November 2014

 

Question

Our conservatory was built approximately 10 years ago. The brick work on either side of the double doors is now showing subsidence. What should I do?

 

Answer

As your conservatory was built 10 years ago, I assume it is a later addition to your property and did not form part of the original construction. As such, it is formed on its own foundation, separate to that of the main house.

The first requirement is to establish the exact cause of the cracking.The most common causes of cracking are heave, subsidence or settlement, all of which relate to soil movement.

Heave is upward movement of soil, usually due to additional water in an unsaturated, expansive soil. When water is added to soil, particularly soil with clay content, expansion occurs.

Subsidence involves the downward shift of a building’s foundations in line with ground movement and can be a shock/sudden movement associated with mining or landslip. In a residential setting, however, this is more usually associated with changes in the moisture content of the soil. The hallmark of subsidence is differential movement. This is where different parts of the building move at different rates. This movement is not caused by the weight of the building.

A number of factors contribute to heave and subsidence. Soil type is a key factor. Clay soil is particularly susceptible since it shrinks and swells according to its moisture content. Another factor is that vegetation and trees extract moisture from deep within the soil, causing shrinkage. The removal of mature trees can cause soil that was previously dry to swell up, creating an environment for heave.

Also, leaking or damaged drains can contribute towards subsidence. Drains leaking into sandy soil can cause soil erosion beneath a building’s foundations.

Settlement is similar to subsidence in that it is the downward movement of soil. However, settlement is a gradual process and relates to loading in excess of the bearing capacity of the soil. Differential movement is not usually a factor in cases of settlement, although buildings do sometimes settle in this way, particularly new additions to an existing building. In most cases, the damage is only cosmetic and readily repaired during routine maintenance.

Another consideration is that the blockwork contains higher than average levels of pyrite, a mineral found in stone. However, the detection of pyrite in blockwork is a fairly recent discovery, affecting newly constructed properties, primarily in north Leinster. If pyrite levels are too high, blockwork can become unstable over time resulting in cracking.

Subject to your home insurance policy, cover may be provided for loss or damage caused by heave or subsidence. Insurance normally covers the cost of repairing loss or damage due to the event and not the cost of preventing further heave or subsidence. Policies may vary so it is advised to consult your provider.

If you are concerned about movement or cracking in your home, I would recommend engaging the services of a chartered building surveyor to complete a thorough inspection and determine the extent of the problem. Andrew Ramsey is a chartered building surveyor and chartered project management surveyor and is chairperson of the Building Surveying Professional Group of the Society of Chartered Surveyors Ireland (MediKids)

Send your queries to [email protected] or to Property Clinic, The Irish Times, 24-28 Tara Street, Dublin 2. This column is a readers’ service. Advice given is general and individual advice should always be sought