Water Resistant Sheetrock
By John R. Schneider
Mar 10, 2008, 18:42
Q. We recently had our bathroom remodeled. The drywall we used was the green board moisture resistant type. When we had the insulation inspected within the framing the inspector said make sure you use "regular" drywall for the ceiling and NOT the green board, but to use the green board on the walls. After verifying this with the building inspection dept. we proceeded as instructed. However, this does not make sense to me. Why would you install moisture resistant drywall throughout a bathroom except the ceiling which in fact probably gets the most moisture on account of steam, etc. rising up?? D. L., East Bay
A. You certainly brought up a good question that most people do not understand.
The surfaces of tub and shower surrounds are required to water resistant. A standard industry practice has always been to install a vapor barrier ( a waterproof membrane behind the surface of a tub or shower surround to prevent moisture from getting into the wood framing members and causing mold and deterioration to occur. The most common type of surround in residential construction was tile set onto mortar.
However, all of this changed in the 1960ís when technology allowed manufacturers to introduce new materials on the market to act as alternatives to the standard tile set on mortar. These new materials included fiberglass, masonite, and water resistant sheetrock.
Originally, water resistant sheetrock, often referred to green board because of its color, was developed as a fast solution for providing a water resistant surface to act as a backing for tub and shower surrounds. By using this water resistant sheetrock, you did not have to install a vapor barrier behind the sheetrock which was required behind a tile and mortar installation. Not installing a vapor barrier and a mortar surface behind a tile finish represented a substantial cost savings to a builder in the installation of tub and shower surrounds.
The reason that green board is often not allowed to be used on ceilings of bathrooms has a bit of a history. Technically, the building code does allow water resistant sheetrock installed on ceilings of tubs and showers, but it must be nailed to ceiling joists that are spaced no more than 12" apart. This is a problem because normal spacing of ceiling joists is 24" on center.
Although green board is not waterproof, it did provide enough water resistance to meet the minimum standards of the code, and was an acceptable backing for a finished surface of tile, marble, or fiberglass. However, the performance limitations of the sheetrock was not fully understood until a few years after its use. The water resistance of green board relies on a wax solution that was added to the gypsum and paper of the sheetrock. The problem was that the mixture of wax and the gypsum had a slight drawback. It weakened the strength of the gypsum when it got wet or remained damp over long periods of time.
Back in the 1960's when green board was first being widely used in residential construction, there were many failures of ceiling applications, because the board would get saturated and delaminate or break apart from the ceiling. The failures would usually occur after 5 to 7 years of use. Moisture would penetrate the sheetrock through cracked or unsealed joints in the surround and saturate the material. Over time the sheetrock would weaken, and with the weight of the tile, would start to loosen and deteriorate.
In response to this, the manufacturers developed specific installation requirements that ensured the material would perform as it was intended. It was shown that water resistant sheetrock could perform well, if it were nailed to ceiling joists that were installed 12" on centers, and this is what the code now requires. The problem is that homes are not framed with ceilings on 12" centers. And if you combine this fact with the sheetrock's history of past performance, you can understand why certain building departments might want to restrict or prevent its use.
The industry is now favoring cement board over green board as a backing to tub and shower surrounds because of its durability and superior performance in wet conditions. The only drawback to using cement board, is you have to install a vapor barrier behind the board to prevent moisture infiltration to the framing members. Although this is what the industry did before they started to use green board, installing the cement board is faster than installing a mortar base.
The truth of the matter is that ceilings without a finished application (of tile, granite, or cultured marble), can perform quite well with just regular sheetrock and a good coating of a primer and enamel paint. If you think about it, a painted ceiling on regular sheetrock or plaster has been the primary surface for bathroom ceilings for over 100 years. I hope this answers your question.
John R. Schneider is a licensed general building contractor and an ICBO certified residential code specialist. He is president of All About Homes, a residential inspection company, and has been performing code and construction consultations since 1985. Readers may address their comments to John Schneider, 24326 Mission Blvd., Suite 7, Hayward, CA 94544, Fax number: 510-537-8666, or on the web at www.allabouthomes.com . Schneider will answer questions of general interest in the paper. He reserves the right to edit the letter for brevity and clarity. Readers are encouraged to contact a competent contractor or code consultant for specific information regarding questions they may have about their home.
Copyright 2005, John R. Schneider, all rights reserved.
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