Old
Houses And Plaster Cracks:
Cracks in the plaster of older houses may signal
disaster or simply reflect the wrinkles of age.
Knowing the difference can save you some
sleepless nights or even from buying a lemon.
Many cracks are standard in older houses but
there are a few which indicate significant
difficulties.
Houses built prior to 1935 normally have plaster
walls and ceilings which were installed first by
nailing many thin strips of wood, called
lathing, to the house framing materials. Three
layers of plaster were then troweled over the
lath strips. A thick brown coat, then an equally
thick coarse scratch coat and finally a thin
white finish coat was applied. This all added up
to quite a bit of weight and a very rigid
surface.
Houses built after 1935 often had perforated
gypsum board installed in place of the wood
lath, with thinner coats of plaster over this.
The board was more firmly attached to the
framing and weighed less. Rock lath houses, as
these are called, have more durable plaster than
the wood lath houses.
The main difference in all this is that the
weight of the ceiling materials in the older
wood lath houses usually causes the ceilings to
crack and sag, necessitating replacement, while
the rock lath ceilings stay more firmly
attached.
Another difference is in the patterns the cracks
make in the plaster. A very serious crack, which
generally means foundation trouble is one which
runs along the ceiling and then down a wall.
This will usually run in a fairly continuous
line in a wood lath house but will almost always
step along the joints of the gypsum board backer
in the rock lath house. Cracks like this usually
occur all at once and are accompanied by some
floor sagging or sloping. They should be
investigated by a professional. These cracks are
usually due to foundation settlement and, in a
masonry house, usually are accompanied by some
cracking of the exterior walls.
When masonry walls crack, it usually sounds like
someone hammering on the outside bricks.
Some cracks in plaster houses are really of
little concern. These include the cracks in wood
lath ceilings. The usual solution is to apply
half inch thick wood strips called firing strips
to the ceiling with 3 1/2 inch common nails.
This holds the plaster in place. Then gypsum
sheetrock is glued and nailed to the strips. The
result is a smooth finished surface which won't
crack again.
Old wood lath houses almost always have cracks
where different surfaces meet. These are common
where walls come together or where a ceiling
meets a wall. The crack usually runs the length
of the joint. These are almost always nothing to
worry about and should simply be spackled or
caulked prior to repainting.
A long crack (sometimes two parallel ones)
lengthwise in the ceiling of the living or
dining room of a rock lath house is almost
universal. This is usually due to the wood
joists or rafters above expanding and
contracting with seasonal humidity changes. The
ceiling actually bows down a bit in the summer
with the increased humidity and then rises in
the winter. These are difficult to repair
permanently, but the best chance is obtained by
using a fiberglass mesh tape over the crack and
then spackling over it. Naturally, applying a
layer of drywall over the ceiling eliminates the
crack as well.
Expect to find some cracks in certain locations.
Thin cracks starting at the top of doorway
corners and reaching the ceiling are very common
but cannot be ignored if the doorway is in an
exterior wall. Likewise cracks emanating from
the corners of windows should not be ignored.
Stairway openings through floors tend to
concentrate the floor's weight at the corner of
the opening. Some slight wood bending and the
resulting plaster cracking in this area is
forgivable.
Houses built after 1950 usually have the walls
and ceilings covered with gypsum drywall. This
material is usually l/2 inch thick and is
installed in boards measuring 4 x 12 feet. It
has less rigidity and weight than plaster and
more flexibility. The flexibility of this
material means fewer cracks. Cracks above
doorways in "drywall" houses are usually due to
sloppy initial framing.
These houses also occasionally get cracks in the
ceiling wall joints on the second floor level. A
hairline crack starts near the end walls and
progressively opens wider toward the middle of
the house. It opens wide in the winter and
closes in the summer. This phenomenon is due to
the ceiling framing actually rising. It usually
occurs when webbed trusses are used in the roof
construction and loose fill insulation covers
the second floor ceiling. It is felt that
moisture rising through the ceiling and
collecting in the loose insulation causes the
bottom piece of the truss to warm and absorb
moisture and expand. While this is occurring,
the low humidity of the winter air circulating
through the attic is drying and contracting the
other pieces of the truss. These forces combine
to draw the bottom pieces upward and produce the
crack in the ceiling below.
There is no known permanent solution though the
problem seems to diminish over time. One
cosmetic remedy is to attach crown molding to
the ceiling alone which then is allowed to ride
up and down the wall as the seasons change.
Another remedy is to detach the ceiling drywall
from the bottom truss chord within 4 feet of the
intersecting wall and secure it to 2 X 6 pieces
nailed to the top plate of the wall.
Cracks will occur in almost all houses. Don't
lose any sleep over the common ones but don't
delay getting expert advice on the more
significant ones.