Porcelain Over a Marginal Wood Floor With a Height Limit
A homeowner wants 12 x 24 porcelain tile in a 14 ft x 22 ft second-floor family room. The joists are 2 x 10 No. 2 SPF at 16 in. on center, spanning 16 ft 0 in. between supports. The subfloor is 5/8 in. exterior-glue plywood with ring-shank nails. The existing finish is carpet over 3/8 in. particleboard. The tile supplier provided modified thin-set and 1/4 in. cement backer board. The homeowner will not accept more than 1/2 in. of additional floor height at the hallway transition. The installer proposes to remove the carpet, screw the 1/4 in. backer board through the 5/8 in. plywood into the joists, and set the tile with a 1/2 x 1/2 in. notched trowel. The room is over a finished ceiling, and the joists are not accessible without cutting drywall.
1. Evaluate the floor structure and subfloor for the proposed porcelain installation. What checks and corrections are needed before tile is ordered?
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The joist span must be checked against L/360 for ceramic or porcelain tile. For a 16 ft span, allowable deflection is 192 in. divided by 360, or about 0.53 in. Published span tables for 2 x 10 No. 2 SPF at 16 in. on center generally do not permit a full 16 ft clear span for L/360, so this floor is likely undersized. Verify with the actual grade, species, and span table, or with a deflectometer. If it fails, sister the joists, add a beam, or reduce the span; tile cannot fix a bouncy floor. The particleboard must be removed because it is not a tile substrate. The 5/8 in. plywood may be acceptable for a backer-board assembly if the manufacturer allows it and the joists are corrected, but the backer board must be embedded in thin-set and fastened to the plywood, not through it into the joists. Fastening into joists can cause differential movement and cracked tile.
2. Can the proposed assembly meet the 1/2 in. height limit? Show the reasoning and recommend an assembly.
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The proposed assembly as written is not acceptable. Screwing 1/4 in. backer board into the joists is wrong, and the height estimate must include mortar and tile. A typical build-up above the plywood is about 1/4 in. backer plus 1/8 to 3/16 in. thin-set plus 3/8 in. porcelain, or roughly 3/4 in. If the homeowner means 1/2 in. above the existing carpeted floor, that fails. Removing the 3/8 in. particleboard recovers about 3/8 in., however, so the net rise may be only about 3/8 in. if the tile top is compared with the old carpet top. Measure at the transition rather than guessing. If the joists need sistering, that does not add floor height. Use a modified thin-set suitable for porcelain, embed the backer board in thin-set, and back-butter the 12 x 24 tile. Confirm coverage; a 1/2 x 1/2 in. notch may be adequate only with back-buttering and good technique.
3. Write the sequence of correction and installation steps, including the perimeter detail.
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First, verify and correct joist deflection. Remove carpet, tack strip, and particleboard. Inspect the plywood for delamination, water damage, and loose fasteners; repair or replace as needed. If the backer manufacturer requires more subfloor thickness, add plywood rather than forcing the 5/8 in. panel. Establish the finished height at the hallway before installing anything. Spread modified thin-set and set the 1/4 in. backer board, fastening it to the plywood per the manufacturer, typically 6 to 8 in. on center in the field and closer at edges. Tape and skim the joints. Set tile with a modified thin-set, back-buttering each large-format tile, and check for at least 80 percent coverage in the dry room. Leave a gap at the perimeter and fill it with flexible sealant, not grout. Grout the field with a sanded grout suitable for the joint width, then clean and cure.