Tub Shower fiberglass units.

I was told by a building inspector that I could not butt my green board sheetrock up to the shower/tub flange (paper side towards tub of course) and then use compound 90 to float the remainder about 1" gap to the shower/tub unit. Is this correct?
Thank you,
Original Post
The back wall is a one hour rated wall with regular gyp board down to the floor then I ran green board over the regular board to the flange of the tub, He is saying that I am not getting a good enough water seal to the tub unless I lap the gyp board over the tub flange!!!!!! I have floated this area many times before, Im just curious if this is a code or just his preferance?
Thank you for your help,
sticking in my two cents... forget for a moment whether this is code or not... if you have only plaster/compound filling the gap you describe, there is a path for moisture leading to the face of the underlying GB and down to the floor... even if the material you use for 'floating' is waterproof, there is a 'joint' between the bottom edge of the green board and the float material that over time can allow moisture to penetrate back to the underlying substrate

even green board is marginal for achieving true waterproofing, although I can't argue that it has been used universally for 'ages'... but when it is installed the lower edge should overlap the flange, as your inspector has noted, so that water hitting its 'face' is directed (in whatershed fashion) in front of the flange and into the tub...

although you may not feel the inspector should be criticizing 'details', he is doing you a favor

a better solution to the two layer GB application you describe (where there is a gap between the bottom of the greenboard and the tub) is to install a true waterproof material (several inches in its vertical dimension) between the two layers and overlapping the tub flange, for a properly 'flashed' application... from a forensic standpoint I can tell you that this is incredibly cheap protection that can help avoid a significant, costly repair

if you have a situation where the work is in place, see if you can resolve by using a one or two piece flashing (depending on the dimension/detail), slipping a piece of flashing material under the bottom edge of the green board, with a full bead of high grade sealant behind it against the regular drywall, and over the tub flange, with a second full bead between the flashing and the flange... then you can go ahead and finish out the installation, with the tile or other material also reaching down to the tub's shoulder

as an aside... in some jurisdictions I believe green board is no longer considered by AHJ's as water-protective, so does not meet code for that purpose... although I can't recall in what jurisdiction I came across this (couple of years ago)

I could ask why the greenboard was cut short in the first place, resulting in the gap you describe... did you just measure incorrectly, or cut incorrectly... and try to salvage the board(s) instead of cutting a new piece(s)? If so, penny wise and pound foolish.


your sketch shows typical configuration, but does not seem to depict the relationship of the parts per Shaneduty's description... for this discussion let's call the 'stud' in your sketch a layer of GB, as in the post (for rated wall)

if I understand the existing condition the framing is 'wider' than the tub, with a gap between the tub flange and the rated GB layer... as a result the green board could not come down straight over the flange and onto the shoulder... in an attempt to solve the problem, instead of furring (the wall w/first GB or the green board) out he simply stopped it above the flange and was going to 'fill in' the remaining space with compound

one item I did not mention was that if the tub does not sit against the rated GB layer, I hope Shaneduty has installed proper support for the tub shoulder full length of the affected sides so that there is no movement (which will defeat any attempts to maintain a proper seal long term, and may damage the finish material as well as the underlying substrate

I have always been under the assumption that green board was to be used in all moisture areas especially above showers and tubs were you have rising warm moisture collecting and settling on the walls ! I could be wrong I have never challenged the issue. Just my thoughts on that issue,
By the manufactures

Greenboard is out! So now what?
International Building Code (IBC) and International Residential Code (IRC) changes made in 2006 approve the use of a tile backer in wet areas (PDF: 232KB/2pgs) such as tubs and showers, where greenboard is no longer approved. DensShield tile backer dramatically outperforms heavy, hard-to-install cement backer boards in moisture protection, strength and ease of installation.
If you do use it for the ceiling, the joist spacing should be 12" or less.

I don't make this stuff up.

Now on the other hand, Purple board is for use...
Oops! I was assuming that greenboard was the same thing as "water resistant gypsum backing board". Thanks for the information.
And, by the way, I was not accusing you of making anything up. I am here to learn and I always appreciate your input.
Point of note, manufacturers details and newer products should be reviewed with caution. Too often they are accepted as gospel and fail in the field. This is a growing issue in newer offices that lack greyhairs to qc details. As the grey hairs deminish in numbers this unfortunately will become even more of an issue. There can never be enough best practices books published, even if there were they would be too costly for most interns.

what you describe is so true... has been for a long time... and is only getting worse... i see the results every day

same problem with construction tradespeople...

in design offices would you like to venture a guess as to how many CAD draftsmen have ever been on a job site... installed any actual work in the field... have any real understanding of 'how things work', or 'why it has to be done a certain way'... most of the time they are borrowing from standard details, from CAD programs, from catalogs, from old sets of drawings that have been handed down sometimes for generations (with the same incorrect detailing)... they don't know how to critically evaluate what they are putting on paper (so don't even know what questions to ask of their peers or mentors)

making it worse is 'computerization' of the process which in many cases just makes it easier and faster to make mistakes (without knowing that a mistake has been made)... then add 'distant' supervision in huge offices where QC is lacking, or off-shoring operations where the CD's are seen once or twice and not adequately critiqued (because the reviewer does not have an in-depth understanding of how the CD's and details have been developed in the first place)...

factor in our younger generation, now coming into the business arena, with the attitude that because they went to college they must already know everything important, are already entitled to a senior job, and should not have to work their way up the ladder they way our 'grey-hairs' did in past generations

i think the design industry is going to be in for a continuing quality downswing, more law suits, and a loss of image in the eye of the public, until we start making hands-on construction experience and 'honest' apprenticeship work predicates to licensing

International Building Code (CBC) states in section 2509.2 Base for Tile:
Cement, fiber cement, or glass-mat gypsum backers in compliance with ASTM C 1178, C1288, or C1325 and installed in accordance with manufacturer recommendations. and shall be used as a base for wall tile in tub and shower areas and wall and ceiling panels in shower areas.
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.
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