Section 2509.2

I am not sure if I understand this section completely. Does this mean that the old method of using water resistant gypsum wallboard under paper, wire lath and cement is no longer permitted for tile in a shower?
Original Post
Water-resistant gypsum board or 'green board' per Section 2509.2, is not allowed to be used behind tile "in tub and shower areas', where there is high water exposure, BUT green board is allowed behind tiles in water closet compartments where it does provide effective resistance for a less severe moisture condition.

Note that Section 2509.3, location 1, does allow green board behind approved water-resistant finishes other than tile in the walls and ceilings of shower and bathtub compartments PROVIDED there is no vapor retarder installed on the inside face of the green board.

The 2006 IBC Code/Commentary indicates that the installation of water-resistant membranes on both sides of the green board traps water inside the wall and causes the gypsum board to decompose and fail.

Further, per Section 2509.3, location 2, the IBC Code/Commentary indicates that green board is not allowed in areas where there are extreme moisture conditions with direct water exposure to water or high humidity, such as saunas, steam rooms, gang showers or indoor pools.

i would throw in the following, which is not really addressed by the 'minimum' standards cited in your three current topical postings:

Use of green board or similar GB products as substrates in the described assemblies (i.e. tile) presumes the tile or stone wall finish material in 'moderate-exposure' locations has been installed with a high level of workmanship, effectively acting as a surface barrier (inhibiting all or almost all moisture to which it is exposed.

Experience shows that a level of workmanship cannot be 'presumed' in the current construction industry environment. Factors include: improperly prepared substrate joints and surfaces; highly porous 'open channel' configurations behind tile or stone, reflecting notched trowel patterns with 'skimpy' thinset material, and lack of back-buttering of tile/stone (and allowing moisture/vapor that reaches the thinset layer to travel freely); porous natural stone (i.e. travertine) that is not properly filled or surface sealed; poor or no sealing of tile/stone grout joints, combined with joint cracking caused by a variety of factors; hard grout at tile/stone perimeters in lieu of flexible sealant (and the attendant cracking and spalling of perimeter grout where it abuts or intersects other materials); poor system perimeter and penetration detailing, flashing, etc. due to either improper design or installation; etc. etc. etc. All of this leads to more moisture within the wall finish material 'system' than is contemplated or acceptable.

Designers need to anticipate real world conditions. Even though codes may allow use of certain materials, if a designer understands the inherent weaknesses/problems associated with the materials and systems being incorporated, he/she should anticipate possible deficiencies and account for them in the construction documents. Whereas a good designer has reason to be aware of potential problems, he/she cannot then 'assume' the work will be performed 'perfectly' by the installers. Designers need to be proactive and understand what is needed to do the job when installed using real world parameters. When a designer fails to do this, and then shifts all responsibility to the installers (who are known to be less than perfect) he/she is not being realistic, nor prudent (in my opinion).

Design 'defensively' to get the best results. Understand what is realistically achievable by the average constructor. Detail adequately and correctly - and with an understanding that the details must be clear (and clearly understood). Do not assume the contractor, subs and crews have looked at, read or understand the details (insist on face-to-face dialogue with the crew 'forepeople' prior to the start of any critical work - and that includes anything that has to do with water). Make sure the installers have read and understand the manufacturer's installation instructions (which are usually never looked at by the contractors, many of whom have been incorrectly installing the items for decades). Make sure somebody that understands the requirements is actually inspecting the work, particularly at the front end. As 'sarge' said (now I'm showing my age), "Be safe out there".

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