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Advice & Guidance

Understanding Why Buildings Decay

The most common failures in building fabric can usually be traced back to one of three basic causes - poor construction, inappropriate repair or neglect.

Although it is tempting to imagine that all old buildings were soundly and robustly built this is not necessarily true. Medieval structures may not have foundations in the sense that we would think of them whilst Georgian or Victorian buildings might have an outer face of stone tied back to the structure with iron cramps, which are susceptible to rust. There can also be a considerable difference in the quality and durability of the stone itself - granite is generally thought to be a very hard wearing stone whilst some limestones, for example clunch or chalk, are very soft and may be more prone to decay. Flint, on the other hand, is a very durable material but its rounded shape can make it difficult to form a strong bond alongside rectangular blocks of masonry.

Crumbling Blue Lias stoneRusting iron armature

The issue of inappropriate repairs frequently crops up when dealing with historic buildings. One of the most common is the use of modern cement mortar to repoint old walls. Whilst walls do need to be repointed from time to time to protect them from the weather, using a cement mortar is likely to increase the rate of decay and cause a great deal of damage. Such work is usually carried out with the best of intentions but is ultimately harmful to the building fabric. In such a case, it would have been better to seek professional advice about the specification for the repointing works and appoint a contractor familiar with traditional lime mortars. Other examples are the use of chemically injected damp proof courses or the application of proprietary water repellent solutions or modern emulsion paints to old walls. Such products will often prevent the proper evaporation of moisture from the wall and can exacerbate any inherent problems of dampness.

Waterproof coating exacerbating decayInappropriate use of cement

However, in most cases neglect is the main cause of premature building decay. The telltale signs of neglect include broken rainwater goods, blocked air grilles and plants growing in gutters. All of these problems will encourage moisture to penetrate the fabric and prevent its evaporation. If the fabric becomes excessively damp this might lead to blistering paint and plaster, increased rates of decay in masonry and the possibility of timber decay and insect attack. High levels of moisture and excessive fluctuations in temperature can also encourage the movement of soluble salts in masonry structures. Salt movement is characterised by blooms of white crystals on the surface of walls (efflorescence) and can cause considerable damage to plaster and paintwork. In extreme cases, problems caused by excess moisture might even lead to walls becoming unstable, a potentially serious structural issue.

Ivy overwhleming a gutter and downpipeBroken gutter and mossy roof

The weather also plays an important role in the decay of structures. Prolonged exposure to acid rain can start to dissolve some limestones and will also corrode metal ties and fastenings. Driving rain can penetrate deep into solid walls where the pointing is missing or decayed and condensation in a poorly ventilated building can lead to mould growth or encourage decay in timberwork. Frost can also contribute to the break down of building materials. If there is too much moisture in porous walling materials, this can freeze in winter. As the moisture freezes it expands and can shatter the surfaces of old bricks and tiles. This process can have a dramatic effect if the cycle is repeated a number of times over the winter season. Storm damage and high winds may also play a part as they dislodge slates, tiles and leadwork.

Bricks damaged by frostSandstone eroded by weather

The natural world can have a devastating effect on old buildings too. Tree roots can disrupt foundations and some climbing plants, such as mature ivy, can be strong enough to force rainwater goods away from the wall if allowed to grow behind them. Although it may look attractive, ivy can cause a great deal of harm to some types of masonry such as flint walling and should not be encouraged. Even small plants can become a problem as their roots will tend to break down mortar and may even force joints open allowing water to penetrate into the fabric.

Though protected by law, some animal species, such as bats, can have a negative impact on fabric and fittings. Evidence of bats roosting in the roof spaces may include scatterings of droppings and traces of urine on furniture and floors. Masonry bees may burrow a nest in soft mortar joints whilst birds can cause problems when they choose ecclesiastical buildings as nesting sites. Twigs, debris and indeed dead birds are common causes of blocked rainwater goods. There are even reports of woodpeckers preferring shingled spires to living trees!

Timber decayDry rot

Under the heading of the natural world, we may also include the wood-destroying fungi commonly known as wet and dry rot. Dry rot (Serpula lacrymans) is the most aggressive wood-destroying fungus and thrives in unventilated voids. It often has a musty smell and can develop into grey or white cotton wool-like sheets with tiny orange spots. Wet rot is a generic term that refers to decay occurring in very damp conditions. An example is the cellar rot fungus (Coniophora puteana), which may be identified by dark brown strands appearing on the surface of the timber. This fungus commonly causes exposed wood to soften and lose strength. Where there is rot we may also expect to find wood boring insects such as furniture beetle (woodworm) and the larger deathwatch beetle at work, as they are attracted to warm, damp, unventilated conditions. Flight holes and bore dust are typical symptoms.

It should be remembered that these fungi and insects all have one thing in common: they generally only cause significant damage where dampness exists. If rot or insects are present, this indicates an underlying building problem. Conversely, because dry timber is less vulnerable to attack, successfully arresting decay involves eliminating moisture and promoting drying.

More extreme occurrences such as fires or floods can, of course, have a devastating effect although these are thankfully rare. It is worth remembering that a constant drip which goes undetected for years may in the end prove to be a more serious and expensive problem.

© SPAB 2008

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