with the building codes for residential applications. The tile figure was never the code figure, it explains: it was measured under a 300-pound concentrated load, which differs from the L/360 deflection limit under uniform load. So a floor that satisfies the Residential Code of Ohio has satisfied a live-load limit inherited from plaster ceilings; it has not thereby been tested for tile, and the body that used to publish a tile number has stopped publishing one.

Stone is held higher. TileLetter, the National Tile Contractors Association's publication, gives the maximum allowable floor member live load and concentrated load deflection for natural stone as L/720, twice the stiffness asked of ceramic tile, attributing the figure to the tile Handbook and to the Natural Stone Institute's design manual. The Ceramic Tile Education Foundation adds the framing consequence: the Natural Stone Institute does not accept a single layer of structural wood panel under a stone floor, because the joints between panels cannot provide the necessary strength, so two layers are required.

Oriented strand board needs both halves of its answer together. The Tile Council's page on the panel says the most current tile installation standards specifically exclude it, and that thin-set manufacturers do not recommend installing ceramic tile directly to it; the Park Service says the same of wood panels generally. The same page then says tile can be installed over the panel in Handbook systems that include it, following the manufacturer's literature. Read as a ban it would condemn every floor laid since the 1980s, which is not what the source says: the distinction is between bonding tile straight to the panel and building a system over it.

Why floor tile cracks and comes loose

The Tile Council lists the causes on its own pages: excessive deflection, moisture induced deterioration of the subfloor, delamination of the subfloor, and thin-set spread in dabs instead of a full bed. Two of the four are about the wood rather than the tile. Over wood, that page says, floor failure is more likely than over concrete. Where tile is bonded to concrete the failure has a different shape: cracks occurring in the concrete can cause cracks in the tile layer, which the trade calls reflective cracking, and even small shrinkage cracks can stay dimensionally active while the slab goes on curing.

Movement joints are the part most often assumed away. The Tile Council calls it a frequent misconception that anti-fracture membranes allow you to eliminate expansion joints, and says soft joints have to be in the tilework regardless. TileLetter, quoting the Handbook detail it names as EJ171, gives the guideline as a maximum of 25 feet in each direction indoors and 12 feet where the tile is in direct sunlight or exposed to moisture, with a joint wherever tilework abuts a restraining surface: a perimeter wall, a different floor finish, a curb, a column, a pipe. In a bathroom that last clause matters more than the spacings, because a bathroom is mostly perimeter.

A heated floor

The membrane under a heated tile floor is usually an uncoupling membrane, and its principle is the old cleavage membrane's. TileLetter's timeline of the methods says such a membrane provides air space between the tile and the substrate to allow independent movement, and names the Handbook methods that use one as F128, F147 and F148. The Ceramic Tile Education Foundation says some manufacturers make versions of their membrane that can carry electric heating wire. Every Handbook number on this page reaches it through a publication quoting the Handbook, which is sold rather than posted free to read.

Ohio's residential energy rule says less than people expect, and what it says is about slabs: a reading of the whole chapter finds nothing about electric floor warming in a wood-framed floor. Where the floor is a slab it speaks twice. For new construction, footnote d to Table 1102.1.2, the table section 1102.2.10 points to, says R-5 insulation shall be provided under the full slab area of a heated slab, on top of the slab edge insulation. The same chapter also carries Table 1107.4.1 as a compliance alternative for existing buildings in lieu of Table 1102.1.2, under its alterations section, and that table's own footnote d words it differently: R-5 shall be added to the required slab edge R-values for heated slabs. Which of the two reaches a given alteration is not something the published rule settles. The wiring is a different code and a different page: the ground-fault rule for electric radiant floor heating is on the bathroom electrical page.

A floor that got wet

HUD's kitchens and baths volume states the thing a tile floor is usually assumed to do and then denies it: the guide recognizes that tile work cannot be made permanently waterproof. Water that gets past will be absorbed by cement backer board, and by the framing if it passes beyond the backer board, so what decides the outcome is whether the wood can dry.

The federal wood laboratory states the mechanism without alarm. Serious decay occurs only when the moisture content of the wood is above the fiber saturation point, which the handbook averages at 30 percent. Below that, the same chapter says wood will not decay if it is kept air dry, that decay already present will not progress, and that 20 percent is a reasonable margin of safety.

HUD's guide orders the work accordingly: deterioration of floor sheathing is often moisture related, it may be rot from a roof or a plumbing leak, and corrective work should not be initiated until the cause of the deterioration is understood and corrected. What comes out when a bathtub does is the subject of the tub-to-shower conversion page.

A floor with no step

A shower with no threshold asks the framing a question before it asks the tile one. HUD's kitchens and baths volume says the floor of a no-threshold shower is set in mud rather than in thin-set and grout, and that the floor should be planned to accommodate the thickness of the mud underlayment. Planned is the operative word: the depth is found in the structure, not added on top of it. What that means in a Central Ohio house, over a basement slab or over the wood joists of an upper floor, is set out on the curbless shower page and the wet room page, both of which treat the floor structure as the first question rather than the last.

The short version. Board sheathing was ordinary into the late 1930s, plywood replaced it from the early 1940s, and the standard covering oriented strand board was approved in 1992. The Residential Code of Ohio holds a floor to its span divided by 360, measured under live load alone; the tile trade struck its own deflection language out in 2007 and pointed at the building codes instead. An old tile floor is thick because its bed was inches deep, and a modern setting bed can be as thin as three thirty-seconds of an inch, which is why thresholds, doors and the flange all move when a floor is redone.

Sources

Every figure, date and quotation on this page comes from one of these documents, read on the date given. Where two of them disagree, the page says so.

Common questions

What is under the bathroom floor in a Central Ohio house built in the 1950s?

Nobody publishes a count of what is under Central Ohio bathroom floors by decade, so the honest answer is national rather than local. The rehabilitation guide the federal housing department published in 1999 says the typical residential floor sheathing was boards, 1x6 and larger, straight-edged or tongue-and-grooved, until plywood began to replace them in the early 1940s once waterproof adhesives arrived. A house from the 1950s may have either, and the only way to know is to open the floor. Oriented strand board came later still: the federal standard covering it was approved in 1992.

Does the Residential Code of Ohio set a deflection limit for a floor that will be tiled?

It sets one for the floor, not for the tile. Table 301.7 of the Residential Code of Ohio puts the allowable deflection of a floor at its span divided by 360, and section 301.7 measures that under the live load listed rather than under everything the floor carries, so the weight of a mortar bed, a stone floor or a filled bathtub does not enter the test at all. The federal housing department's rehabilitation guide records where the figure came from: the deflection at which a plaster ceiling in the space below would crack. It was never a tile rule.

Why is the tile floor in an old bathroom so much thicker than a new one?

Because it was set in a mortar bed. The National Park Service's brief on historic ceramic tile floors, quoting an 1857 description of the work, gives a two and a half inch bed of concrete, or of Portland cement and clean sharp sand, laid first as the foundation for the tile. The Park Service also records how far setting beds have thinned since, from several inches to as thin as three thirty-seconds of an inch, and a tile mortar manufacturer's technical sheet gives the same low figure. An old floor sat low because its bed was thick; a new floor on the same joists usually sits higher.

Can floor tile be laid over an oriented strand board subfloor?

Both halves of the answer come from one page. The Tile Council of North America says the most current tile installation standards specifically exclude oriented strand board, and that thin-set manufacturers do not recommend installing ceramic tile directly to it. The same page then says tile can be installed over the panel in Handbook systems that include it, following the manufacturer's literature. The distinction is between bonding tile straight to the panel and building a system over it, so the exclusion is not a verdict on every floor laid since the 1980s. That page cites the 1999 edition of the installation standard, so it is not a recent statement.

What makes a tile floor crack or come loose?

The Tile Council of North America lists the causes on its own pages: excessive deflection, moisture induced deterioration of the subfloor, delamination of the subfloor, and thin-set spread in dabs instead of a full bed. Two of the four are about the wood rather than the tile. Over wood, that page says, floor failure is more likely than over concrete. Where tile is bonded to concrete, the same body says cracks occurring in the concrete can cause cracks in the tile layer. It also calls it a frequent misconception that anti-fracture membranes remove the need for movement joints.

What does Ohio's residential code allow to be cut out of a floor joist?

It depends on what the joist is. For sawn lumber, section 502.8.1 of the Residential Code of Ohio says a notch may not exceed one sixth of the depth, may not be longer than one third of the depth, and may not fall in the middle third of the span, and it caps a bored hole at one third of the depth, kept 2 inches clear of the top, the bottom and any other hole. For trusses, wood I-joists and structural composite lumber, section 502.8.2 prohibits cuts, notches and bored holes unless the manufacturer allows them or a registered design professional has accounted for them.

What happens to the subfloor under a bathtub that has leaked?

It depends on how wet the wood got and for how long. The federal Forest Products Laboratory states the mechanism plainly: serious decay occurs only when the moisture content of the wood is above the fiber saturation point, which it averages at 30 percent, and wood will not decay if it is kept air dry, with decay already present not progressing once it dries. The federal housing department's rehabilitation guide adds the order of the work, saying deterioration of floor sheathing is often moisture related and that corrective work should not begin until the cause is understood and corrected.

Does a heated bathroom floor change what Ohio's code asks?

Ohio's residential energy rule speaks to heated slabs rather than to heated wood floors. For new construction, the footnote to Table 1102.1.2 says R-5 insulation shall be provided under the full slab area of a heated slab, on top of the slab edge insulation; the same chapter offers another table as a compliance alternative for existing buildings, whose own footnote instead adds R-5 to the required slab edge values. A reading of the whole chapter finds nothing about electric floor warming in a wood-framed floor. The wiring rules for radiant floor heating sit in the electrical code instead.

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