Tile Assemblies Over Wood and Concrete
Why this matters
A tile floor is a stiff, brittle skin on top of a structure that moves. When it cracks, the crack is almost never the tile's fault: it is the floor under it bending more than the tile can follow, or a slab crack running up through it. Choosing the assembly, meaning everything from the joists or slab up to the tile, is the decision that decides whether the floor lasts, and the deflection check is the part of it most often skipped because it takes a calculator.
Two assemblies
OVER WOOD FRAMING
[tile] [tile] [tile] [tile] tile
############################## thinset
============================== backer board set
in thinset and
screwed, or an
uncoupling membrane
------------------------------ plywood subfloor
| | | | joists: span and
| | | | size set the bend
OVER A SLAB
[tile] [tile] [tile] [tile] tile
############################## thinset
============================== crack isolation
membrane
////////////////////////////// concrete slab
| shrinkage crack
| in the slab below
Each layer has one job. The tile and grout are the wear surface. The thinset bonds the tile to whatever is under it, at the coverage the setting how-to owns: see related: How to Lay Out and Set a Tile Floor. The layer under the thinset is there to give it a stable surface to bond to and, in the membrane cases, to keep the substrate's movement from reaching the tile. The structure underneath has to be stiff enough that none of that is asked to bend.
Why a stiff tile on a springy floor cracks
When a floor bends under load, its top surface changes length: over a joist span, the top of the floor shortens toward midspan. A wood subfloor tolerates that easily. Tile and cured grout cannot change length with it. The stress has to go somewhere, and it goes into the weakest point: the bond line releases and the tile sounds hollow, or the tile cracks, or the grout cracks in a line. A floor that is merely noticeable underfoot to a person can still bend far more than tile can follow.
That is why the tile industry's rules are written as deflection limits, a fraction of the span.
The deflection rule
The TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, current edition, states in substance that the floor framing under ceramic and porcelain tile should deflect no more than L/360 under live and concentrated loads, and under natural stone no more than L/720, where L is the clear span. L/720 is twice as stiff as L/360. Stone gets the stricter number because it is generally weaker and more variable than porcelain and cracks along its own natural veins. The Handbook is a consensus document; it binds you where your contract, the tile or setting-material maker's warranty, or the project specification adopts it, and setting-material makers routinely do.
Joist deflection is not the whole stiffness story. The subfloor panel also bends between joists, and the Handbook's wood-frame methods set minimum subfloor thickness and joist spacing alongside the deflection limit, with stone methods commonly calling for two layers of subfloor. Checking joists and subfloor in the field is covered in Subfloor Assessment + Repair Reference.
Over wood: backer board or membrane
Cement backer board (cementitious backer units under ANSI A118.9) gives the thinset a dimensionally stable, water-tolerant surface to bond to. It is bedded in thinset and screwed to the subfloor in the pattern its maker specifies, with seams taped and filled. It adds little stiffness: a springy floor is still springy with backer on it. Cutting it makes respirable crystalline silica dust; score and snap where the maker allows, and for any power cut use a saw with dust collection or water as 29 CFR 1926.1153 Table 1 describes. See related: Respirable Crystalline Silica and the Construction Standard, and How to Install Cement Backer Board on a Wood Subfloor.
An uncoupling membrane is a thin sheet, bonded with thinset to the subfloor and to the tile, designed to let the substrate move a little in-plane without passing that movement to the tile. It does not change the framing's deflection. Some membrane makers publish tested assemblies that allow a thinner subfloor or a wider joist spacing than a backer-board method would; those listings are the membrane maker's, with the conditions stated in them, and do not relax the deflection limit. See related: How to Install an Uncoupling Membrane for Floor Tile.
Over concrete: the slab's own movement
A slab is stiff, so deflection is rarely the question. Its movement comes from shrinkage cracks, control joints and seasonal temperature.
- Shrinkage cracks with no vertical offset can be bridged with a crack isolation membrane meeting ANSI A118.12, run over the crack and some distance either side as its maker states. The membrane handles movement in the plane of the floor.
- Cracks with vertical offset (one side higher than the other), or a crack that is still opening, are a structural or settlement problem. No membrane carries tile over a slab that is still moving up and down; that crack goes to whoever evaluates the slab.
- Control joints and cold joints in the slab are carried up through the tile as movement joints, not tiled over. See related: Movement Joints in Tile Floors.
Thinset choice follows the membrane and tile makers' instructions, often a modified mortar. See related: Mortar + Grout Selection Reference.
Worked example: porcelain or marble over the same joists
A second-floor bathroom with a clear joist span of 14 feet, so L = 14 x 12 = 168 inches. The joists are 2x10 Douglas fir-larch No. 2 at 16 inches on center. The customer is choosing between porcelain and marble.
The check below is a simple-span bending calculation under a uniform load, ignoring the stiffening effect of the subfloor and the small shear deflection. It is an estimate for deciding what to ask, not an engineering stamp.
Inputs.
JOIST DEFLECTION, SIMPLE SPAN, UNIFORM LOAD
Load: 40 psf live + 10 psf dead = 50 psf
w = 50 psf x 16/12 ft spacing
= 66.7 lb/ft = 5.56 lb/in
E = 1,600,000 psi (DF-L No. 2, NDS supplement)
I = 1.5 x 9.25^3 / 12 = 98.9 in^4 (2x10)
L = 168 in, L^4 = 7.97 x 10^8 in^4
deflection = 5 w L^4 / (384 E I)
= (5 x 5.56 x 7.97e8) / (384 x 1.6e6 x 98.9)
= 2.22e10 / 6.08e10 = 0.364 in
span ratio = 168 / 0.364 = L/461
The 40 psf live and 10 psf dead loads are the common residential floor design loads. The Handbook's limit is read, in substance, against live and concentrated loads; the calculation above uses the total, which is conservative. On live load alone, 0.364 x 40/50 = 0.291 inches, or 168 / 0.291 = L/577. Neither answer below changes: porcelain passes either way, and L/577 still falls short of L/720.
Against porcelain. The L/360 limit at this span is 168 / 360 = 0.467 inches. The joists deflect 0.364 inches, which is under 0.467: pass, at L/461.
Against marble. The L/720 limit is 168 / 720 = 0.233 inches. The joists deflect 0.364 inches, over 0.233 by 0.131 inches: fail. Checked from the other direction, L/461 is less stiff than L/720.
What stone would take. Sistering a second 2x10 to each joist doubles I to 197.8 in^4 and halves the deflection to 0.182 inches, which is 168 / 0.182 = L/923, better than L/720. A beam at midspan that halves the clear span would also pass by a wide margin. Either is structural framing work: it goes to a carpenter or the trade the shop uses for framing, under a permit where the jurisdiction requires one, and not to the tile crew on the morning of the install. The subfloor still has to meet the stone method's own requirements on top of that.
What the customer is told. Porcelain works on the floor as it stands. Marble needs the framing stiffened first, and the quote shows that as a separate line of work with the reason written beside it.
Where the check goes wrong
- The wrong span. L is the clear span between supports, measured from below, not the joist's length on a lumber list and not the room's width. A joist that bears on a beam a few feet in from the wall has a shorter span than the room suggests; one that runs past an interior wall with no bearing has a longer one.
- The joists the plumber cut. Bathrooms are where joists get notched and bored for drains, and a toilet drain run through a notch at midspan can leave a joist far weaker than its nominal size. Look at the framing from below before trusting a span table or a calculation.
- Assuming the old tile proves the floor. A previous porcelain floor that never cracked says the framing met porcelain's needs, not stone's.
When the assembly changes
- Heated floors. An electric mat embedded in the thinset or a self-leveling layer adds a layer with its own maker's rules. See related: Self-Leveling Underlayment Application.
- Wet areas. A shower floor or a wet room adds a waterproofing membrane and slope, which are their own assembly.
- Old resilient flooring under the tile. Tiling over an existing sheet or tile floor is a maker-specific question, and any resilient floor or black adhesive of unknown age is presumed to contain asbestos until tested, so it is not ground or scraped to prepare it until the result is known. See related: Nine-Inch Floor Tile and Black Mastic.
References
- TCNA Handbook for Ceramic, Glass, and Stone Tile Installation, current edition (deflection limits, floor methods)
- ANSI A118.9 (cementitious backer units) and ANSI A118.12 (crack isolation membranes)
- 29 CFR 1926.1153 (respirable crystalline silica in construction)
- See related: Subfloor Assessment + Repair Reference
- See related: Mortar + Grout Selection Reference