Why building structures experience movement
All building structures experience movement and most buildings develop cracks in their fabric at some stage. Only rarely does cracking indicate serious structural failure.
Many owners are unaware of a problem until their house goes through the selling process, when a valuation report for a lender recommends that an Engineer's report is obtained.
This can be an unwelcome surprise for many vendors In this article we will discuss some of the main causes of movement and outline how surveyors assess visible evidence to establish if remedial work is needed.
This is one of the main causes of movement in buildings and in most cases does not mean that major works are needed. Settlement is mainly downward movement caused by the inherent weight of the building structure and its various elements It is normally limited in nature and does not threaten the structural stability of the building. Often occurring shortly after the building is completed or when a new extension is added, the building or the individual element normally settles to a new equilibrium and movement stops enabling making good to be completed.
Subsidence is the downward movement of the ground, often caused by clay subsoils drying out When the ground beneath a building subsides or compacts, it rarely does so evenly. Consequently, part of the building and his uneven movement puts stress on the structure and causes it to crack.
House built on clay
There is clay distributed throughout the U.K., but the drier warmer climate in the southeast makes subsidence more likely in that area.
As clay soil dries out it will shrink allowing the building to settle. This usually occurs in long dry weather periods (e.g. the drought in the mid 1970’s); physical changes in drainage schemes; growth of trees taking up more water etc. When the clay becomes rehydrated, it swells which can cause heave, which may affect a structure by lifting the foundations. This can occur after a dry spell when the weather changes to a wet period; by the action of frost causing swelling by freezing and the moisture content expanding; or by the removal of trees causing water content to rise as the roots no longer take up moisture etc.
House close to trees
Trees draw moisture from the soil. During dry spells, their roots spread out in search of water, accelerating the drying out of the subsoil.
Cracking is common in dwellings on shrinkable clay soils close to trees. When trees are close enough their roots take up moisture, drying and shrinking the clay soil below foundation level. Conversely, where trees have been removed swelling of the clay can also cause damage known as heave.
Peat subsoils will compact under a building load, as the moisture is squeezed out allowing the structure to settle. Unlike clay, however they are not prone to expansion in wet periods. The building will normally settle and find an equilibrium position.
Subsidence can also be caused by leaking or collapsed drains, particularly on sandy soils. If built on sandy soil and drains leak, the soil will be washed away under the foundations. Older drains earlier than the early 1970’s are likely to be rigid jointed and very susceptible to fracture. In most cases involving structural movement affecting houses built on sandy soils the first step in establishing the cause is to undertake a drain survey
Made up ground can consolidate where there are voids and organic matter and variations in the water table or surface water collecting in the fill can cause ground movement, particularly with loosely backfilled ground. Some type of material can react badly with groundwater and can expand or contract. This problem is often seen within the first ten years of the life of the building.
The instability of sloping ground can result in gradual and sometimes sudden movements known as landslip. Some types of subsoil are more susceptible and in particular clay is at risk of movement on sloping ground. Generally, steep slopes are more problematic..Slopes created by recent ground works are most suspect.
Subsidence due to mining operations is a special case and a topic in itself. Mining subsidence occurs when the mining excavation or supports collapse.
Alterations including extensions, new openings in walls etc., will result in changes to the distribution of loads. This often causes cracking at the ends of lintels and is usually a once and for all movement, which ceases as the building achieves a new equilibrium.
Expansion and contraction occurs in brickwork and other materials due to thermal effects. Good design should allow expansion joints. Vertical cracking patterns in brick walls is often related to expansion and contraction.
Most commonly affecting Portland cement products, sand lime bricks and timber products. Shrinkage cracks are probably amongst the most common but normally dont involve significant movement. Normally making good is required.
Defective lintels cause distress to the brickwork over. Fracturing can occur in houses where replacement windows usually PVCu, have been installed, affecting particularly bay windows. In some cases lintels over openings are inadequate or missing altogether leading to fracturing and adversely affecting the operation of windows and doors. Soldier courses were often built with a mild steel rod passing through for support, which can corrode and fail and may provide less stable support than a modern lintel. The remedy is to cut out affected areas of masonry, insert a modern lintel and make good.
Current requirements should ensure that all elements of a building are tied together. The older the building the more likely it is to suffer from problems associated with lack of restraint. Walls were often butted up with minimal or no bonding; floor joists running parallel to a wall would have no lateral ties to support the wall. Many Victorian houses have suffered outward bowing of particularly flank walls and tie bars were often introduced to restrain further movement
Mainly a problem found on older properties and may affect all or only part of the structure (problems with garages, porches and bay windows and conservatories are particularly common) Strip foundations may be too narrow to adequately support imposed loads. Foundations need to be constructed to a depth suitable for subsoil conditions
Assessment of Damage
Assessing damage to the building is fundamental to establishing the correct repair and surveyors and engineers will refer to BRE Digest 251 - Assessment of damage in low-rise buildings with particular reference to progressive foundation movement to help classify movement and establish the need for repair
This Digest discusses the assessment and classification of visible damage resulting from structural distortion in conventional masonry low rise buildings. The assessment is based on a description of work considered necessary to repair the building fabric; classification into six categories is recommended, taking into account the nature, location and type of damage
The six categories are:
Hairline cracks of less than about 0.1 mm which are classed as negligible. No action required
Fine cracks which can be treated easily using normal decoration. Damage generally restricted to internal wall finishes; cracks rarely visible in external brickwork. Typical crack widths up to 1 mm.
Cracks easily filled. Recurrent cracks can be masked by suitable linings. Cracks not necessarily visible externally; some external repointing may be required to ensure weather-tightness. Doors and windows may stick slightly and require easing and adjusting. Typical crack widths up to 5 mm.
Cracks which require some opening up and can be patched by a mason. Repointing of external brickwork and possibly a small amount of brickwork to be replaced. Doors and windows sticking. Service pipes may fracture. Weather-tightness often impaired. Typical crack widths are 5 to 15 mm, or several of, say, 3 mm.
Extensive damage which requires breaking-out and replacing sections of walls, especially over doors and windows. Windows and door frames distorted, floor sloping noticeably. Walls leaning or bulging noticeably*, some loss of bearing in beams. Service pipes disrupted. Typical crack widths are 15 to 25 mm, but also depends on number of cracks.
Structural damage which requires a major repair job, involving partial or complete rebuilding. Beams lose bearing, walls lean badly and require shoring. Windows broken with distortion. Danger of instability. Typical crack widths are greater than 25 mm, but depends on number of cracks.
The Digest is intended to help building professionals, property valuers and insurance advisors both in putting building damage into its true perspective and in determining necessary action, either in the form of seeking expert advice or in recommending repairs. In the case of the most serious structural movement the only certain way of confirming the progression of damage is to take a series of measurements with time and monitoring will be recommended before repairs can be specified.