FAQs

Before this century, building techniques and materials were very different from those employed today. Traditional properties need to “breathe” to allow moisture inherent in a solid wall construction without a damp proof course to evaporate from the external stonework or render. Many old buildings are constructed from materials such as brick, cob and stone which are relatively porous and often of lower strength. Lime mortars were normally used for bedding and plastering. Lime mortar is a relatively softer mortar and therefore it is able to withstand a certain amount of movement (without cracking) that comes with settlement and seasonal changes in ground conditions. Lime mortar is porous and allows moisture to evaporate, helping to keep a building free of damp.

Apart from the adverse effect it has on the environment in general, cement mortar is usually hard, brittle and less porous than lime mortar. It often contains additives to make it sometimes completely waterproof and is damaging to traditional buildings for several reasons.

Cement mortar is often harder than old bricks, cob or some types of stone. Therefore, when movement occurs it may damage these softer traditional construction materials. Hard cement mortar can trap moisture behind it causing damage to the structure and encourages ground water to rise up a solid wall by capillary action.

Trapped water in the wall can cause poor insulation, decay and crumbling. In severe cases a cob wall can fail. The brick, cob or stone can also be subject to frost damage if moisture levels are too high.

The materials cost a little more and some firms will also charge a premium for lime work. In the long run using the right materials is less expensive than causing damage to the building.

We have comprehensive guidance notes on our website together with YouTube videos to assist you. We are always more than happy to give advice over the telephone.

We offer free traditional building consultancy advice over the phone.

Natural hydraulic limes set even when wet and come in a range of strengths. They are useful for building with stone or brick where the earlier set may speed up construction. You can also add a pozzolan such as Argical to a lime putty mortar.

We suggest that for external bedding and pointing late in the year natural hydraulic lime should be used. It is capable of a faster initial set in cold weather.

For most external rendering and internal plastering jobs, the fattiness of lime putty makes a superior mortar that allows coarser sands and thicker coats to be applied without shrinkage.

This is the raw material that is used to make lime putty. Quicklime is made by burning limestone or chalk in a kiln. This drives carbon dioxide into the atmosphere, leaving a very reactive material called quicklime (calcium oxide). It is made in a range of sizes from lumps down to very fine powder. We use the finest grade of high calcium quicklime from Derbyshire.

This is quicklime that has been slaked. Lime putty is sometimes also called non-hydraulic lime or ‘fat lime’.

It is the process for making lime putty. Quick lime is added to water and a chemical reaction occurs resulting in the release of a large amount of heat from the quick lime creating a boiling liquid. The liquid is drained off through a sieve into a settling tank. Chemically the calcium oxide is converted into calcium hydroxide. The longer the slaked lime is matured in the tank the better the lime putty. We leave ours for a minimum of 3 months.

For lime putty mortars, the sand used for building, pointing and backing coats of render and plaster should be a washed sharp coarse sand. We use a sharp sand to BS882 which is free of vegetable matter, clay and salts. For finishing coats of internal plaster we mix a very fine, kiln dried sand with the lime putty.

These are mixes of lime putty and coarse sand, used for building, pointing, rendering and plastering. The mix we use for our lime mortar is 3 parts sand to 1 part lime putty by volume.

We supply our premixed lime mortars in sealed dumpy bags or sealed tubs. It can potentially last indefinitely as long as long as air cannot react with it and carbonate the lime. This means keeping it from drying out and storing it free from frost. Mortars with hair teased in will gradually lose the hair as it dissolves in an alkaline wet mortar and you will end up with an unhaired mortar.

Both lime putty and lime mortar should be protected from frost. If frosting should occur during storage, it may be possible to reuse frozen lime mortar so long as it is completely thawed out and thoroughly mixed before use.

Animal hair, usually horse, cow or goat, is added to the backing coats of lime mortar used for external rendering or internal plastering onto stone, cob, brick or timber lath. Adding hair gives extra strength and minimises shrinking and cracking. This allows thicker coats to be applied to uneven walls and holds the plaster keys in place when plastering onto lath.

Lime plaster is the fine top coat skim used internally. We supply two grades of lime plaster – Heritage and Regency.

Heritage Lime Plaster is our standard lime skim which produces a traditional match to many historic plasters.

Regency Lime Plaster is an ultra-fine skim using marble dust which is more in keeping with Victorian/Georgian properties and the closest match to gypsum finishes.

This is typically a powdered additive that reacts with lime to produce a quicker and harder set. Historically, ash or brick dust was used as a pozzolan but we use metakaolin which is produced  burnt clay.  We recommend a pozzolan is always used wit traditional lime putty mixes when using externally to offer from future frost and adverse weather conditions. It can also be useful for damp environments internally.

Natural Hydraulic Lime (NHL) is made from limestone that contains impurities such as clay or silicates. Unlike lime putty which is non-hydraulic lime, NHLs can set in damp conditions. They also have some free lime available for carbonation. There are three European classifications NHL 2, NHL3.5 and NHL5 based on the compressive strength of laboratory mortars after 28 days.

These are often somewhat misleadingly termed feebly hydraulic, moderately hydraulic and eminently hydraulic. We generally recommend the use of NHLs where the need for breathability and lower strength is outweighed by the desire for an earlier and harder set such as working on wall copings, chimneys and slate floors.

Wattle are strips of wood, often hazel, woven between timber studs in a timber frame house. A mixture of clay and chopped straw was used as the plaster to coat the wattle.

These are thin strips of wood used as a key for the lime plaster. Chestnut, oak and larch are popular choices of timber for the lath. Riven lath is hand split along the grain using traditional tools and provides extra key on the surface of the lath so it is best for repairing lathed ceilings. Sawn lath is straighter with a less textured surface than riven and is best for repairing lathed walls. In traditional buildings the lath would be lime plastered and limewashed.

Lath can deteriorate over the years due to insect infestation or rot, especially if modern less breathable plasters or paints have been applied.

Cob blocks are made from a clay-based sub-soil with straw and are about the same size as a concrete block.. They are used for repairing holes and damage to cob walls or rebuilding new areas. They are already dried and hence will not shrink in use.

A coating of lime (or cement), usually referring to the outside of a structure.

A coating of lime (or cement or gypsum), usually referring to the inside of a structure.

Filling the gaps between stones or bricks in walls with a lime (or cement) mortar.

Also known as scat coat or rough cast coat.

Harling is a technique for applying a coat of render or plaster. The lime mortar has extra water added to bring it to a porridge-like consistency. It is then cast on the wall using a special tool called a harling trowel which has a curved blade. It provides a rough texture when cured that gives extra key for the next trowelled coat and also controls suction with very thirsty materials such as cob and brick.

Limewash is a traditional, breathable paint made from lime putty which is thinned with water. Our limewash is approximately 1 part mature lime putty to 1 part water. Other suppliers may use different concentrations of putty to water and sometimes other additives are included.

Limewashes are coloured with pigments and can be used internally or externally on lime plaster, lime render, stone or brick. It works best on porous surfaces and hardens as it absorbs carbon dioxide from the atmosphere to form calcite crystals, giving it its unique appearance. Typically four coats are needed on new external render and three coats on new lime plaster.