How do you repair more serious damage to a ceiling? There may be serious cracks, areas of plaster which are completely missing, or perhaps the ceiling has been plasterboarded but should have a cornice. For more general restoration, see here.
Use this diagram to work out what approach to take:
If there are cracks in the ceiling or cornice; have a closer look. If there is a 'step' from one side to the other, then this suggests that the plaster has failed.
One remedy is to screw the dropped side to the joists; use 'penny' washers on the screws. The risk is that this work will cause more areas to fail; this damage may not be immediately obvious so monitor the situation.
If you have a completely plain ceiling, the best option is to replace the ceiling. You could take the existing ceiling down, remove the laths and nails and then fit plasterboard and skim with plaster. Alternatively, remove the plaster but retain the laths and re-plaster in the original way.
If you have a plain ceiling but with cornice where this is intact, with care the main part of the ceiling can be removed and replaced, making good any minor damage to the cornice.
If it is the cornice which is cracked, bear in mind that this is usually also attached to the top of the walls and so is less likely to fall. You can wipe PVA glue into the cracks, and make good the plaster. More significant damage can only be repaired by making replacement cornice; an expert can either 'run' it in situ or make a mould to prepare a replacement section.
If you have an elaborate ceiling with serious cracks or which has fallen in one area, you can try a repair with screws and washers but success is not certain. If the remaining plaster has clearly separated from the laths, then the best, and safest, option is replacement but this is expensive. Before taking the rest of the ceiling down, have an expert in to make the necessary moulds.
Plastering is one of those skills which is difficult for most DIY-ers to master. Ceilings are even harder than walls. You could attempt a small areas of, say, less than a square metre and you may be able to achieve a reasonably level and smooth finish.
Ideally the new plaster should match the composition of the original; subsequent fine cracks between the old and new areas are less likely. You can use a modern plaster instead but expect cracks.
You can carefully remove only the damaged ceiling plaster, and then get a plasterer in to replaster. This is usually a good approach if you want to avoid disturbing cornices.
If you opt for plasterboard, remove the old plaster. Remove the laths, and all the nails. Prepare a piece of plasterboard, cutting it to size. Nail loosely and then complete the nailing at intervals. If necessary add a second sheet to bring the level close to the original. Gaps between boards are then taped and the whole area skimmed.
Before you consider your options, you need to determine whether the coving or other detail is modern or old; if the plaster is modern, it is not usually worth attempting to match it. It is better to remove the plasterwork, make good and fit off-the-shelf or custom-made plaster.
If the plaster is original, it may be possible for you to make minor repairs using plaster of Paris from an art shop.
Large areas of straight runs, for example of cornice, are best left to a specialist firm. They will make a mould and supply a replacement length. Some will also fit it.
Small, complex shapes, such as a modillion, can be made using a latex or silicone rubber product to make a mould from an existing pattern. Ensure the original is stripped of all paint. Coat it with a release agent and then paint several coats of the moulding product over it. Remove it once it has dried.
If the resulting mould is not very stiff, set it in a box of sand to provide support.
Dab plaster of Paris into the details and pour in more to fill the mould. Tap it to draw out bubbles and level the top. As it sets, score the top to make it easier to fix in place.
PVA or tile adhesive can be used to fix the plaster shape in place.
For companies specialising in this area, see the Products and Services Directory.