Reproduced with kind permission from Mike Wye & Associates, Natural and Sustainable Building and Decorating Specialists.
Frequently Asked Questions
1Why use lime on old buildings?
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.
2What is wrong with cement mortar?
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 to high.
3Are these traditional lime products more expensive than modern cements and gypsum plasters?
The materials cost a little more and some firms will also charge a premium for lime work. We run a regular practical course and over the last 12 years many hundreds of owners have been inspired to tackle many repairs themselves. In the long run using the right materials is less expensive than causing damage to the building.
4When should I use a natural hydraulic lime or non-hydraulic lime?
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.
5What is quick Lime?
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.
6What is lime putty?
This is quicklime that has been slaked. Lime putty is sometimes also called non-hydraulic lime or ‘fat lime’.
7What is slaking?
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 around 4 months.
8What sands are suitable?
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 sand with the lime putty.
9What is lime mortar, or coarse stuff?
These are mixes of lime putty and coarse sand, used for building, pointing, rendering and plastering. The mix for mortar is normally 3 or 3.5 parts sand to one part lime putty by volume.
10How long can lime mortar be stored?
It will last indefinitely as long as long as air cannot get in to 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. Mortar left for many months will take more “knocking up” to plasticise the lime mortar.
11Should I protect lime putty or mortar from frost?
Both lime putty and lime mortar should be protected from frost. If frosting should occur, it is possible to reuse frozen lime mortar so long as it is completely thawed out and thoroughly mixed before use.
12What is haired lime mortar?
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. It allows thicker coats to be applied to uneven walls and holds the plaster keys in place when plastering onto lath.
13What is lime plaster?
This is a finishing plaster used as a final skim internally. We supply three grades of lime plaster – 2/1, 3/2 or 1/1 which are the ratios of a fine sand to mature lime putty. The 2/1 grade is suitable for plastering onto haired coarse float coats, the 3/2 is better for feathering into existing plasterwork and the 1/1 is appropriate for matching the finest historic finishes
14What is Limewash?
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 make much more watery limewash. 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.
15What is Natural Hydraulic Lime (NHL)?
This 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.
16What is wattle & daub?
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.
17What is lath?
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.
18What is plaster?
A coating of lime (or cement or gypsum) mortar, usually referring to the inside of a structure.