Why Important
What to look for
When viewed from above, a litter of broken pieces of plaster between the branders is sufficient warning that the ceiling is no longer ‘keyed’ and may therefore fall, although many badly bulging and cracked ceilings remain in position for years, whereas others collapse without warning.
Where a ceiling has developed a slight bow, cradling pieces can be formed between the joists to secure it in the existing alignment. This is sometimes preferable to attempting to press the ceiling back to its original alignment.
Inspection, preferably carried out by an architect, is usually from below by tapping with the knuckles to establish the ring of the ceiling, but visual inspection from above is the only certain way to detect loss of key.
This involves lifting floorboards and clearing existing insulation (originally used for deafening, not heat retention) which can be both time-consuming and inconvenient, particularly when the building has been divided into separate apartments/flats.
An additional test is to gently push the ceiling with a brush covered by a duster and to watch for any sign of movement.
When to look
After fire or flood damage, saturated plasterwork must be allowed to dry out.
Saturated plasterwork must be allowed to dry out. This drying-out process will take some months, and there is always the danger that the laths, expanded with water, will crack the plaster and subsequently fail once drying and shrinkage has taken place. For this reason, a ceiling will often collapse unexpectedly many months after a flood has occurred.
How to fix
It is better to retain and repair a sagging original ceiling, particularly if it has enrichments or mouldings, rather than replace it with new plasterboard.
A lath-and-plaster ceiling which has lost its key can be propped on planks supported on temporary trestles or struts while full inspection is undertaken.
Felt pads on the planks will protect the plaster from accidental damage when the ceiling is wedged back to its original position; permanent support can then be provided by metal discs countersunk into the plaster and attached by long brass screws to the branders above. The countersinking can then be filled flush with plaster of Paris.
If water is trapped above a ceiling, a hole should be made in the plaster to allow it to drain away and relieve the load. Holes should be drilled both at floor and ceiling level to allow air to circulate behind the lathing and to allow water to drain away.
If a small part of a ceiling has collapsed, it may be possible to repair it by re-plastering on to the cleaned lathing, although this is not generally recommended. Where plasterboard has been used for a past repair, it should be covered with at least 9 mm of plaster, rather than a mere 3 mm skim coat.
Where some keys are broken, it may be possible to repair from above, using a slurry of lime and water, hessian and plaster of Paris, or glass fibre and resin glues. A patch repair may also be affected by fixing the fallen section of plaster to the branders with long countersunk brass screws and stainless-steel gauze washers.
Generally, new plasterwork is supported on expanded metal lathing, which should be galvanised, weigh not less than 1.6 kg/sq m, and be fixed by galvanised nails or staples at 100 mm centres, in both directions. Stainless-steel lath should be used in areas which are susceptible to damp.
Any patching should be flush with the old plaster and not with the wallpaper. A patch can be hidden by applying lining paper to the whole wall or ceiling before decorating. When re-plastering, three-coat work is recommended in spite of the delays involved in drying each coat.
The use of dehumidifying plant will reduce drying time to a matter of days and at the same time reduce the risk of plaster failure.
Avoiding creating problems
The use of plasterboard for repairs/replacement is not totally unacceptable, although the quality of finish is undoubtedly inferior, and the slight difference in thickness between plasterboard plus plaster and traditional three-coat work may well cause problems at junctions with existing cornices, etc.
Historical background
Ceilings in Georgian homes were traditionally formed with plaster on timber lathing fixed to branders or ceiling joists. Hand-riven laths, 38mm wide by 6mm thick to 12mm thick, were considered to be longer-lasting and less likely to warp than sawn laths.
The first ‘scratch’ coat of plaster was pushed up through the gaps between the lath to form ‘rivets’ or ‘keys’ to take the weight of the ceiling, then scratched when soft to provide a key for the straightening coat, which was finally levelled by the finishing coat.
When the houses were built in the eighteenth century, master casts of the ornaments were taken from clay models, or patterns were carved in timber. Many of these casts are kept by specialist firms, which can supply copies of accurate mouldings.
In the nineteenth century fibrous plaster, known as ‘rag and stick’, was developed for very ornate designs which required an elaborate supporting framework. Fibrous plaster is still available.