Gyp-rock and other gypsum or plasterboard panels used for building wall & roof sheathing:
This article describes the use and durability & history of water resistant gypsum panlels used in construction of building walls & roofs. We include advice for renovating or repairing gyp-rock sheathed buildings and a discussion of the mold resistance of this material.
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Gypsum board has been used for non-structural wall sheathing (photo at above left) and even roof sheathing on buildings (photo at above left). Above is a row of 1940's buildings constructed at Stewart Air Force Base in Newburgh, New York, apparently as military housing. Some of these structures are still in use while others (foreground) are being demolished.
Originally a brick veneer covered the structure, behind which was this black paper-covered gypsum board sheathing. An ongoing demolition project ca 2012-2014 has removed the brick veneer from several of these buildings, leaving the gypsum-board sheathing exposed to the weather.
Look closely at the photo at above-left [click to enlarge] and you'll see that the interior wall (visible as white through the hole in the gyp-rock exterior) appears to be cement asbestos millboard.
At above right, from a different but similar-aged building, also located in New York State, is the same water-repellent sheathing stamped as in compliance with ASTM Standard ASTM C79/C79M-04a (withdrawn in 2005). Other current pertinent standards include ASTM C1278 / C1278M - 07a(2011) (fiber-reinforced gypsum board) and ASTM C1396 / C1396M - 14 (standard specification for gypsum panels).
Despite having been exposed to full weather in a northern U.S. climate for at least two years, the gypsum board sheathing on the under-demolition buildings at Stewart Field was remarkably intact except for having been smashed about by the demolition crew. Below we include a photograph of the gypsum board sheathing in cross-section.
On buildings where gypsum board was used to cover walls or roofing (photo at left), for structural stiffness we expect to find either plywood or let-in bracing nailed at the corners of building walls.
Initial versions of this product have not performed well on buildings where they might be exposed to dampness or leaks. We have found this material installed under asphalt roof shingles, hardboard siding, and other exterior siding materials.
Gypsum board continues to be marketed as a less costly alternative to plywood or OSB building sheathing.
These panels are intended for use under brick veneer and stucco exterior building wall finishes. Later versions of the material are called cementious board sheathing and can be expected to have been treated with water repellant chemicals.
Producers of gypsum panels used for building sheathing include Georgia Pacific Co. (Densglas gold™), US Gypsum Co. (Fiberock™), and National Gypsum Company (Gold bond™).
Our photo (left) shows a close up of gypsum sheathing board used on building exterior walls and on some roofs.
Georgia Pacific's DensGlas™ exterior building sheathing includes this product description: "The product features a moisture-resistant core and enhanced fiberglass mats, instead of paper facings, to resist the effects of moisture exposure during and after construction.
It is so weather-resistant that Georgia-Pacific backs it with a 12-Month Weather Exposure Limited Warranty. " The company indicates that contemporary gypsum board sheathing such as their DensGlas™ product is intended to serve as a building " substrate behind brick, siding, EIFS, stucco and other permanent claddings."
Watch out: OPINION: unless your building is only expected to last 12-months, you should not leave gypsum board exposed to the weather. In fact few building sheathing products are intended to be left exposed to weather. For example, OSB sheathing board also deteriorates if it is repeatedly wet.
We estimate that from 1942 to 1965 gypsum board was used as exterior wall sheathing on a variety of buildings, especially low-cost structures and in panelized construction as we explain along with our photographs shown below.
We also find gypsum board sheathing used on some roofs, believe it or not.
In some applications a water repellent paper was used to improve the product's durability, as we show in this wall cavity side photograph of identifying marks on gypsum board sheathing.
When gypsum board was used for exterior wall sheathing, as we show in this interior photo (above left), let-in cross bracing was required at building corners. The white paint on the wall cavity side of the gypsum board shown in this photo was added during building renovations to address water damage and to improve water resistance.
Notice the pair of wall studs in the left hand photo? That stud pair marks the abutment of two panelized wall sections in this building. The corner panels were built flat in the panelized home factory and measure just 1/2" under 8' x 4'. Larger 8' x 8' wall panels were also produced and were used for this home.
Our photo at above right shows additional bracing that was incorporated into the gypsum-clad wall panel bottom, along with an assembly wire.
Details of this panelized construction home are at PANELIZED CONSTRUCTION
An example of a drywall/gypsum board identification number appear on this 1950's product shown at left. This is another example of gypsum board used for exterior wall sheathing.
Mold growth on the wall cavity side of drywall is common when there have been leaks into the wall cavity. Our photo (left) shows mold growth on the wall cavity side of gypsum board used as exterior wall sheathing on a 1960's condominium in New York.
See SAMPLING DRYWALL for more information about mold growth on drywall and gypsum board products.
The photographs above show two different gypsum board wall sheathing products that employ a textured paper surface. At right is a modern product found in a home built in the 1990's.
(Sept 9, 2012) Derek said:
I am doing a renovation on my home (built in the mid 1950's) and have come across an exterior sheathing product called Gyproc Sheathing made by a company called Gypsum, Lime and Alabastine in Canada.
It appears to be a drywall type product with a black paper coating. The entire exterior of the home is sheathed in this material. My question is whether you are familiar with this product and whether it contains asbestos.
Yes indeed, Derek. A number of buildings used exterior wall sheathing made of a weather-resistant gypsum board, as we discuss and illustrate here.
In the photo my wrinkled hand is showing a cross section of gypsum board that was installed as exterior sheathing on military housing built at Stewart Air Force Base in Newburgh, NY in the 1940's. More about this material is in the article above.
Continue reading at at SHEATHING, FIBERBOARD Celotex Homasote & Other or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
For plaster type surfaces used on building exteriors, see STUCCO WALL METHODS & INSTALLATION
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