Brickwork
- identifying common defects in residential buildings
The property management function requires that the manager can identify and report on defects to a property as they become apparent, and report them to the property owner. This article examines brickwork and its associated properties and defects. By Roger Heath - Senior Lecturer in Building Surveying, Faculty of the Built Environment,University of the West of England, Bristol.
Background
The external walls of a domestic building perform three key functions - they support the roof, provide weather protection and, more recently, thermal protection. They are constructed with a variety of materials including timber, mass concrete and concrete block work, and brickwork. They may be covered with other materials such as timber, plastic, metal or render.
The use of bricks as a construction material has a long history in this country. The earliest remaining residential brick buildings date from the late medieval and Tudor periods and were mainly timber framed. The 17th and 18th centuries saw increasing numbers of brick buildings constructed for residential, commercial and industrial occupation as brick manufacture became more efficient and, with the development of suitable industrialised processes, millions of brick buildings were erected in the 19th and 20th centuries.
Until the First World War, these brick buildings had solid external walls, although a small number of cavity walls were constructed in the Victorian period. The thickness normally ranged from 112mm (half brick) thick for extensions and outhouses, through 225mm (one brick thick) for general domestic two or three storey construction, to 337mm and sometimes 450mm thickness for the lower walls of taller buildings. Stone detailing and lime render finishes (rather than cement render) were both common features.
After World War 1, the cavity wall gradually became the commonest form of external wall so that, after the 1930s, very few solid-walled buildings were constructed. The 1930s cavity wall had two skins of brickwork normally connected by galvanized vertical twist ties or butterfly ties. Concrete block internal skins became popular in the 1950s, and thermal block from the 1970s as insulation values became important. In the last twenty years or so, the wall ties have changed to stainless steel or plastic and the internal skin is likely to be either thermal block or timber frame.
The quality and types of bricks have varied immensely. In the 19th century they have included: sun-baked bricks (often used for cheaper housing, generally of poor quality and requiring a render finish to provide weather protection); ordinary quality bricks (common Flettons and similar bricks) for use in non-exposed areas; higher quality facing bricks; and semi-engineering and engineering bricks of great strength that could be used below damp proof course (DPC) level and for retaining walls.
Modern bricks are manufactured as common bricks (for general building purposes), facing bricks (which come in a variety of finishes, colours and strengths) and engineering bricks. They are also classified by their resistance to frost, and range from those that are totally resistant (F2) for use in exposed positions such as parapets walls, garden walls and below the DPC, to those that are vulnerable (F0), which are generally for internal use only.
The bricks are bonded together with mortar. Until the 20th century this was lime mortar, which is relatively soft and porous but is able to expand and contract without cracking. Cement mortars were developed in the 20th century and are much stronger than lime mortars. They are useful where a wall requires high strength, e.g. for the sub-structure or for a retaining wall. Unfortunately, cement mortars tend to shrink and this allows moisture penetration of the wall and, in some cases, the degradation of weaker bricks as they are defaced. The modern approach for new brickwork is to carefully link the mortar mix and strength to the type of brick.
The finish to the mortar joint is also important, in terms of both appearance and performance. The most effective joints are weathered (or struck) so that they shed water effectively and seal the joint. This can also be achieved by a bucket handle. Recessed joints are to be avoided as they allow water to rest on the edge of the lower brick. Flush joints are also a risk as they often leave a gap between the bricks and the mortar.
Weathered/struck pointing
|
 Bucket handle/hollow key pointing
|
 Recessed pointing |
Flushed pointing
|
Common defects and repairs / treatments
The most common factor in the defects experienced by brick walls is water. A wall that is exposed to the elements, e.g. an exposed south west facing wall on top of a hill, or a wall that is not protected at the top by an overhang or coping and at the bottom by a DPC, is most at risk of a water related-defect.
Thermal movementExternal walls are subject to considerable temperature variation leading to expansion and contraction. Excessively long walls suffer worst and the problem is normally avoided by constructing them in relatively short lengths (maximum 6m) and incorporating vertical expansion joints. Adversely affected walls tend to slide outwards over any DPC when warmed, fail to return to their original state when cooled, and develop full height vertical cracks above the DPC. |
|
|
|
Moisture movementBricks undergo initial expansion of between 0.1mm and 1.0mm as moisture is re-absorbed after production from the construction process and weather conditions. The amount of expansion appears small but can result in vertical cracking in large areas of wall, similar in appearance to thermal cracking. Where the brickwork is restrained by a frame, it can also lead to bowing of the wall. Avoid long lengths of brickwork and incorporate expansion joints. |
Frost attackWater turning to ice leads to a 9% increase in volume. This expansion can lead to the surface of the brick crumbling or spalling, especially with older softer bricks or those in exposed positions. Mortar can also be affected. If left un-remedied, the problem can lead to total disintegration of the brick and the need to re-construct. Boundary and retaining walls are particularly vulnerable, as are older brick cavity walls that have had cavity insulation treatment. Silicone and similar wall treatments can also lead to the problem as they inhibit the drying out process of the brickwork. |
|
|
|
EfflorescenceA common occurrence with new brickwork, soluble salts are brought to the surface as water in the wall dries out, leaving an unsightly white staining. It is generally harmless and temporary and normally ceases over time. A persistent problem may indicate a design or construction fault. A more serious problem is crypto-florescence where the crystallisation of salts just below the surface of older brickwork causes spalling. |
Sulfate attackThis is a serious problem that can lead to crumbling of the mortar joint as well as expansion and instability of brick walls. It is linked to the use of cement mortar and is due to a reaction between sulfates in the brickwork and a constituent of the cement, and is triggered by water saturation over a relatively long period. Parapet walls, retaining walls and chimney stacks are particularly at risk because of exposure to relatively high levels of moisture. The problem manifests itself by attacking every horizontal mortar joint (unlike cavity tie failure). Chimneys can take on a banana shape because sulfate expansion affects the side facing the prevailing weather. Once identified, the only solution is demolition and reconstruction. |
|
Cavity insulation
Many new buildings incorporate cavity insulation at the time of construction and these generally present few problems. The main risks are associated with those cavity walls that are retro-fitted with cavity insulation, especially if they are in an exposed situation. The filling of the cavity restricts evaporation and, if the outer leaf becomes saturated through prolonged rainfall, there is an increased risk of frost attack. There can also be penetrating damp problems if the cavity was not completely filled, as well as cold bridges where there are mortar droppings. In a severely affected building, it may become necessary to remove the insulation.
Wall tie failure
This is the result of the rusting of wall ties in older cavity walls. The zinc coating of galvanized ties degrades naturally and for many years was insufficiently applied to provide really long-term protection. The problem is also accelerated by black ash mortar (commonly used for early cavity walls), chloride salts and carbonation. In 1981 the British Standard for such coatings was raised to incorporate a thicker coating. The problem is normally identified by horizontal cracks in every 4th or 6th mortar joint due to the expansion of the rusting ties in the outer skin. Occasionally, the ties degrade within the cavity and the skins are no longer linked. Once discovered, the only solution is to replace all wall ties in the external walls.
Recommended further reading:
Understanding Housing Defects; Marshall, Worthing and Heath; Estates Gazette; 2003
House Inspector; Marshall and Dann; Estates Gazette; 2005