SB326 Balconies with Tile & Grout


Simply put, both the tile and the grout are porous.

Because of their porosity, either the tile discs themselves or the cementitious grout between the tiles may be the source of water entry under the tile’s mortar bed and on top of the plywood’s original waterproofing membrane. Once the water has reached the top of the plywoods’ original waterproofing membrane, the waterproofing membrane can deteriorate over time due to the prolonged chemical interaction between impure water and the membrane’s material. Any water in contact with wood will create conditions for dry rot.


According to the American National Standards Institute (ANSI) “There is a common-held belief that ceramic tile is naturally water-resistant. However, this is not the case.”[1] Tile is porous. According to American Society for Testing & Materials (ASTM) C373 a tile’s porosity varies depending on the density of the tile. Refer to Figure 1.

Figure 1: Porosity of Tiles according to American Society for Testing & Materials (ASTM) C373


As many already know, the cementitious grout between tiles may be as much as 14% porous (i.e., any grout without latex additives).[2] Some recently installed grout may have latex additives to decrease porosity. However, this type of additive has only been available in the last 2 decades.


Impervious porcelain tile and latex-additive grout are the only forms that are close to being impervious. In these cases, any absorbed water is thought to evaporate up and out into the environment before gravity pushes it down through the impervious tile and latex-additive grout.

Other Installation Related Factors Affecting Plywood’s Water Exposure

In addition, even in the best tile installation, the contact area between the troweled mortar (under the tile) and bottom face of the tile disc is only 95%.[3] There would be at least 5% remaining area where the bottom face of the tile and the mortar are not in contact. These air void regions between the uncollapsed ridges/notches of the troweled mortar provide cavities for water to collect in under the tile. Eventually water collected in these regions will also permeate down to the bottom of the mortar bed.


Water from the sky carries with it chloride ions from oceanic wind-blown airborne salt spray. Acid rain carries sulfuric and nitric acids from atmosphere. The water in contact with building materials is impure.

Overall Conclusion

For most ceramic tiles and cementitious grout, because of their porosity, downward water permeation can occur. Therefore, a saturated tile, saturated grout, and eventually, a saturated mortar bed can occur with different tile/grouts and installation techniques. Prolonged water saturation of the mortar bed can deteriorate the plywood’s water proofing membrane. Eventually the plywood can be exposed to water and dry rot can occur.


Where we encounter high levels of moisture detected with our non-invasive moisture meter during initial inspections, we recommend a step-by-step destructive testing approach during our re-inspections as follows:

  1. Contractor to drill a small 3/16” diameter hole through grout at specified locations where initial moisture readings are highest. Drilled hole must not penetrate the plywood.
  2. Use an invasive moisture meter (with prongs) at the drilled hole locations to check the plywood’s moisture content.
  3. Determine if the plywood is over-saturated (>20%) by reading the invasive moisture meter (with prongs).
  4. If not over-saturated, fill up drilled hole with latex additive cementitious grout, epoxy, mastic, or other similar solutions, as close as possible to the existing grout color. Final written notes will be provided stating the moisture content of the plywood is acceptable.
  5. If over-saturated, contractor to remove local tiles to determine the extent of the moist plywood and if any dry rot exist. Further recommendations will be written in our reinspection notes once the impacted region of the plywood is exposed.

We trust this information helps provide clarity on our work and how to move forward.

[1] ANSI Blog: ANSI A108/A118/A136.1:2020 ANSI A108/A118/A136.1:2020 – Installation of Ceramic Tile – ANSI Blog

[2] According to the Ceramic Tile Foundation and American National Standards Institute (ANSI) A108 & A118.6 documents

[3] According to the Ceramic Tile Foundation’s technical advisor Scott in private correspondence

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