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Here we provide a photo guide to identifying types of plaster installed in buildings, using building ceilings as a photo and investigation guide. In this article series we describe and discuss the identification and history of older interior building surface materials such plaster, plaster board, split wood lath, sawn lath, and expanded metal lath, Beaverboard, and Drywall - materials that were used to form the (usually) non-structural surface of building interior ceilings and walls. Page top photo provided courtesy of Minneapolis home inspector Roger Hankey.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
How to Identify & Diagnose Pillow Effect Bulging Found on Plasterboard Lath (Rock Lath) Ceilings & Walls
Shadow Effect in Bulged Plaster Ceilings
Compare bulged pillowed plaster ceiling with sagged 16 x 32-inch ceiling tiles?
Doubts about plaster shadow sagging cause & a vote for bulging
OPINION-DF: The SmartTrack explanation above leaves us a little unsure of the sagging plasterboard diagnosis even though we agree that visually, it's a "sag". Take another look at our bulged wall photograph above.
And check the Carson Dunlop Associates explanation of the shadow effect on building walls (left).
The identical bulging pattern appears on a vertical surface, with the convex side of the bulge facing into the room.
While it's natural to suspect weight-driven sagging on a bulged plaster ceiling, gravity cannot not explain the roomwards bulge of the same pattern in a gypsum-lath wall.
Indeed in their Gypsum Construction Handbook, 2d ed. p. 339, USG describes a cause of ceiling panel sagging but does not address the identical bulging in wall panels:
But these causes do not address wall bulging, nor are the remedies complete as cause and cure of excessive interior moisture are not addressed. Further not all mistakes that might cause bulging or failures in board-lath plaster walls and ceilings are addressed, for example use of portland cement based plaster on gypsum board lath, or using perforated board lath on ceilings - two mistakes we discuss at PLASTER TYPE IDENTIFICATION.
We offer an alternative explanation may explain the plaster pillow bulge, at least on walls: a too-wet, too thick, too-slow-drying second or finish coat of plaster applied over the gypsum board lath may have caused both softening (the sagging theory) and swelling-buckling away from the nailed edges (swelling-buckling theory). Wet, humid weather conditions at the time of installation may have been a factor.
We also considered exposure to subsequent wetting events as a possible explanation for plaster sagging. The regular rectangular pattern surely has at its root the original panel size and the fact that panel edges were nailed to ceiling or wall joists or studs.
Modern plaster board joint shadowing
Buckled gypsum board products, including modern drywall, are often observed to have deformed when wet along with their refusal to return to a flat position when dry. Indeed shadowing is described in gypsum product manufacturer's literature such as USG's "Plastering", and shadowing remains a cosmetic concern at joints in modern gypsum-board based plaster systems. To avoid modern shadowing at plaster board joints, two coats of veneer plaster are required at the tape joints and must be allowed to harden and dry before the plaster application is started. But we think that applying this description to the bulged plaster board lath panels above may be an error.
An accurate understanding of the etiology of construction defects is important in forming a reliable opinion about their import as well as their cure or prevention. Many construction explanations mistake confidence for authority and expertise. We're researching this question and will post further results here - Ed.
Other Interior Wall "Shadowing"
US Gypsum in the Gypsum Construction Handbook (2d Ed. p. 339) provides a completely different definition of shadowing, quoting:
[This definition and explanation are incomplete, see THERMAL TRACKING Indicates Heat Loss for details - Ed.]
Watch out: although the ceiling in Mr. Hankey's photo (above) may be soundly secured, other cases of bulged plaster are unsafe. See PLASTER, LOOSE FALL HAZARDS for examples of bulged plaster that may be danger signs, including an example of a collapse of an expanded wire lath ceiling that had been improperly installed.
Details about exterior stucco and metal lath are at STUCCO WALL METHODS & INSTALLATION.
Frequently Asked Questions (FAQs) about plaster ceilings, ceiling materials, and pillowing or sagging pattern diagnosis
Question: is it ok to plaster over "beaver board" ?
Richard, as our photo (above left) shows, people have indeed installed plaster directly over wood-fiber insulating boards, such as in the concrete building in our photo. But in general, I'd be concerned that the beaverboard may not not a good base for plaster finished walls or ceilings installed in a wood-framed structure for these reasons
Questions & answers or comments about how to recognize & diagnose loose or bulged plaster ceilings or walls, causes, hazards, recommendations.
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Technical Reviewers & References
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