A Systematic Method for Assessing Wood Lath & Plaster Ceilings in

Provider: Conference for Catholic
Facility Management (“CCFM”)
Provider No.: G460
Course Title: A Systematic Method for
Assessing Wood Lath & Plaster Ceilings in
Historic Churches and Heritage Buildings
Course No.: SEA 1305
Historic Plaster Conservation
Services & John Tiedemann Inc.
Speaker: Rod Stewart
May 7, 2013
Credit(s) earned on completion of
this course will be reported to AIA
CES for AIA members. Certificates of
Completion for both AIA members
and non-AIA members are available
upon request.
This course is registered with AIA CES
for continuing professional
education. As such, it does not
include content that may be deemed
or construed to be an approval or
endorsement by the AIA of any
material of construction or any
method or manner of
handling, using, distributing, or
dealing in any material or product.
Questions related to specific materials, methods, and
services will be addressed at the conclusion of this
Copyright Materials
This presentation is protected by US and International
Copyright laws. Reproduction, distribution, display and use of
the presentation without written permission of the speaker is
Historic Plaster Conservation Services Limited &
John Tiedemann Inc.
Conference for Catholic Facility Management
Participants will learn how to
safely access and work in
attics above historic wood
lath and plaster ceilings, and
how to methodically assess
and quantify the condition of
a plaster ceiling as a system.
Learning Objectives
At the end of this course, participants will be able to:
Understand how to manage the risks involved with working in an attic
environment above a wood lath and plaster ceiling, and various
occupational health and safety considerations to take into account,
using examples and photographs.
Understand how to select, mark out and prepare specific test
locations within a large plaster ceiling, using examples and
Understand the skill of pull-testing plaster keys and lugs, and how to
systematically record the test results, using examples.
Understand how to tabulate and extrapolate the test results to
quantify the overall condition of a plaster ceiling, which will lead to a
decision on the need for preventive maintenance treatment, using
sample results.
In plain language:
• Learn to work safely within attic space in order
to assess the plaster condition
- select sample test locations
- learn how to pull-test plaster
- learn how to record the test results
• Learn how to interpret the results
The Cultural Importance of Historic Plaster
A systematic assessment: the application
of a system of analysis to obtain data
which can be empirically evaluated.
The Current Art of Assessment
Ancient technique of sounding
In scientific world – “Impact Echo Testing”
Use of sound waves
Uniform density = uniform wave patterns
Variable density = variable wave patterns
Used extensively on aircraft fuselages
Sounding in Architecture
• Much less effective in architecture
• Good for cast plaster ornaments
• Very limited value with plain face plaster
The Challenges of Assessment
Church buildings are huge
Full surface assessment is prohibitive
Conflicts of interest
Facility managers need good, reliable data
How do we do this?
Another Method of Assessing Plaster is
• Easily learned
• Repeatable and verifiable
• Simple premise: the wood lath and plaster in
your building is part of a system
• System deterioration process starts from
What Holds Up a Wood Lath and Plaster
• A suspended system
• Mechanical keys and lugs
• Plaster is not adhered to the wood – that is it’s
• If it was “pasted” on – it would have soon
cracked and collapsed
How Wood Lath and Plaster Ceilings
Were Made
• Wood lath - the base
• Lath was green and soaking wet
• Typically, 3/8th inch gap between each strip of
• Wet plaster applied in several coats, forming a
hard, brittle covering
• Wet plaster “slumps” to form keys and lugs
• Wood lath shrinks – leaves plaster shell
suspended…with flexibility
Two Types of Wood Lath
Sawn Lath
Split Lath
What Can We Learn From Close
• Lugs and keys hold the ceiling up
• They support the ceiling collectively
• Need to understand how individual keys
and lugs fail in order to understand the
deterioration process
The keys are pushed up through the lath spaces
The lugs are nicely folded over, “locking “ the
suspended ceiling in place
How the System Breaks Down
Five Things That Go Wrong
1. Water infiltration and erosion
2. Trauma from unsafe working procedures – highly
3. Big structural issues in the framing of the ceiling come
to a head
4. Ongoing stress from building micro-movements
5. Moisture vapor transmission mobilizes salts
How plaster deteriorates on its own:
• Small continuous movements of building
• Transmission of moisture vapor
When Plaster Ceilings Collapse
• From looking at the surface, no sign of trouble
• Never any warning – collapse is sudden and
• Apprehended collapse - burden of loadbearing weight shifts to fewer and fewer keys
• Failure is decades in the making
The Benefits and Advantages of the
Proposed Assessment Method
• Assesses structural integrity of plaster system
• Tells us where plaster system is in its “service life”
• Helps us understand immediate, short and longer
term implications
• Contributes important information to maintenance
• Vital for budget planning
The Importance of the Other Side of the
Ceiling - the Attic
• Only from the attic can you see the
mechanical connection on which the ceiling
• Hence, a proper assessment must be
conducted in attic
Overview of a Typical Church Attic Space
The Challenges of Working in an Attic Above a
Plaster Ceiling
• Darkness, dirt and dust
• Bat and/or bird guano sometimes present –
potential risk of histoplasmosis
• Steep cavernous arches
• Poor ventilation
• Repository for garbage
• Enormous square footage, especially in churches
Steep cavernous arches difficult to navigate
The Method
Objective: to safely assess and quantify the
condition of the plaster ceiling as a system
For today’s discussion:
• Large mid-19th century gothic, heavy timberframed church with approximately 10,000
square foot ceiling area
• High nave and sanctuary with side aisles and
clearstory windows
Step 1: Establish that the attic is a fit place to go
Preliminary Measures
Determine the conditions of the attic
Is there lighting?
Are ladders and guardrails in safe order?
Are there any bio-hazards present?
Are there electrical hazards?
Is there debris?
Is the ceiling insulated?
Is there flooring covering the ceiling access?
Is the attic a “confined” space?
Do we need specialized professional assistance?
Other Issues and Basic Safety Precautions
• Liability insurance for contractors and additional professional
assistance – case by case considerations
• Personal Protective Equipment (PPE) and Fall Arrest training
required - same as any construction site
• Once the attic is deemed safe to operate in, assessment can
Step 2: Select Sample Test Areas
• In our case study, approx. 10,000 square feet of
ceiling surface
• Sample test area - roughly 3’ by 3’
• Objective: to assess between 1% and 2% of the
square footage
• Sample areas must be representational
Step 3: Mark Out Test Location
• Mark out test areas using red/green/blue
painters tape
• Take photographs
• Number each location – L1, L2, etc. – on Test
Data Card
• Staple or tack card within test location.
Test Data Card
Sample Test Location
Step 4: Clean Sample Test Areas
• Gently scrub with bristle brush
• Vacuum clean
(Wear safety mask and goggles - this is dirty work)
Sample Test location (after cleaning)
Step 5 – Measure All Keys Within Test Area
• Measure all keys (intact, missing and broken)
within test area
• Record the total number of linear inches on
the Test Data Card
Step 6: Pull-test Keys and Lugs
Pull-testing Keys and Lugs
Testing plaster keys is an acquired skill and a vitally important one:
• Hold a well-formed key between thumb and index finger and exert
slight upward rotational pressure
• One of three things will occur:
1. The key will move under slight pressure with no snapping
2. The key will stand firm against the pressure applied
3. The key will stand firm for a time and as pressure is increased,
will snap off.
• After pull-testing, measure all missing and broken keys within the test
area and record total number of linear inches on the Test Data Card
Note: every key and lug within the sample area must be tested
Step 7: Collect Broken Keys
Step 8 – Re-clean Test Area
Step 9: Measure functioning keys, apply
formula and complete Test Data Card
Step 10: Tabulate Test Results
Calculate the Results
• Average the test results (% “As-Built”
Strength) from all test areas
What Do The Results Mean?
Snap shot of conditions as of today
The larger the sample size the more reliable the results
Some Cautions
1. Throwing out the high and low test results
2. The nature of arches
When To Act
• Many ceilings exhibit losses of 10-15%
strength but are not at risk of failure
• Substantial loss can exist without risk
• Preventive maintenance planning should
begin below 75%
• Test results below 60% probably represent a
safety problem
This concludes The American Institute of Architects
Continuing Education Systems Course. At this time, the
course participants are free to ask questions.
Rod Stewart , Historic Plaster
Conservation Services (HPCS) & John
Tiedemann Inc. (JTI)
Contact: Andy Guljas
HPCS: 1-888-6242854
[email protected]
JTI: 1-877-600-2666
[email protected]

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