1. The document discusses methods for assessing and treating deteriorating wood lath and plaster ceilings. 2. A key part of assessment involves measuring wood lath components called keys and lugs from within attics and pull testing their strength. 3. Various treatment methods are described, with plaster consolidation presented as the preferred approach for strengthening the ceiling as a unified system while extending its lifespan.

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Systematic Assessment &
Treatment of Wood Lath & Plaster
Ceilings

2. • 30+ years conserving plaster – it’s all we do
• Proprietary products and tools developed in-house
• 100+ important buildings in Canada and USA
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About HPCS HPCS

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Surface of Wood Lath Before Plastering

4. • Plaster is mechanically suspended from the
wood lath by keys & lugs
• Plaster is not adhered in this system
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What Holds Up a Wood Lath and Plaster Ceiling?
What Holds Up a Wood Lath and Plaster
Ceiling?

5. The keys are pushed up through the lath spaces
The lugs are nicely folded over, “locking “ the
suspended ceiling in place
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6. 1. Water infiltration
2. Trauma from unsafe working procedures – preventable.
3. Damage caused during installation of M&E systems
4. Big structural issues in the framing of the ceiling come
to a head
5. Continuous stress from building micro-movements
6. Moisture vapor transmission mobilizes salts
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t Six Things That Go Wrong

7. How the System Breaks Down
(Exponential Deterioration)
Wood Lath and Plaster Ceiling?
1 If Key A breaks it overloads
Keys B and E by 50%

8. When Plaster Ceilings Collapse
• Clients are always taken by surprise
• Always unanticipated unless it has been
inspected
• These failures are decades in the making
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When Plaster Ceilings Collapsen

9. Plaster Assessment as Practiced by
Others
• Sounding: an informal, unscientific variation of
Impact Echo Testing
• Unreliable
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Plaster Assessments as Practised by Others

10. • Systematic assessment: the application of a
system of analysis to obtain data which can be
empirically evaluated.
The system is easily learned
The results can be Verified and Repeated
But it is a very dirty job.
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Plaster Assessments as Practised by HPCS

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It seldom looks like this… This is more typical
Many consultants don’t want to get this “close” to the problem.
Back Side of Plaster on Wood Lath

12. • Fall arrest gear is often needed.
• In-situ ladders are often unsafe and should be
avoided.
• Lighting is usually abysmal.
• Guano is often present with histoplasmosis risk.
• Broken light bulbs buried in old insulation are very
common.
SUMMARY: Old attics are dangerous places.
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COMPULSARY SAFETY SLIDE

13. Since the objective is to put a numeric value on the present
condition of the plaster, relative to its
“as built” condition,
it is essential to inspect the “structural” components of the
system and evaluate how they have deteriorated since
construction.
These structural components are only found in the attic.
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A Systematic Method of Plaster Assessment

14. • From the attic you can see the mechanical
connection on which the ceiling depends
• From the attic you can also learn about all the other
contingent issues that the owner needs to be aware
of to properly care for the building.
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A Proper inspection must therefore be
conducted from within the attic

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From within the attic you can see the mechanical
connections that the ceiling plaster depends on .
Overview of a Typical Church Attic
AFTER Cleaning

16. Step 1: Select & mark out sample test areas - roughly 3’ by 3’
Step 2 : Measure all keys and record on Data Card for each test
area
Step 3: Pull test (an acquired skill) every key & lug in each test
area – record results on Data Card
Step 4: Calculate Assessment Results for each Test Area and
record on Data Card
Step 5: Tabulate Test Results
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The HPCS Plaster Assessment Method

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Step 1: Select & mark out sample test areas - roughly 3’ by 3’
(Objective: to assess between 1% and 2% of the total square footage)

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Step 2: Measure all keys and record on Data Card for each test area

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Step 3: Pull test (an acquired skill) every key & lug in each test area –
record results on Data Card

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Step 4: Calculate Assessment Results for each test area and record on
Data Card

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Step 5: Tabulate Test Results

23. Average the test results (% “As-Built”
Strength) from all test areas
What do the Results Mean?
• Good snap shot of conditions as of today
• The larger the sample size, the more reliable the
data
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The Final Assessment Results

24. 1. Throwing out the high and low test results
2. The nature of arches
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Some Cautions

25. • Addresses structural integrity of the plaster system
• Numerically quantifies the ceiling condition
• Tells us where plaster system is in its “service life”
• Helps us understand immediate, short and longer
term implications
• Contributes important information to maintenance
planning and budgeting
• Easily learned, repeatable and verifiable
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Benefits and Advantages of a Systematic
Assessment

26. • Many ceilings exhibit losses of 10-15% strength but are not
at risk of failure
• Losses can exist without risk
• Preventive maintenance (i.e. consolidation
treatment) planning should begin below 75%
• Test results below 60% probably represent a safety problem
- Evaluate each situation independently
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When To Act

27. • Keys & lugs are either
missing or broken
• Cracks go all the way
through to substrate
• Plaster is in state of
apprehended collapse
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Example of Damaged Plaster

28. Hole drilled into wood lath above the plaster.
Acrylic resin sprayed into hole.
Acrylic paste then injected into hole
The acrylic paste bonds to and strengthens
the plaster immediately below.
Most of the plaster remains untreated
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Ill-advised Treatment Method Example #1:
Isolated applications of acrylic resin & paste.

29. Wood screws & washers are installed
across ceiling surface
Wood screws & washers deface the ceiling
surface.
The plaster continues to deteriorate around
the points of contact.
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Ill-advised Treatment Method Example #2:
Wood Screws & Washers

30. Acrylic resins are infused into the
base coats with multiple applications
across the entire ceiling
Decaying plaster is significantly
strengthened.
Plaster is effectively converted
from being mechanically
supported by the keys and lugs,
to being fully adhered to the
wood lath substrate.
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HPCS Treatment Method #3:
Plaster Consolidation

31. After consolidation, missing keys
are replaced
This process begins a new era for
the building.
In the absence of UV light, the
resin component of this
methodology will last and
continue to perform indefinitely
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HPCS Treatment Method # 3:
Plaster Consolidation (cont’d)

32. • Addresses the ceiling as a system
• Converts ceiling from a fragile suspended
system to a strong, unified adhered system
• Does not deface the ceiling surface
• Addresses safety concerns
• Greatly extends useful service life
• Often does not require scaffolding
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Benefits and Advantages of Plaster Consolidation

33. http://historicplaster.com/content/plaster-
conservation-projects-united-states-and-
canada
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Projects

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