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T W E N T Y F I R S T E D I T I O N
INTERIORS
5
Repair using a tray system
Re-fixing original enrichment
will help to draw out moisture if the ceiling
is water-saturated. Another good reason for
using sheep’s wool is that it can be reused as
insulation between joists after the repairs
have been completed, provided that it has
been reliably treated against clothes moth.
Modern closed-cell insulation or packing
foam can be used in some cases but should
be avoided if the ceiling is damp because it
will trap moisture against the surface of the
ceiling.
Once the ceiling is safely supported, the
repair work can begin.
Principal joist failure
Structural repairs
should be undertaken with the guidance of a
structural engineer.
Secondary joist failure
A new joist can
be fixed either to the side of the existing joist
or alongside, but not necessarily touching
the original. All fixing should be carried out
using screws rather than hammer and nails.
Lath/nail failure
In the past the ‘tray’
system was commonly used to reconnect the
lath with the joists. In this method stainless
steel wire mesh was fixed to the inside of
each joist across the back of, but not touching
the ceiling. This formed a suspended bridge
through which a casting plaster mix could be
Back of ceiling showing missing laths due to woodworm
Soft propping using sheep’s wool insulation, timber support and adjustable props
poured. The plaster acted as a stabiliser for
the ceiling by using the mesh as a support,
thus restoring strength and preventing
further movement and cracking.
An alternative method is ‘back
plastering’. In this method the back of the
ceiling should be carefully cleaned with a
vacuum cleaner to remove dust and loose
debris. Care must be taken not to damage
the plaster nibs. The surface will need lightly
dampening to control suction. Sheets of
hessian or fibreglass mesh soaked in casting
plaster are laid over the joist across the back
of the ceiling and over the adjacent joist. This
approach uses much less casting plaster than
the tray system but, like the tray system, it is
still considered an ‘old school’ repair because
it adds significant weight and is irreversible.
The suspension system is preferred as it
involves less alteration and is much lighter.
This system uses stainless steel washers
fixed to the bottom of a wire or threaded bar
which is fixed through the plaster ceiling
from below and suspended on metal straps
attached to the joists above. The washer is
countersunk into the underside of the plaster
and the face of the ceiling is made good
with appropriate materials. One drawback
to this approach is that the repair will need
decoration.
Fire damage
If the ceiling is intact
but blackened by smoke damage it may
be possible to clean it using warm water,
sponges and a suitable cleaning product.
If the fire has caused partial collapse
but the overall strength/structure of the
ceiling is retained then it would be possible
to replace the damaged section alone. This
would involve cutting back the plaster
and laths around the damage to the joists
to form a square opening, fixing new lath
to the ceiling joists, and building up new
plasterwork to match the original.
If the fire has caused major structural
damage and all the supporting joists require
replacement then the entire ceiling may
have to be replaced. As much of the fabric
of the ceiling as possible should be saved,
including flat plaster and ornamentations.
These pieces should be used to make moulds
to cast replicas of the decorative elements of
the ceiling.
Water damage
The repair solutions for
water damage are similar to those described
above. However, a lime plaster is much more
resilient to water damage than one based on
