Flat-head wood screws and plaster washers were used to reattach loose ceiling plaster to the wood lath. After the crack is covered with fiberglass mesh tape, all will be skim-coated with a patching compound. Photo: John Obed Curtis. |
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.
Patching Holes in Ceilings
This beaded ceiling in one of the bedrooms of the 1847 Lockwood House, Harpers Ferry, West Virginia, is missing portions of plaster due to broken keys. Photo: NPS files. |
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.
Establishing New Plaster Keys
If the back of the ceiling lath is accessible (usually from the attic
or after removing floor boards), small areas of bowed-out plaster can be
pushed back against the lath. A padded piece of plywood and braces are
used to secure the loose plaster. After dampening the old lath and coating
the damaged area with a bonding agent, a fairly liquid plaster mix (with
a glue size retardant added) is applied to the backs of the lath, and worked
into the voids between the faces of the lath and the back of the plaster.
While this first layer is still damp, plaster-soaked strips of jute scrim
are laid across the backs of the lath and pressed firmly into the first
layer as reinforcement. The original lath must be secure, otherwise the
weight of the patching plaster may loosen it.
Loose, damaged plaster can also be re-keyed when the goal is to conserve
decorative surfaces or wallpaper. Large areas of ceilings and walls can
be saved. This method requires the assistance of a skilled conservator--it
is not a repair technique used by most plasterers.
The conservator injects
an acrylic adhesive mixture through holes drilled in the face of the plaster
(or through the lath from behind, when accessible). The loose plaster is
held firm with plywood bracing until the adhesive bonding mixture sets.
When complete, gaps between the plaster and lath are filled, and the loose
plaster is secure.
When ceiling repairs are made with wet plaster or with an injected adhesive mixture, the old loose plaster must be supported with a plywood brace until re-keying is complete. Photo: John Leeke. |
Replastering Over the Old Ceiling
If a historic ceiling is too cracked to patch or is sagging (but not
damaged from moisture), plasterers routinely keep the old ceiling and simply
relath and replaster over it. This repair technique can be used if lowering
the ceiling slightly does not affect other ornamental features. The existing
ceiling is covered with 1x3-inch wood furring strips, one to each joist,
and fastened completely through the old lath and plaster using a screw
gun. Expanded metal lath or gypsum board lath is nailed over the furring
strips. Finally, two or three coats are applied according to traditional
methods. Replastering over the old ceiling saves time, creates much less
dust than demolition, and gives added fire protection.
"Repairing Historic Flat Plaster" an Historic Preservation Brief
