Installation Guide

8
Lath & Plaster Problems and Solutions
Conditions/Causes/
Prevention/Correction
Plaster problems are rarely caused by
a gypsum plaster product deficiency.
In general, most conventional plaster
construction problems can be
classified into one of the following
categories.
m
Cracks*
m
Surface defects, blemishes, stains
m
Weak or soft plaster
m
Bond failure
m
Finish coat problems
m
Working qualities and setting
action
m
Painting plaster
The cause(s) of these generally are
the result of:
m
Conditions relating to plaster set
m
Poor quality or incorrect propor-
tioning of aggregate
m
Structural movement
m
Improper mixing, application or
thickness of basecoat or finis
m
Incorrect or improper lathing or
substrate
m
Dirty mixing equipment
m
Environmental job conditions
m
Poor or improper plastering
practices and workmanship
*Conventional plaster, although not con-
sidered to be a structural member, is not
delicate construction and can endure con-
siderable abuse without cracking. However,
cracks can occur when excessive external
forces caused by structural or component
movement over which the plaster is applied
are transmitted to the plaster or by extreme
environmental changes.
Condition
CRACKS
Connected Vertical
And Horizontal Cracks
Occurring At Somewhat
Regular Intervals,
Usually In A Stepped
Or Grid Pattern
Cracks Around And
Over Openings Such As
Doors Or Windows
Fine Cracks Occurring
In A Random Pattern
Usually Less Than
6" Apart. Shrinkage
Cracks, Crazing,
Alligatoring, Chip
Cracks
Random Cracks In Wall
Or Ceiling, Generally
Running Full Length
Of Wall Or Width Of
Ceiling. May Adjoin
Other Cracks And
Are Generally Widely
Spaced
Construction
Plaster applied over
gypsum or metal lath.
Plaster over masonry, cement
block, gypsum tile or gypsum,
wood or other sheet lath.
1. Basecoat over masonry,
cement block, etc.
2. Lime putty finish over
gypsum basecoat.
Plaster over any substrate.
Probable Cause
1. Plaster too thin.
2. Weak plaster (too much
or improper aggregate.)
3. Lath improperly applied.
1. Poor construction of lintel,
door bucks improperly
constructed or installed.
2. Plaster too thin.
3. Failure to use reinforcing.
4. Expansion of wood or
metal framing.
5. Weak plaster, too much
aggregate.
1. Basecoat too rich,
insufficient aggregate.
2. Basecoat too dry when
finish applied – excessive
suction.
3. Insufficient or slowset
gauged finish.
4. Finish applied too thick.
5. Insufficient troweling of
finish.
Structural movement.
Preventive Action
Properly install lath, use
properly sized and propor-
tioned aggregate and apply
plaster to the required
thickness.
Follow proper construction
installation practices.
Follow proper plastering
practices.
Use a properly designed
and installed construction
such as a suspended or
floating plaster system.
Corrective Action
1. Apply another coat of
plaster to proper thickness.
2. Remove defective plaster
work and replace with
properly proportioned
plaster.
3. Check Lathing Specifica-
tions, remove and reapply
lath properly and replaster.
1. Patch cracked areas.
2. Apply coat of plaster to
proper thickness.
3. Remove plaster and provide
expanded metal rein-
forcement and replaster.
4. Remove plaster, provide
control joints, replaster.
5. No correction – remove
and replaster.
1. Remove plaster and redo
properly.
2. Prepare basecoat by spraying
with water to decrease suction.
3. Strip finish, replaster with
proper gauging.
4. Strip finish and correctly
refinish.
5. Strip finish and tightly scratch,
apply 1/32" - 1/16" thick.
Repair cracks.
Large Cracks: Remove
plaster about 6” on each side
of the crack down to the sub-
strate. Place a strip of metal
lath in the cleared area, wet
with water and replaster.
Small Cracks:
Open the
crack slightly, wet and fill
flush with a setting type
joint compound.