Plaster lath board © Daniel FriedmanBulges & Shadow Effects in Plaster Ceilings & Walls - the "Shadow Effect"

  • PLASTER BULGES & PILLOWS - CONTENTS: Plaster & plaster lath ceiling types, history, age determination. Plaster System identification and history of use. Photo guide to split wood lath, pit-sawn lath, circular blade sawn wood lath, expanded metal lath, "rock lath" or plasterboard, drywall, & tainted Chinese drywall. Photo guide to plaster coatings, cracks, hazards. Plaster ceiling collapse hazards & photographs.
  • POST a QUESTION or READ FAQs about how to recognize & diagnose loose or bulged plaster ceilings or walls, causes, hazards, recommendations

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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.

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How to Identify & Diagnose Pillow Effect Bulging Found on Plasterboard Lath (Rock Lath) Ceilings & Walls

Plaster lath board © Daniel Friedman

Shadowing bulged walls traced to gypsum lath installation: as our photos show, below, the wall in this New York home was bulged in a regular rectangular pattern. The building owners were concerned for possible exterior leaks into the wall cavity and a hidden mold problem.

Because the home had experienced leaks in other areas and had other mold problems we investigated further. But the bulged wall problem in this case was essentially a cosmetic one identified and described as plaster wall shadow effect discussed below.

If you shine a flashlight along, rather than directly at a wall surfaces, both regular details (such as regular, rectangular bulges in a plaster wall or ceiling) as well as irregular surfaces and defects are easily observed.

In this photo you can see the convex vertical plaster wall bulges and the indentations at the plaster lath-board points of nailing to the wall studs. More about using oblique light to find building conditions is at FLASHLIGHT HELPS FIND MOLD.

Minnesota inspection expert Roger Hankey has provided our next photograph, below, where the oblique light source was from a window at the right side of the picture.

Shadow Effect in Bulged Plaster Ceilings

Plaster lath board © Daniel Friedman

ASHI home inspector Roger Hankey has provided us with the image of regularly-bulged rectangular plaster ceiling panels shown at left in a home built in 1947. Mr. Hankey includes the following explanation:

Here is the image for the "pillow effect" ceiling from the rock lathe panels. From the ASHI SmartTrack lesson on this topic Hankey quotes:

Shadow Effect A common problem with plaster applied over gypsum lath is the shadow or bulge effect. This was created when the plaster was applied too quickly. The finish coat was sometimes applied before the first coat dried completely. The moisture was driven back into the gypsum lath which sagged. The result is a pattern visible in the wall or ceiling that shows seams every sixteen inches in one direction. Sometimes seams are also visible perpendicular to these, at thirty-two or forty-eight inch intervals.

Compare bulged pillowed plaster ceiling with sagged 16 x 32-inch ceiling tiles?

Bulged water damaged 16 x 32 ceiling tiles - possible asbestos (C) InspectApedia C.H.

At ASBESTOS CEILING TILES - in the article FAQs section we discuss the possibility that the material shown at left, apparently 16" x 32" water-damaged ceiling tiles may contain asbestos and what to do about it.

The ceiling shown above was found in home built in the 1940's.

Doubts about plaster shadow sagging cause & a vote for bulging

Shadow effect from plaster pillowing or sagging (C) Carson Dunlop AssociatesOPINION-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:

Panels - Board Sag:

a. Cause: too much weight from overlaid insulation; exposure to sustained high humidity; vapor retarder improperly installed or wetting causes ceiling panels to sag after installation. Also caused by installing board too thin for framing spacing.

Remedy: remove sagged board or fur ceiling using RC-1 Resilient Channels and apply another layer of board.

Prevention: ... proper frame spacing and application procedures

b. Cause: Water-based textures wet face paper and weaken gypsum core, causing ceiling panels to sag after installation

Remedy: same as above.

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.

A useful ingredient in understanding observations of anomalies or defects buildings is the recognition of the presence or absence of uniformity. We must ask, why is this particular wall or ceiling bulged and not that one? Were they built at the same time, using the same materials? Really identical materials? What is different in and out of the anomalous area? If we can understand completely all of the forces at work, individual building defects will no longer include an element of chance.

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:

[Definition of] Finish-Shadowing:

Cause: Temperature differentials in outside walls or top-floor ceilings causes collection of airborne dust on colder spots of interior surface, resulting in photographing or shadowing over fasteners, furring, or framing. Most severe with great indoor-outdoor temperature variation.

Remedy: Wash painted surfaces, remove spots with wallpaper cleaner, or redecorate surfaces; change air filters regularly.

Prevention: Use double-layer application with adhesively applied face layer. Use separately framed free-standing interior wall surface and insulate in void to reduce temperature difference between steel or wood components and panels.

[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.

Useful Plaster Wall & Ceiling Standards & Texts

  • ASTM C 842
  • ASTM C 841 Metal Lath or gypsum lath installation
  • Plastering, PM 5, Product & Systems Technology, US Gypsum, May 1998, web search 10.5.2010, original source:
    United States Gypsum Company, 125 South Franklin ST., PO Box 806278, Chicago, IL 60680-4124,
    Paraphrasing from this document: USG uses the term shadowing in this document in describing the visual effect over gypsum board joints caused by the lower moisture absorption rate (take-up) and lower capacity than gypsum base face paper. Shadowing at joints occurs where veneer plaster is applied over tape joints, requiring a second coat to completely hide the tape, providing a visually uniform surface. USG Advises: "This [second] cover coat must be allowed to harden and dry before plaster application is started.
  • Gypsum Construction Handbook [purchase at] H17, Technical Folder SA920 and PM2, PM3 and PM4, United States Gypsum Company, 125 South Franklin ST., PO Box 806278, Chicago, IL 60680-4124,
  • Plastering Skills, F. Van Den Branden, Thomas L. Hartsell, Amer Technical Pub (July 1, 1985), ISBN-10: 0826906575, ISBN-13: 978-0826906571 [purchase at]
  • Lath & Plaster Systems, 092300/NGC, National Gypsum Lath and Plaster Systems, National Gypsum Corporation, 800-628-4662 describing National Gypsum's Kal-Kore brand plaster base
  • Gypsum Construction Guide, National Gypsum Corporation
  • Metal Lath Specifications, Specification for metal lath and accessories, Lath and Plaster from Amico, a lath and plaster accessory producer.

For plaster type surfaces used on building exteriors, see STUCCO WALL METHODS & INSTALLATION.

More Reading

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Frequently Asked Questions (FAQs) about plaster ceilings, ceiling materials, and pillowing or sagging pattern diagnosis

Question: is it ok to plaster over "beaver board" ?

Wall test cut shows construction (C) D FriedmanCan I plaster over beaver board ? - Richard


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

- beaverboard is likely to be too flexible- leading to plaster cracking

- possibly you may also have bond strength problems; normally the plaster base coat depends on mechanical adhesion (plaster ears pushed through the lath or through holes in the plasterboard)

- you may find severe bleed-through and brown staining of the finish plaster surface

I'd consider installing a laminate of drywall over the beaverboard instead.

Also see

Weaver: Beaver Board and Upson Board: Beaver Board and Upson Board: History and Conservation of Early Wallboard, Shelby Weaver, APT Bulletin, Vol. 28, No. 2/3 (1997), pp. 71-78, Association for Preservation Technology International (APT), available online at JSTOR.


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