Advice & Guidance

Understanding Traditional Materials

In order to look after an historic building it is important to understand how it was constructed. There are many differences between medieval buildings with their thick masonry walls and modern buildings, which are often constructed with cavity walls built of bricks or concrete blocks. If we wish to care for older buildings appropriately we have to understand how these differences affect our approach to maintenance and repair. Choosing an appropriate approach to the maintenance and repair of older structures will certainly require us to think about the materials we intend to use. A key point to remember is 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. One of the reasons why old buildings are often pleasing to the eye relates to the materials with which they were constructed and subsequently repaired in previous centuries. Traditional materials often seem to have a natural affinity with each other so that a new stone placed in an old wall in the time-honoured manner will soon blend in and become part of the whole.

Breathing buildings

Traditional buildings are usually built of stone, brick, timber and earth (cob or wattle and daub) held together with earth or lime-based mortars. These materials are absorbent and allow moisture to penetrate the fabric and then evaporate away harmlessly when conditions are favourable. For this reason, traditional buildings are said to ‘breathe'. In such buildings, dampness is controlled by the building's ability to allow moisture to evaporate. The wind and sun aid the evaporation of water from the external surfaces whilst internal air movement through the roof covering, walls, windows and other openings helps moisture evaporate from internal surfaces. As long as the moisture can evaporate freely, the traditional performance of the structure will function as intended and the walls of the building will remain acceptably dry.

Thorncombe St MaryRatcliffe on Soar Holy Trinity

To aid this natural process, traditional buildings were carefully detailed to encourage the shedding of water from their surfaces. Features such as deep overhanging eaves, protective lime renders, lead flashings, wide gutters and sturdy plinths at ground level were used to protect the wall surfaces as much as possible. If such details are altered or unable to function the building might be put at risk. A common example is excessive dampness caused by gutters or downpipes becoming blocked. You might also encounter signs of dampness if the external ground levels have risen above the internal floor level as a result of burials in the churchyard over many centuries.

Waterproof buildings

Modern building materials, such as hard bricks; cement-based mortars and renders; modern masonry paints; and external sealants are specifically designed to keep moisture out of the building by providing an impervious physical barrier. Cavity walls and cement renders protect the building from driving rain and damp-proof courses prevent moisture rising from the ground. 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). However, it is important to understand that these methods are rarely appropriate for older buildings.

Cracked cement in brickwork jointCracked concrete

The following table summarises the differences between traditional and modern building construction.

Traditional mass wall construction

Modern cavity wall construction

Relies on the mass of the wall for ‘weatherproofing'.

Relies on ‘waterproof' materials.

Built with soft, porous, flexible, ‘breathable' materials.

Built with hard, impervious and inflexible materials.

Absorbs moisture and allows quick, natural drying.

Physical break (cavity) to prevent moisture transferring to the inside of building.

Relies on natural ventilation to control the internal environment and prevent condensation and mould growth etc.

Relies on mechanical extraction and physical ventilation to control the internal environment and prevent condensation and mould growth etc.

Mixing old and new

Whilst it may be tempting to use impervious modern materials on old buildings this is not recommended because there is a serious risk that the balance between water entering the fabric and water evaporating from it will be disturbed. The use of impervious materials, even as part of a diligent maintenance or repair programme, can change the way the building functions and have an adverse affect on its performance. For example using cement renders, masonry paints or sealants on the walls of an old building will substantially reduce its ability to allow water to evaporate. As the amount of moisture in the wall rises, the possibility of decay increases. Vulnerable materials, such as timber and soft bricks or stones, are particularly at risk. If you look closely at an old building that has been repointed with modern cement mortar you may be able to see where the original fabric has started to decay more rapidly. This happens because the moisture that is naturally present in the historic fabric cannot escape through the hard and impervious cement mortar. Instead, it evaporates through the softer stone or brick accelerating the rate of decay.

Another important factor is the hardness and inflexibility of modern cement-based renders, mortars and plasters in comparison to traditional lime-based products. The inherent brittleness of cement mortars makes them vulnerable to cracking, particularly where they are bonded to softer, more flexible traditional materials. Even a fine hairline crack in a cement render can cause a problem, as moisture is drawn through the crack by capillary action and is then trapped behind the impervious coating. If this process continues over time, the moisture content of the core of the wall will rise dramatically and may provide the ideal conditions for decay in adjacent timber or plaster.

Damaging cement in a sandstone wallDamage caused by cement

If you do identify signs of dampness and decay related to the use of impervious modern materials in your building, seek advice from your architect or surveyor. Ideally, inappropriate materials should be removed, but only if this is less harmful than leaving them in place. The removal of inappropriate materials can 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

Further information:

© SPAB 2008