BUILDING BRIEFING: 
Mind the Gap!
Peter Ryland, a surveyor specialising in residential buildings, describes cracks and foundation movements, and explains some monitoring and remedial techniques that property managers may encounter.
Cracks are always cracks, but their cause, effect and remedy can vary considerably. In the main, the cause of a crack is movement, shrinkage, vibration or bad workmanship/design. In order to understand the importance of a crack, it is first necessary to understand the cause, and then consider whether it is a one-off situation that can be patched, or whether there is an inherent problem that needs remedy.
In the later decades of the 20th century there was much press coverage of the effect on buildings of the excessive wet and dry conditions that emanated from climate change, particularly on clay soils. The majority of the problems caused were the differential movement of foundations as a result of the ground beneath them contracting or heaving. These problems, however, were not unique to this period and have been a cause of cracking for many centuries. They are also not the only cause of cracking but are the one which can cause the greatest expenditure on remedy.
Movement
But, how do you decide if a crack is moving? Tell-tales are the answer and there are two main types or methods that can be used to determine movement.
The glass tell-tale is a sliver of glass which is affixed to the wall either side of the crack and passing across it. If movement occurs the glass will break.
A more modern alternative is two pieces of steel affixed to the wall(s) either side of the crack abutting each other in the centre of the line of the crack. When fixed the scale markings on their edges are aligned, allowing a reading as to the extent of any movement over a period of time.
Foundations
A foundation is the base upon which all structures are built and can take one of three main forms: strip, raft or pile.
Strip foundation
The majority of one to three storey residential and commercial properties are built on a strip foundation. Diagram 1 below shows a typical strip foundation, which is the base from which all structural walls of the property are erected. The size of the strip is dependent on the nature of the ground and the size of the structure to be built upon it. The foundation is therefore bearing the load of the structure, and its failure will be evidenced by a crack in the wall above it.
A collapse crack is wider at the bottom than the top whereas the reverse is the case for a heave crack.
The regular cause of the collapse of the foundation is that the ground below the foundation is washed away or is decomposed. This creates a void below the foundation into which the foundation then collapses.
The heave of the foundation can be the result of frost, or pressure on adjacent ground causing it to be compacted with comparable reverse pressure on the ground under the foundation. Frost heave is as a result of the expansion of the water in the ground over a period of time. Although not so prevalent in the U.K. today (we do not have lengthy periods of excessively cold weather), it was prevalent in the 1960's when there were winters with several months of below zero temperatures.
Although the effect is different, the solution is similar - rebuilding the foundation. However, in the case of collapse, this will involve creating a larger foundation than previously (the void has to be filled or built across).
Remedying a heave foundation will usually result in the removal of any excess ground in the affected area and rebuilding a foundation of a similar size to previously.
If there is extensive cracking there may be a need to underpin the wall. This process is carried out by removing sections of the foundation and the wall and rebuilding/reforming the foundation. Lengths of approximately one metre, with at least one metre between them, are removed and reformed as above. The remaining areas between the renewed sections are then themselves removed and reformed.
Raft Foundation
A raft foundation is usually used on sloping sites or where there is made up ground. It comprises a platform of foundation with a "toe" at the end on the lower side of adjacent ground and a "heel" at the upper end. The whole of the raft acts as the foundation of the property, with the heel being the anchor at the back and the toe being the resistance at the front, very much like the use of our own feet to stop ourselves going forward. Diagram 2 shows a typical raft foundation.
Obviously, in view of the extensive size of the foundation, only a major problem would create cracking of the structure above since, in effect, any movement of the foundation will take the property with it. Unlike a strip foundation, heave or collapse in any particular area will not cause cracking as the whole raft acts as the foundation. However, a common cause of cracking over a raft foundation is used is as a result of extensions or peripheral elements of the structure, e. g. Porches, being built on standard strip foundations. If the raft foundation does then move, cracks can form between the main structure and the extensions. The usual form of crack in this case is vertical with has no taper.
In extreme cases it may be necessary to rebuild the extension, as the structural tie between the extension and the main structure may have breached. However, in the majority of instances the remedy is one of making good the crack by filling. The type of filling is dependent upon whether the differential movement is continuing or was a one-off situation. If the movement is continuing a flexible filler is necessary; if not then a non-flexible filler would suffice.
Pile Foundation
Piled foundations are rarely used in smaller residential property but are normally used only in commercial property e.g. warehouses, industrial units. They can be used where there is extensive loading of the structure e.g. blocks of flats.
A piled foundation is normally used where the condition of the ground does not give a stable base for a normal strip foundation e.g. development on filled land or, for example, on an old riverbed.
In simplistic terms the pile is driven into the ground to a depth that allows the base of the pile to rest on top of the bedrock. The structure is built on top of the pile with walls constructed around cross beams between the pile caps.
Assuming the piles are bedded onto the bedrock, there is not normally any movement of the structure except subsidence of the floor slab due to compaction of the sub-ground below. The only solutions are, therefore, either to re-level the floor or to replace it.
Not to be ignored 
In conclusion therefore, cracks to the structure of a building need to be monitored, but some are of a greater significance than others. It is necessary to establish the cause, the effect, the significance and whether it is one-off or ongoing. Once these have been determined, then remedy can be instigated. But it must be remembered that failure to remedy even minor cracking can result in further deterioration as, for example, a crack to rendering can allow water penetration, thereby giving the potential for deterioration of the elements of the structure behind the crack.
Further Information
Understanding Housing Defects by Duncan Marshall, Derek Worthing & Roger Heath, 2nd edition. Published by Estates Gazette Books.
Cracks Caused by Foundation Movement. Good Repair Guide No. 1 - published by the Building Research Establishment. (www.bre.co.uk).
Monitor that crack !
The property manager plays a vital part in monitoring a building. They are the eyes and ears of the owners. Often the tenants will carry on regardless even if a gaping great crack appears just above the front doorway and they have to walk by it every day. An understanding of building movement and its causes can assist in the location of developing problems and their underlying causes before it leads to serious and irreversible damage.
All buildings move. In the majority of cases, this movement will be minor and controlled and have no adverse effect on the structure and performance of the building. On the other hand, other types of movement such as subsidence are more destructive and need to be identified at an early stage and the cause determined before too much damage occurs. The function of the foundations of a building are to support the load of the building and transfer this loading onto the subsoil below. It is here that many of the serious problems start when buildings move.
There are a wide range of reasons for foundation failure and building movement but often it is of inadequate design and poor construction – especially when building on certain types of sub-soil. Some of the common causes are:
Initial settlement
This is normal and occurs as the building is being built, loads are increased and the sub-soil consolidates under the newly applied load. Normally the effects of such movement are expected, will be at their greatest in the first few years after construction and are within acceptable levels of damage to the structure. They might result in cracks up to 5mm across which can be filled for decorative purposes.
Shrinkage – Various building materials have different rates of expansion/contraction. It is common, therefore, to find cracking between various elements of the structure e.g. wooden windows abutting block/rendered walls. The remedy in these cases is merely filling the cracks and making good using a flexible filler where expansion/contraction is likely to re-occur e.g. with summer heat and winter cold.
A further cause of shrinkage is in materials where the drying process has been accelerated and not allowed to happen naturally e.g. plastering on walls. Again the solution is to fill and make good.
Seasonal movement – a number of subsoils are affected by seasonal change. This might occur because the building us built on shrinkable clays which expand and contract in summer and winter as their water content varies. This type of movement is not always destructive; in many cases, the movement in the dry spells is matched by the movement closing back up as sub-soil conditions return to their norm in subsequent seasons.
Differential settlement
This occurs where different parts of the building move at different rates. This might be caused by inadequate foundations – especially for example, where extensions have been built onto the original building.
Trees and Shrubs - Trees and their roots can cause severe damage to neighbouring buildings due to the drying out of the subsoil leading to shrinkage and the ensuing movement of the sub-soil. Again, water-sensitive subsoils such as clay may be more affected by this dessication process than other more stable sub-soils. Mature trees such as oak and poplar can take up to 50,000 litres of water each year from the underlying sub-soil. For this reason, there are now recommended planting distances that should be observed by planting a new tree near an existing building (see Letting Update Jan 2003…. and BRE guidance ). An approximate rule for planting a large tree, is that the tree should lie at least as far from a nearby building as its height at maturity – although there are clearly many cases of existing trees sited much closer to buildings that have had no adverse effect.
A well-documented case is Delaware Mansions Ltd v Westminster City Council, where, in the early part of 20th century Westminster City Council planted a plane tree in the pavement outside Delaware mansions. In 1989 cracking was found in this mansion block and the finger of suspicion pointed to the plane tree. The structural engineers who investigated the cracking devised a scheme to underpin the building and trim back the tree roots. The underpinning turned out to be extensive and expensive. The owners of the building succeeded in recovering the costs, some £570,000, from Westminster City Council. Other problems can be caused simply by the physical presence and action of the roots themselves which can cause damage as they grow and expand through foundations and displace the sub-soil. Problems with trees and buildings are far from simple, and even the removal of the tree can cause further problems known as 'ground heave' due to re-absorption of water into the area around the tree causing a localised upmovement.
Vibration - Most building materials are stable and do not allow for movement. Movement can be caused by vibration e.g. plant not being fitted with adequate dampeners, thereby allowing the vibration to pass into the structure. Remedying the cause is the first concern and once remedied making good.
If, however, the vibration is caused by external influences, e.g. heavy traffic on an adjacent road, then remedy is not a solution; you can only monitor the situation and make good if the problem is minor.
Drains - It is not uncommon to find that subsidence and building movement is the result of faulty or damaged drains (perhaps caused in turn by tree root damage) or similar underground drainage problems. The action of a leaking drain or underground stream or culvert can be to gradually erode away the sub-soil from under or around the foundations, or increase the water content of a shrinkable clay.
Damage can occur to drains by natural settlement of the building simply where they have been built too close to the foundations, or even by the action of tree roots which can have a surprisingly aggressive nature, breaking into drain runs and defective joints in their search for moisture. In a recent example, a faulty underground drain was suspected when differential movement was noticed in a rear patio area. Fortunately the problem was noticed early enough that the action can be taken before more serious and costly damage occurs.