Wood Framing To Support Tile Flooring
by John F Mann, PE
Design of a wood-framed floor to support tile flooring requires careful consideration of deflection to minimize potential for cracks in tile flooring.
Numerous homeowners have been greatly disappointed by a new tile floor that cracks extensively after only a few weeks or months. To avoid such result, make sure that floor framing is properly designed.
Recommendations for home buyers are provided after discussion of industry standards for limiting deflection.
Tile floors crack for many reasons. However, if cracks occur in a new house, the floor framing designer will likely be considered a prime suspect if the floor was not designed properly, using industry standards for deflection.
For a renovation project, the contractor (or owner) should obtain evaluation of floor framing before tile installation. Floor reinforcement is very often required to ensure that floor framing satisfies deflection limits.
Floor framing deflection limits included in the building code do not specifically address tile flooring. Therefore, industry standards must be considered.
Design standards for tile flooring installation are published by the following two organizations;
Marble Institute of America for stone tile flooring
Tile Council of America for ceramic tile flooring
Prior to the mid 1990s, the Marble Institute specified (in their Design Manual) a deflection limit of Span / 360 for total deflection (dead load + live load). This was more conservative than the Span / 360 limit for live load specified by the general building code.
However, due to increasing occurrences of cracked tile installation, the Marble Institute changed the limit to Span / 720 for total deflection (dead load + live load). This limit requires much greater framing stiffness compared to the general building code.
Manufacturers of I-beam type floor joists (such as Trus Joist / Weyerhauser and Georgia Pacific) have recommended a deflection limit of Span / 480 for live load to minimize complaints about bouncy floors and vibration. This limit results in 75 percent of the live load deflection using the limit of Span / 360. However, this does not come close to the requirement of the Marble Institute limit.
COMPARISON OF REQUIRED FLOOR JOIST STIFFNESS
Consider a residential first floor joist supporting wood flooring. Dead load is typically taken as 10 psf for design. We can use an index (baseline) value of 1.00 to represent deflection due to live load (40 psf) as well as required moment of inertia (for floor joist) to limit live load deflection to Span / 360. Total deflection will be 1.25 since dead load deflection is 25% of live load.
Now consider the same floor joist supporting stone tile flooring, such that floor dead load is now 20 psf. If we use the same joist, total deflection will be 1.50 (1.25 + 0.25). However, the limit on total deflection is Span / 720, resulting in the need for total deflection to be only 0.50 (half of the 1.00 value for Span / 360). Therefore, moment of inertia for the joist must be 3 times greater (1.50 / 0.50) than moment of inertia required to support wood flooring. Using the Marble Institute standard to govern design, there will be a major design deficiency (for stone tile flooring) if the architect designed for wood flooring using the maximum deflection limit per the building code.
Using the Span / 480 live load limit recommended by I-beam joist manufacturers, total deflection would be 0.94 for wood flooring (0.75 + 0.19). However, total deflection for stone tile flooring would be 1.12, more than 2 times greater than the allowable limit of 0.50.
These results raise the question of whether the Marble Institute deflection limit is excessively conservative. However, without any other standard, design professionals (and contractors) can reasonably be considered at least potentially liable if cracks develop and floor framing stiffness does not meet the industry standard for stone tile flooring considered most valid.
Until recently, the Tile Council had specified a standard (for ceramic tile only), based on using test equipment, that was intended to limit deflection of floor sheathing. However, the limit of Span / 360 was often misunderstood as the same limit for framing members specified by the building code. Therefore, the Tile Council has eliminated the Span / 360 limit from their design guidelines and now references an ANSI standard that requires conformance to the same standard. However, there is (apparently) no practical way for a designer to ensure that design of floor sheathing meets the standard.
There remains much debate about design requirements for wood-framed floors to support tile flooring. Various reports have been published that point to curvature of the floor surface as being the main problem, not deflection. However, unless the industry organizations change focus to curvature, deflection should be the primary criteria used for design, especially for stone tile flooring.
Most important is the need to use much more conservative design criteria compared to design without tile flooring.
Without doubt, two key reasons for cracks in tile floors are flexibility of floor sheathing and discontinuity in the subfloor surface. This is the reason that the Marble Institute recommends installation of two layers of sheathing, with the following conditions;
(1) Total sheathing thickness of 1-1/4 inches (minimum)
(2) Joints in the upper layer ("underlayment") offset with respect to joints in the lower layer (structural subfloor).
Discontinuity in floor joists should also be considered, such as at the ends of joists at or over a girder.
RECOMMENDATIONS FOR BUYERS OF NEW HOUSE
When purchasing a new house that will include large area of tile flooring, the buyer / owner should obtain the following information in writing from the builder (if architect is working for builder) or from the architect (if working for owner);
(1) Floor framing to support tile flooring has been designed specifically (by architect or engineer if applicable) to limit deflection in accordance with industry standards. This requirement is especially important if stone tile flooring is to be installed. Notes on plans should specify; (a) weight of tile flooring used for design, and (b) deflection limit used for design. If clear notes about design for tile flooring are included on design plans, request a copy of design plans used for construction (signed & sealed by architect). If notes are not included on design plans, request a written statement from the architect (signed & sealed) confirming design criteria.
(2) Thickness of tile flooring and thickness of any mortar bed.
(3) Verification that architect (or engineer) has used actual weight of tile flooring for design.
(4) Drawing or sketch showing details of tile floor installation, including; (a) number & thickness of sheathing layers, (b) thickness of mortar bed and (c) any wire mesh installed.
(5) If floor trusses are to support tile flooring, obtain copies of floor truss diagrams showing that; (a) dead load includes weight of tile flooring and (b) proper deflection limit has been used for design. The truss diagram is prepared by the truss manufacturer and should be signed and sealed by a professional engineer working for truss manufacturer.
All too often, tile flooring is installed by the builder without prior consultation with architect to ensure that floor framing has been designed for weight of tile, using industry deflection limits. If this has occurred, the buyer (or new owner) should insist that the builder obtain evaluation by the architect or other qualified design professional (such as structural engineer). If the builder refuses, the buyer or owner should obtain their own independent evaluation from a qualified structural engineer.
If tile has been installed on a floor that was not designed to support tile flooring, remedial work may be necessary to ensure the floor framing satisfies industry standards (especially if stone tile was installed). Floor joists must be reinforced or a header beam must be installed under floor joists. A header beam requires at least two supports, such as columns, which may also require new footings. Cost of this work could be substantial.