The Lime Plaster Guide

What is Lime Plaster?

Lime plaster is the name given to a lime mortar that is applied to cover internal walls and ceilings, it has been used to finish interior surfaces for thousands of years.

It is a mixture of traditional lime, sand and water. It may also contain a reinforcement fibre such as animal hair which can serve to strengthen the material and bind it tightly to the underlying structure.

The Benefits of Lime Plaster

Unlike the modern gypsum plaster found in most homes today, lime plaster is breathable. Not only does it allow moisture to escape and evaporate from the structure, it effectively draws moisture out from the structure thanks to its exceptional capillary properties.

This makes lime plaster hugely beneficial in old buildings with solid walls.

Solid walls behave like a large sponge and can absorb a large amount of water in the event of a leak or flood; there is no cavity to stop the transfer of moisture as you'd find in a modern building.

If you own a property that was built before 1950 then there's a good chance it is finished with lime plaster. In that case, you may have avoided long term problems with damp in the building. Damp issues tend to be short lived where masonry is fully breathable. The moisture is eventually expelled and the masonry gradually dries once the root cause of the damp is located and fixed.

If you are experiencing damp issues in an older building, it could have been replastered with a modern gypsum plaster or may have been covered with a non-breathable coating like an emulsion or even a wallpaper. Once a non-breathable material is added to solid wall, large amounts of moisture can become locked in. The moisture will generally find a way out in some isolated areas and manifest itself as a damp problem.

Types of Lime Plaster

There are three types of lime plaster:

Hydraulic Lime Plaster (Water Lime, Hydraulic)

Made with hydraulic lime, this product is better suited for damp conditions and may be required in extremely damp conditions such as cellars where a non-hydraulic lime plaster would not carbonate sufficiently. Hydraulic lime plaster is less fatty than other lime plasters and may require more effort to apply to laths and ceilings. However, it cures in around 90 days (typically) which means it can be painted with a breathable coating much sooner than other lime plasters.

Non-Hydraulic Lime Plaster (Air Lime, Non-Hydraulic)

Made with non-hydraulic lime, commonly known as lime putty. This plaster is extremely soft and flexible, perfect for conservation of delicate masonry. It sets very slowly by carbonation and typically cures in around 12 months. Non-hydraulic lime plaster can stay wet for an extremely long time in damp conditions where there is no opportunity to carbonate. However, it is quite "fatty" and therefore sticky, it is easier to apply to ceilings and laths.

Three Coat System

Lime plasters are always applied and built up in thin layers called coats, the traditional three coat system is the recommended method:

† Non-hydraulic lime plaster assume warm, dry weather and may be significantly longer.

Scratch Coat

The scratch coat is the first coat applied to the substrate, this could be bare masonry or laths. It is made with a coarse aggregate to leave a rough surface, this is then heavily scratched (hence the name scratch coat). The rough scratched finish provides a good key for the following coat to adhere.

Float Coat / Brown Coat

The float coat (also known as brown coat) is applied on top of the scratch coat and is also made with a coarse aggregate. The surface is ruled to a relatively flat finish and is scratched lightly to leave a key for the finish coat.

Finish Coat

The finish coat is the final, visible surface. It'a purpose is decorative and is made with a fine aggregate to produce a smoother, flatter finish.

Adaptations

The three coat system may not suit every scenario. However, the general principals can be adapted and the plaster can be applied using two or more coats of different thicknesses.

Build up gradually in thin coats

A coat should not exceed 15mm thick.

Lime plaster is heavy and dries slowly compared to modern gypsum plasters. Thick layers are less likely to adhere due to the excess weight and are prone to shrinkage and cracking with the increased amount of moisture.

Don’t proceed until it’s dry

Allow sufficient time for each coat to dry before applying the next.

The recommended drying time is 1 calendar day per millimetre; therefore a 10mm coat would be left to dry for 10 days before applying the next coat. This may vary slightly depending on site conditions but should not be less than 4-5 days in any scenario.

Products

Conserv® lime plaster is created with the traditional three coat system in mind. Lime plasters is available in two formats:

• Base coat
  Suitable for scratch coat and float coat, this material contains a coarser aggregate.
• Finish coat*
  Suitable for finish coat only, this material contains a finer aggregate.

* Please note: Although lime plaster finish is described as fine, it is coarser than a modern gypsum plaster.

Preparation

The lime plaster must be applied to a suitable background, preparation of the masonry is so important. This can require more effort than the plastering itself but shortcuts here can be very expensive.

Non-breathable barriers

Existing non-breathable surfaces or barriers must be removed. This may include previous plasters, cladding, acrylic or other non-breathable paints. Lime plaster is highly breathable but that is of no benefit if the material to which it applied is not.

Vegetation

Remove climbers, vegetation or other kind of organic growth from the masonry. Wherever biocides are used to remove vegetation ensure these will not react with the plaster and produce patches or staining.

Damp causes

The root cause of any damp problems should be identified and remedied before any plaster is applied.

Clean, solid background

The background for the plaster must form a solid base. Remove any loose masonry, mortar and plaster. Cut back masonry where in doubt.

Clean thoroughly to remove dirt and dust.

Cracks and cavities

Cracks and cavities should be cleaned out, filled and sealed with an NHL pointing mortar. Large cracks may be packed with low fire clay tiles, slate or natural rubble before the NHL mortar is applied. Such repairs should be allowed to cure for 1 – 2 days before plastering starts.

Dubbing out

Heavily defaced surfaces or areas with a lot of damaged joints may require a "dubbing out coat" to even out the surface.

Where joints, cracks or holes are severe they may be backfilled with natural rubble.

It is most efficient to apply the "dubbing out coat" by harling. When set sufficiently it can be keyed (approx. 10 hours).

Allow sufficient time for the "dubbing out coat" to dry before applying any plaster. Site conditions will dictate, this could take 5 days or more.

Bordering different surfaces

Wherever the plaster meets different materials e.g. timber frames, stone lintels – it is necessary to insert a metal mesh at the joint (at least 100mm each side).

Hair or alkali resisting fibres can be added to the base coats or applied to metal rib lath to increase the bond and tensile strength.

Key for first coat

A plaster won’t bond well to smooth or dense surfaces. It may be necessary to provide a suitable key for the first coat - there are several techniques at your disposal.

• Raking out
  Where masonry is reasonably porous, a sufficient key may be established by raking out joints. At least every other course of brickwork would be raked out. This may not be adequate for larger units of dense masonry.
• Key coat
  Key coat (also known as a "stipple coat" or "slurry coat") is a product we sell separately on request.

Timber laths

Lath and plaster is a traditional combination still in common use. As well as providing a key the laths also form a gauge to keep the plaster relatively even.

You will require approx. 100 linear feet of lath for every m² of wall.

Laths are fixed to the wall horizontally with 10mm spacing between laths (no more than twice the largest particle size in the plaster base coat – 5mm in this product).

Traditionally fixed using stainless steel annular ring shank nails. Nail guns are more popular these days although non-ferrous nails must be used.

The type of lath is generally specified by the architect or conservation officer. It is most typically sawn larch, sawn oak, riven chestnut or riven oak.

Dampening

Supporting masonry is often very porous, particularly old bricks. This must be severely saturated to reduce suction before applying the first coat.

Masonry

Timber laths is a traditional method still commonly used. As well as providing a key the laths also form a gauge to keep the plaster relatively even.

The process of soaking the masonry would typically start the day before. This may require several iterations to saturate adequately, the last of which just before the plastering starts

Dampening timber laths

Timber laths are soaked for at least 7 days before fixing as recommended. If that is not achievable then immerse in water for one day before the fixing and saturate further 2 hours before work proceeds. This ensures that the lath is wet when fixed. It will then shrink and pull tight so when the plaster is applied it can only expand back to its original fixing position.

Reinforcement Fibres

Fibres can be added to base coat to strengthen the material, this is recommended for plastering on laths. The fibres should be curly or wiry to form an effective mesh and tie into nooks and crannies around the timber laths.

• Natural Goat Hair
  Preferably from animals wintered outdoors for the maximum strength and durability.
• Coir / Coconut Fibres
  A natural waste material with the tensile strength of steel, works very well in practical terms.

These materials will endure the extreme alkalinity of a lime plaster as it cures. Coir is an excellent modern choice; it is completely natural but offers several practical advantages.

Adding fibres

Fibres are added to base coats only, they are never added to the visible finish coat.

Protecting the plaster

The work is at risk until the plaster has adequate time to cure, protection is critical in the meantime, although often overlooked:

• Cold temperatures may cause the plaster to freeze.
• Heat may cause it to shrink and de-bond.
• Sunlight can accelerate the drying process.
  This leads to colour alterations (bleaching) and cracking (shrinkage).

The work should be protected with a breathable membrane such as hessian sheeting.

In temperatures above 10°C (with no risk of frost), the sheeting should be sprayed regularly with water to increase humidity and control moisture loss. This process is known in the trade as "cherishing" and is recommended for at least 5 days per coat.

Coatings

Lime plaster can be painted with a breathable paint when it is fully cured. Standard paints and emulsions must be avoided as they will compromise the plaster's vapour permeability and lock in moisture.

There are some excellent breathable coatings available, in particular we recommend:

• Earthborn Claypaint
• Lime Wash
• Hydraulic Lime Wash

We recommend the lime plaster is fully cured before applying a paint or coating. Please consult the paint manufacturing if you plan to apply it before the plaster is cured.