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Understanding Decay in an Older Home

No two houses are the same, but knowing where to look and what to look for will help you spot potential problems of decay before they develop into something more serious.

A close-up of decaying stucco and plant growth around a broken drainpipe
The exterior of this wall has been damaged by faulty rainwater collection

What causes decay in older buildings?

Before you start work on any kind of repair you should always try to find and treat the cause of the problem, not just the symptoms.

Weather and its effects are the fundamental cause of decay, leading to problems such as damp, mould, woodworm and fungi. Wood, brick and stone are the three most common building materials, and each of these reacts differently to the effects of weathering.

Regular maintenance will ensure you catch problems at an early stage, before full-scale repair is needed.

If you keep rainwater goods clear of debris, there’s little chance of an overflowing gutter leading to damp walls. If your walls are damp, there could be further damage from frost and changes in temperature, which could cause bricks to flake or decay. Salts in this moisture can also harm plaster and paint surfaces.

If wood gets damp and has no chance to dry out because there isn’t enough ventilation, wood-boring insects and fungi can quickly cause damage. A complete lack of ventilation can cause damp timbers, particularly floor timbers, to rot.

If the source of the damage remains unclear after you have made your own investigations, or the problem continues, seek advice from a building surveyor or other professional with experience of older houses. If you want to make repairs to your home you may need permission and should seek advice, especially if your home is listed or in a conservation area (see Who Do I Contact?).

The paint has flaked off a wooden windowsill. leaving it exposed to weather damage
Regular painting will protect window joinery

Some alterations can also damage your home

Ill-considered or inappropriate alterations can also cause decay.

A very common problem with older houses is that external levels can build up over many years. Eventually they become higher than the level of the internal floor structure or damp proof course. This means that the damp proof course, if there is one, is bridged and can no longer work properly. It can also mean that timber suspended ground floors remain constantly damp and can begin to decay because there is little or no ventilation.

It's important to understand the materials from which your home is made. A key characteristic of older buildings is the widespread use of 'breathable' (or 'permeable') materials, which are able to absorb moisture and release it again. If you use modern impermeable materials, such as cement mortars, during repair you may just be causing more damage by trapping moisture. Find more information about using compatible materials.

Brick gateposts lean at different angles as a result of tree roots
Tree roots can cause movement in ground conditions

Not all causes of decay are obvious

Some problems are more difficult to diagnose. For example, copper and lead are incompatible in close proximity, and rainwater washing over copper roof flashings can corrode lead-lined gutters below. You may need to replace the flashings with those made of lead or another compatible material.

Very acidic rainwater - often caused by pollution - can dissolve limestone and corrode metal fastenings. Seek professional advice on how to minimise the damage.

Tree roots can damage the shallow foundations of old houses, particularly in areas of clay soils. Equally, removing fully grown trees in clay soil areas can also cause structural damage through 'ground heave', where the soil expands as it absorbs the extra moisture that the trees used to take up.

Seek advice from a building surveyor or structural engineer if you think a nearby tree might be causing problems. You will need to check whether there is a tree preservation order (TPO) in place in any case. If your tree is in a conservation area or has a TPO you will need consent for its removal. 

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