The term 'traditional buildings' is used to describe buildings, constructed before about 1900, with solid walls, shallow foundations and (before about 1875) no damp proof course. They make up about 25% of dwellings in Britain . Almost all historic buildings are of traditional construction.
Traditional buildings differ from modern buildings because of their method of construction and the materials used. A proper understanding of these differences is essential to make informed decisions about the sympathetic and appropriate care, repair and maintenance of historic buildings.
Modern buildings are built to be waterproof - they incorporate impervious materials (hard dense bricks, cement based mortars and renders, modern masonry paints and external sealants), which rely on providing physical barriers to keep out driving rain, and damp-proof courses to prevent rising dampness. Used correctly in the construction of new buildings, such materials and methods are perfectly acceptable and will exclude the elements, as long as they are maintained.
Traditional buildings are usually built of stone, brick, timber and earth (cob or wattle and daub) held together with earth or lime-based mortars, often covered with earth or lime based plaster, render or paint. These materials are absorbent and allow moisture to penetrate the fabric and then evaporate away harmlessly when conditions are favorable.
The levels of dampness in the building are controlled by this ready evaporation of moisture. Externally the porous materials are dried out by the wind and sun. Internally, air movement - through the roof covering, windows and openings - promotes evaporation of moisture from the internal surfaces. Where moisture can evaporate freely the walls of traditional buildings will remain relatively dry.
This ability of older buildings to allow absorbed moisture to evaporate is often described as "breathability". This can be rather confusing, since the process doesn't involve air (as the word implies) but water. Breathability relates to how water moves through structures (water vapour permeability), the ability of materials to absorb and release water as vapour (hygroscopicity) and as liquid (capillarity). This is critical to the performance of a building because water affects everything in a building from the health or decay of building fabric, through thermal performance, to the health of occupants
In circumstances where the building fabric is subject to excessive wetting the rate of evaporation from porous materials may not be sufficient. Traditional buildings were designed with good details to keep water out of the building - wide gutters, deep eaves and verges and plinths at ground level. If those features are altered or made ineffective, e.g. by blocked gutters or raised external ground levels, then the building will be at risk. The degree of risk will be greatly increased in parts of a building exposed to the prevailing wind and rain.
Choosing an appropriate approach to the maintenance and repair of historic buildings requires careful selection of materials. It is important to use only materials that are sympathetic to the building and compatible with its construction. This will generally mean using traditional materials such as lime mortars and plasters.
If impervious modern materials are used on a traditional building the balance between water entering the fabric and water evaporating from it will be disturbed. The "breathing" performance will be adversely affected and problems, including dampness and decay, will occur. Vulnerable materials, such as timber and soft bricks or stones, are particularly at risk
Modern cement based renders, mortars and plasters are hard and inflexible. If they are bonded to softer, more flexible traditional materials these modern materials are susceptible to cracking. Even the finest hairline crack will allow water to enter and any moisture drawn in will become trapped behind the impermeable finishes.
Where modern impervious materials are causing problems of dampness and decay they should ideally be removed. However, t he removal of these materials can itself cause considerable damage to historic buildings so the work must be carefully specified and carried out by skilled craftspeople in order to minimise the risk.