Internal Lime-Plastering

Tim Ratcliffe


	Gently undulating medieval plaster with carved stone details under a coat of limewash

The promotion of modern gypsum-based plasters has led to the almost complete demise of lime plastering, and of many of the traditional skills associated with the craft. This has been exacerbated by the plastering trade being divided into flat and decorative work, with new 'fibrous plasterwork' being made in workshops. Many youngsters entering the trade are now just taught the basic skills to enable them to stick up plasterboard and skim plaster onto it. We are told that it is all down to 'supply and demand'; if this is the case, those of us involved in work on old buildings need to be more demanding.

There is a real need for skilled plasterers who can plaster with lime, and also turn their hands to repairing and reinstating dado and cornice mouldings in situ. The current training system works against anyone gaining this set of skills.

An article like this cannot resolve this skills shortage, nor attempt to even describe the range of skills that a traditional plasterer should have. There are, however, some general principles which anyone involved with lime plastering should be aware of. Sadly there are too many cases of lime plasters failing because the people who have specified the work or the people carrying out the work don't have adequate knowledge or experience.

THE RIGHT MATERIALS?

Most people using lime in old buildings have a vague understanding of the benefits of a 'breathing' mortar or plaster, but if they perceive that lime is too difficult to use they may decide not to bother with it. We need to make it clear that the revival of the use of lime is not some 'airy fairy' idea dreamt up by a bunch of idealists. On the contrary, it is driven by the realisation that buildings are suffering because they have been coated with inappropriate materials, and the people living in them may be less healthy as a result. There is compelling evidence that modern gypsum plasters encourage condensation and consequent mould growth if used on walls that are supposed to 'breathe'. We are beginning to see a revival in the use of lime plasters and we need to encourage a revival of the skills required to use them.

Having bemoaned the lack of proper training for anyone wanting to learn traditional plastering skills, it has to be said that lime plastering is not rocket science. With a basic understanding and a willingness to learn, most plasterers can pick up the skills required to produce a reasonable job in a few days. This is not the same as the skill required to repair plaster in a fine quality country house, or the experience required to match a range of historic finishes, but these things come with time, and we can hope that as more plasterers learn to use lime they will be inspired to develop their skills and understanding further.

There are two characteristics that differentiate lime plasters from modern plastering materials. The first is that they set slowly by absorbing carbon dioxide from the air, in the presence of moisture. The second is that they will shrink as they dry.

Although hydraulic limes, which set more quickly than white/fat limes, are occasionally used for plastering in damp conditions, they are less flexible and breathable than the latter, and their use internally should generally be avoided. All the evidence on old buildings and in written documentation indicates that for centuries, if not millennia, plasterers have chosen to use white/fat limes for internal plastering.

There is some debate about whether we really need to use traditionally slaked lime putty, or if bags of dry hydrated white lime from the builders' merchant are just as good. Although chemically they are the same (both are calcium hydroxide), a traditionally slaked lime, which has been matured for three months, will have broken down into much smaller particles and started to form crystal chains. This gives it better adhesive qualities, helping it to grip the wall more tenaciously. The difference seems to be in the maturing process; so if recently hydrated lime from a fresh bag is left to soak in water for three months, it should be as good as a traditionally slaked and matured lime. However, in practice most of us find it is easier to buy the matured stuff from a specialist supplier.

APPLICATION

Lime plaster in most buildings from the second half of the 17th century onwards was applied in three coats, which enabled a flat finish to be achieved. In agricultural buildings two coats of plaster are common, or even one single coat may be found where an undulating surface was acceptable. Similarly, pre-18th century buildings often have undulating plaster finishes, and this usually indicates that fewer than three coats were used.

Before embarking on any plastering project it is worth assessing the number of coats used originally and/or the quality of finish required. On the basis that three-coat work is the most common in historic buildings, it is best to understand how to apply this and then reduce to two or one coats where appropriate.

SCRATCH COAT

The first coat is known as the 'scratch coat', because the surface is scratched with lines to give a key for the next coat. The mix used is usually one part lime putty to two and a half parts of coarse, sharp, well-graded sand. If the grading of the sand includes more or less of a particular grain size the amount of lime may need to be varied slightly. An experienced plasterer will be able to tell instinctively whether another half part of lime or sand needs to be added. Another way to tell is to take a sample of the dried sand and measure the volume of water required to fill all the voids between the grains; the amount used is equal to the amount of lime required.

Scratch coat on the underside of a spiral staircase

Hair can be added to the mix to give it tensile strength. Although this isn't absolutely necessary when plastering onto stone or brick, its benefit in the long term is that if the building moves or any patches of plaster detach from the substrate, the hair will help bridge over any gaps. Old plaster can sound hollow in places when tapped, but is usually still sound if it contains hair. When plastering onto laths, the addition of hair becomes a necessity, because plaster does not adhere well to timber once it has dried; it relies on interlocking fingers formed as the wet plaster squeezes though the laths and slumps over, so the tensile strength imparted by the fibres is vital.

It is important, before applying new lime plaster, to ensure that it isn't going to be sucked dry by the background it is applied to, as this will cause it to shrink and potentially fail. The suction can be reduced and controlled by wetting the substrate before applying the plaster. If the wall is very dry and porous it may need to be sprayed with a hose pipe a couple of times on the day before, and then once again on the day of application, but if it is less porous and the environment is relatively humid, spraying with a hand-held spray on the day of application may suffice. There needs to be enough moisture in the wall for it still to be damp to the touch after an hour, but no longer glistening with droplets of water.

The scratch coat should be no thicker than 15mm (5/8 inch). Any deep recesses or holes should be 'dubbed out' beforehand, using a stiffer (drier) mix, and allowed to dry to avoid deep pockets in the scratch coat. If plastering onto laths it is important to apply the plaster diagonally to the line of the laths, joining up each time with the previous area laid, to achieve a consistent key between the laths.

While still wet, the surface should be scratched with a three pronged lath scratcher or a single pointed lath (which is slower but gives a better job). The scratching should be in straight lines, diagonally to the laths or the line of the wall, in both directions, to create a diamond or lattice pattern. The quality of the scratching affects the keying of the next coat, so it should be done carefully to achieve an even pattern, and, on laths, particular care should be taken not to cut through to the lathing.

The scratch coat should then be left to dry and shrink before attempting to apply the next coat. In most circumstances it will need two weeks to dry out, but can take up to four weeks in some cases. Shrinkage cracks are likely to appear as it dries, but this is not a problem. The important thing is to avoid it drying too rapidly, which can cause it to fail. Exposed areas of plaster (adjacent to open windows, for example) may need to be covered with hessian or polythene, and the use of dehumidifiers should be avoided.

Once the pad of a thumb can no longer indent the scratch coat it is ready to take the next coat. At this stage the surface should still be slightly damp to the touch and will just need brushing down to remove any loose grains and then lightly dampened with clean water, using a hand-held spray. If it has been left too long and has dried out completely, more water will be required.

FLOATING COAT

Floating coat with devil-float scratch marks

The second coat is known as the 'floating' or 'straightening' coat, and is used to bring the surface to a level plane. The mix is usually slightly less rich than that of the base coat, typically one part of lime putty to three parts of coarse, sharp, well-graded sand, and normally without any hair. Again, it should not exceed 15mm (5/8 inch) in thickness. A level surface is achieved using long 'floating rules' or 'straight edges', passed over the wet surface to remove undulations.

In the best quality work, wooden blocks (known as dots) are temporarily applied and plumbed and levelled; lines of plaster (known as screeds) then join between the dots, and are levelled using a floating rule; finally the spaces between are filled using trowels and levelled with a floating rule with its ends bearing on the screeds. This method was used in finer quality Georgian and Victorian buildings.

Once it has begun to stiffen up, the floating coat needs to be consolidated by 'rubbing up' the surface using a wooden float to counteract shrinkage. This is likely to be required once or twice on the day of application, and may be necessary on the following day as well. The timing depends on the speed of drying. Sprinkling the surface with water, using a brush, assists the circular rubbing action if it has dried out too much.

Once the surface has been compacted, a 'devil float' (wooden float with nails or screws driven through the corners to project about 2mm) is rubbed over the surface to form a key for the finish coat. It should then be left for about a week or so before it is ready for the final coat.

FINISHING COAT

The final coat is known as the 'setting' or 'finishing' coat. It is usually thinner than the other two coats and uses a finer sharp sand. The mix can vary depending on the hardness and the type of finish required; the richest mix being three parts of lime to one of fine sharp sand, and the leanest mix being one part of lime to three parts of sand. More sand will give a harder finish and is more suitable for open textured float finishes; more lime will give a softer surface but allows it to be polished smoother. For standard work, a mix of one to one is suitable.


Setting coat finished with sponge floats and lined out to imitate ashlar

Patch repair (top left) to fine 18th century plaster, gauged with gypsum to achieve a completely flush finish between old and new

The thickness of this coat can vary between 2mm (1/16 inch) and 5mm (3/16 inch). In order to achieve an even finish on the surface, it needs to dry out consistently, so is applied in two or three very thin layers. The additional time and labour required for this is well worthwhile because it controls the drying of the surface and allows the plasterer to achieve the finish required.

Before applying the setting coat the floating coat needs to be lightly dampened with water to control the suction. Each layer is skimmed on as thinly as possible, working in alternate directions each time, and is laid over the previous one as soon as it has had a chance to 'steady-up' (usually in about half an hour). When the work is firm enough it should then be scoured to compact and consolidate the surface.

A straight-grain float (that is to say, one with the grain running the length of the float) can be used for the scouring process, but if a very flat surface is required, a cross-grain float is better. The 'cross-grain' prevents the edges wearing down and ensures that any projections are shaved off as the float passes over the surface. Cross-grain floats are only used to rub or scour over a surface and any specification that refers to using them to apply lime plaster is clearly wrong.

Depending on what is required, the surface can then be worked over using either a trowel, to achieve a fine closed finish, or a combination of wooden and sponge floats, to create an open textured finish. Some water is likely to be required in this process, splashed on with a brush.

The rubbing and scouring process required to achieve a suitable finish can present a problem when patching up to old plaster. The first important point to note is that the exposed edge of the old plaster is likely to suck more moisture out of the new plaster than the wall itself, so more water will be required to control this suction at the edges. Once the area has been patched, particular care will be required when rubbing over the setting coat, to avoid forming an indent at the junction with the old plaster.

In some cases, where an absolutely blemish free surface is required, the setting coat mix can be gauged with Plaster of Paris (a form of gypsum) to minimise the need for scouring. However, the decision to add another material needs to be taken carefully, and the visual compatibility of the repair needs to be balanced with its technical compatibility. This brings us back to the need for traditionally skilled plasterers who understand the materials they are working with. Without these skills we are lost.

This article is reproduced from The Building Conservation Directory, 2006

Author

TIM RATCLIFFE spent two years as a labourer with masons and plasterers working on historic buildings, and 12 years with two leading architectural conservation practices, before setting up his own practice in Oswestry in 2000. He was awarded the SPAB Lethaby Scholarship in 1987, and regularly gives lectures and demonstrations on lime-related subjects.

tim@ratcliffe9.fsnet.co.uk

Further information

Plasterwork