Building with Lime: Frequently Asked Questions
We are asked many questions regarding the advantages of using lime-based building products on traditional buildings. Here are answers to some of the most frequently asked questions:
Many old buildings are constructed from relatively soft and porous materials such as brick, cob and some stone. Lime putty based mortar was normally used for bedding and plastering. The walls are usually solid without a damp proof course. Lime mortar is softer and weaker than stone, brick or cob 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 freely evaporate. It is this permeability that is sometimes referred to as ‘walls being allowed to breathe’. Lime mortar helps to keep a building dry inside.
Cement mortar is usually hard, brittle, less porous than lime mortar and sometimes completely waterproof. It is damaging to traditional buildings for several reasons.
Cement mortar is often harder than old brick, some stone or cob, therefore when movement occurs the edges of the stone or brick are pushed against the hard mortar resulting in the masonry being damaged and the mortar itself cracking.
Hard cement mortar can trap moisture behind it causing damage to the structure or encourage ground water to rise from the base of solid walls. Trapped water in the wall causes decay and crumbling, in severe cases the cob, brick or stone fails. Masonry or cob is also susceptible to frost damage if water is trapped in the wall.
Over time lime mortar decays but not the cob, stone or brick. It is much simpler and cheaper to re render or repoint a wall than repair the fabric of the building.
This is the raw material used to make lime putty. The quick lime we use is a made from, high calcium carbonate limestone from Buxton, Derbyshire.
This is slaked quicklime. Lime putty is sometimes also termed as non-hydraulic lime or ‘fat lime’.
Slaking is the process for making lime putty. Quick lime is added to water. A chemical reaction occurs resulting in the release of a large amount of heat from the quick lime. When this has cooled sufficiently the liquid is drained off through a sieve (to remove unslaked quicklime granules, stones or other impurities) into a settling tank. The longer the slaked lime is left in the tank the better, we leave ours for a minimum of 3 months.
Should be a washed sharp sand – one that crunches in the hand. Sharp sand is angular in grain (hence the name), free of vegetable matter, clay and salts. Also know as coarse sand.
Lime Mortar, or Coarse Stuff
Mortars are used for rendering and pointing. The mix for mortar is normally 3 parts sand to one part lime putty.
Haired Lime Mortar
Plasterers hair, (animal hair, usually goat), is added to the mortar. Hair is used to give extra strength minimising shrinking and cracking. This is used for plastering onto cob, stone, brick or lath.
Fine Plaster, or Fine Skim
This is a finishing plaster as the name suggests, it is used as a top coat. The ratio of a fine lime plaster is 2 parts sand (finer sand than is used for the mortar) and one part lime putty.
Limewash is made from lime putty. The mix is approximately 1 part lime putty to 2 parts water. The more dilute the limewash the more durable the finish will be. The resulting material is little more than the consistency of water, hence limewash. Pigments can be added to colour the limewash. White limewash will dry to a brilliant white. At least 3 coats of lime wash is required on new plaster.
This is made from limestone that contains impurities such as clay. Unlike non-hydraulic lime they have the ability to set underwater or in the absence of air. There are three classifications NHL 2 (feebly hydraulic) NHL 3.5 (moderately hydraulic) and NHL 5 (eminently hydraulic). There are situations where the use of non-hydraulic lime is not suited, particularly in severe exposure situations i.e. chimneys, copings, wall heads and paving. In these cases NHL 3.5 or NHL 5 should be used. However hydraulic lime may be stronger than non-hydraulic lime therefore it may not be suitable for use on soft, weak materials such as cob and perished stone or brick.
A mixture of clay (we source ours locally) and chopped straw. This was traditionally used as in ‘wattle & daub’. Wattle is strips of wood formed as panels. Daub is used as the plaster. Daub makes a good first coat of plaster onto straw bale. Then subsequent coats of lime plaster can be added to make a truly resistant surface.
Are strips of wood that can be oak, chestnut or softwood. These are often found in traditional buildings. They would be lime plastered. When the plaster has failed the lath can be clearly seen. Sometimes the laths need replacing because they have suffered insect attack or moisture has caused rotting. Moisture behind the wall quite often occurs when inappropriate cement repairs have been made to lime render. Lath can be replaced with new.
These are pre shrunk blocks about the size of a concrete block. They are ideal for cob repairs where holes have occurred in the cob and infill is required.
Lime Product Safety Data Sheet
Walls coating of lime or cement mortar.
Smoother than render, usually termed for internal work.
Fillets of mortar to fill the gaps between stones or bricks in walls.
Scat Coat, Harling, Thrown or Rough Cast
These terms all mean the same thing. They describe a technique of applying plaster. The mix is very runny and is literally thrown at the wall. Creates a rough textured finish.
A square slightly curved trowel designed especially for throwing plaster.
Riven or Sawn Lath
Riven is lath that has been hand split along the grain and is stronger and has less tendency to move over time. Sawn lath is machine cut and is consequently half the cost of riven lath.