Repairs
When plaster
dries, it is a relatively rigid material which should last almost indefinitely.
However, there are conditions that cause plaster to crack, effloresce, separate,
or become detached from its lath framework These include:
Overloading.
Stresses within a wall, or acting on the house as a whole, can create stress
cracks. Appearing as diagonal lines in a wall, stress cracks usually start at a
door or window frame, but they can appear anywhere in the wall, with seemingly
random starting points .
Builders of
now-historic houses had no codes to help them size the structural members of
buildings. The weight of the roof, the second and third stories, the furniture,
and the occupants could impose a heavy burden on beams, joists, and studs. Even
when houses were built properly, later remodeling efforts may have cut in a
doorway or window without adding a structural beam or "header" across
the top of the opening. Occasionally, load-bearing members were simply too small
to carry the loads above them. Deflection or wood "creep" (deflection
that occurs over time) can create cracks in plaster.
Overloading
and structural movement (especially when combined with rotting lath, rusted
nails, or poor quality plaster) can cause plaster to detach from the lath. The
plaster loses its key. When the mechanical bond with the lath is broken, plaster
becomes loose or bowed. If repairs are not made, especially to ceilings, gravity
will simply cause chunks of plaster to fall to the floor.
Settlement/Vibration.
Cracks in walls can also result when houses settle. Houses built on clay soils
are especially vulnerable. Many types of clay (such as montmorillonite) are
highly expansive.
In the dry season, water evaporates from the clay particles,
causing them to contract. During the rainy season, the clay swells. Thus, a
building can be riding on an unstable footing. Diagonal cracks running in
opposite directions suggest that house settling and soil conditions may be at
fault. Similar symptoms occur when there is a nearby source of
vibration-blasting, a train line, busy highway, or repeated sonic booms.
In addition to
problems caused by movement or weakness in the structural framework, plaster
durability can be affected by poor materials or workmanship.
Poorly
proportioned mix. The proper proportioning and mixing of materials are vital to the
quality of the plaster job. A bad mix can cause problems that appear years later
in a plaster wall. Until recently, proportions of aggregate and lime were mixed
on the job. A plasterer may have skimped on the amount of cementing material
(lime or gypsum) because sand was the cheaper material. Over sanding can cause
the plaster to weaken or crumble. Plaster made from a poorly proportioned mix
may be more difficult to repair.
Incompatible
base coats and finish coats. Use of perlite as an aggregate also presented
problems. Perlite is a lightweight aggregate used in the base coat instead of
sand. It performs well in cold weather and has a slightly better insulating
value. But if a smooth lime finish coat was applied over perlited base coats on
wood or rock lath, cracks would appear in the finish coat and the entire job
would have to be redone. To prevent this, a plasterer had to add fine silica
sand or finely crushed perlite to the finish coat to compensate for the
dramatically differing shrinkage rates between the base coat and the finish
coat.
Improper
plaster application. The finish coat is subject to "chip cracking" if it was
applied over an excessively dry base coat, or was insufficiently troweled, or if
too little gauging plaster was used. Chip cracking looks very much like an
alligatored paint surface. Another common problem is called map cracking--fine,
irregular cracks that occur when the finish coat has been applied to an over
sanded base coat or a very thin base coat.
Too much
retardant. Retarding agents are added to slow down the rate at which plaster
sets, and thus inhibit hardening. They have traditionally included ammonia,
glue, gelatin, starch, molasses, or vegetable oil. If the plasterer has used too
much retardant, however, a gypsum plaster will not set within a normal 20 to 30
minute time period. As a result, the surface becomes soft and powdery.
Inadequate
plaster thickness. Plaster is applied in three coats over wood lath and metal
lath--the scratch, brown, and finish coats. In three-coat work, the scratch coat
and brown coat were sometimes applied on successive days to make up the required
wall thickness. Using rock lath allowed the plasterer to apply one base coat and
the finish coat--a two-coat job.
If a plasterer
skimped on materials, the wall may not have sufficient plaster thickness to
withstand the normal stresses within a building. The minimum total thickness for
plaster on gypsum board (rock lath) is 1/2 inch. On metal lath the minimum
thickness is 5/8 inch; and for wood lath it is about 3/4 to 7/8 inch. This
minimum plaster thickness may affect the thickness of trim projecting from the
wall's plane.
Proper
temperature and air circulation during curing are key factors in a durable
plaster job. The ideal temperature for plaster to cure is between 55 to 70
degrees Fahrenheit. However, historic houses were sometimes plastered before
window sashes were put in. There was no way to control temperature and humidity.
<PDry outs, freezing, and sweat-outs. When temperatures
were too hot, the plaster would return to its original condition before it was
mixed with water, that is, calcined gypsum. A plasterer would have to spray the
wall with alum water to reset the plaster. If freezing occurred before the
plaster had set, the job would simply have to be redone. If the windows were
shut so that air could not circulate, the plaster was subject to sweat-out or
rot. Since there is no cure for rotted plaster, the affected area had to be
removed and replastered.
Plaster
applied to a masonry wall is vulnerable to water damage if the wall is
constantly wet. When salts from the masonry substrate come in contact with
water, they migrate to the surface of the plaster, appearing as dry bubbles or
efflorescence. The source of the moisture must be eliminated before replastering
the damaged area.
Sources of Water Damage. Moisture problems occur for several reasons. Interior plumbing leaks in older houses are common. Roofs may leak, causing ceiling damage. Gutters and downspouts may also leak, pouring rain water next to the building foundation. In brick buildings, dampness at the foundation level can wick up into the above-grade walls. Another common source of moisture is splashback. When there is a paved area next to a masonry building, rainwater splashing up from the paving can dampen masonry walls. In both cases water travels through the masonry and damages interior plaster. Coatings applied to the interior are not effective over the long run. The moisture problem must be stopped on the outside of the wall.
Repairs
Hairline
cracks in wall and ceiling plaster are not a serious cause for concern as long
as the underlying plaster is in good condition. They may be filled easily with a
patching material. For cracks that reopen with seasonal humidity change, a
slightly different method is used. First the crack is widened slightly with a
sharp, pointed tool such as a crack widener or a triangular can opener. Then the
crack is filled. For more persistent cracks, it may be necessary to bridge the
crack with tape. In this instance, a fiberglass mesh tape is pressed into the
patching material.
After the first application of a quick setting joint compound
dries, a second coat is used to cover the tape, feathering it at the edges. A
third coat is applied to even out the surface, followed by light sanding. The
area is cleaned off with a damp sponge, then dried to remove any leftover
plaster residue or dust.
When cracks
are larger and due to structural movement, repairs need to be made to the
structural system before repairing the plaster. Then, the plaster on each side
of the crack should be removed to a width of about 6 inches down to the lath.
The debris is cleaned out, and metal lath applied to the cleared area, leaving
the existing wood lath in place. The metal lath usually prevents further
cracking. The crack is patched with an appropriate plaster in three layers
(i.e., base coats and finish coat). If a crack seems to be expanding, a
structural engineer should be consulted.
For small
holes (less than 4 inches in diameter) that involve loss of the brown and finish
coats, the repair is made in two applications. First, a layer of base coat
plaster is troweled in place and scraped back below the level of the existing
plaster. When the base coat has set but not dried, more plaster is applied to
create a smooth, level surface. One-coat patching is not generally recommended
by plasterers because it tends to produce concave surfaces that show up when the
work is painted. Of course, if the lath only had one coat of plaster originally,
then a one-coat patch is appropriate.
For larger
holes where all three coats of plaster are damaged or missing down to the wood
lath, plasterers generally proceed along these lines. First, all the old plaster
is cleaned out and any loose lath is re-nailed. Next, a water mist is sprayed on
the old lath to keep it from twisting when the new, wet plaster is applied, or
better still, a bonding agent is used.
To provide more reliable keying and to strengthen the patch,
expanded metal lath (diamond mesh) should be attached to the wood lath with tie
wires or nailed over the wood lath with lath nails. The plaster is then applied
in three layers over the metal lath, lapping each new layer of plaster over the
old plaster so that old and new are evenly joined. This stepping is recommended
to produce a strong, invisible patch. Also, if a patch is made in a plaster wall
that is slightly wavy, the contour of the patch should be made to conform to the
irregularities of the existing work. A flat patch will stand out from the rest
of the wall.
Hairline
cracks and holes may be unsightly, but when portions of the ceiling come loose,
a more serious problem exists. The keys holding the plaster to the ceiling have
probably broken. First, the plaster around the loose plaster should be examined.
Keys may have deteriorated because of a localized moisture problem,
poor quality plaster, or structural overloading; yet, the surrounding system may
be intact. If the areas surrounding the loose area are in reasonably good
condition, the loose plaster can be reattached to the lath using flathead wood
screws and plaster washers. To patch a hole in the ceiling plaster, metal lath
is fastened over the wood lath; then the hole is filled with successive layers
of plaster, as described above.