Causes of Cracks in Buildings

Substructure

This is the area of a building below ground in which the foundations are placed and on which the building is constructed.

The ground below foundation can be rock, clay or granular soils such as sand or gravel.

Commonly used traditional foundations have been strip concrete and spread masonry footings.

In unstable ground where the ground has been filled or worked engineers design deep pile and ground beam systems or in less severe situations concrete rafts.

Settlement and Subsidence

Settlement is the gradual settling of the ground below a building and is not usually covered by insurance.

Subsidence is damage caused to a building by the failure of a foundation due to external factors.

Defects to Foundations

Clay soils will expand (hydrate) with increased moisture content and shrink (dessicate) as they dry. Where there is differential movement of clay below foundations cracking will be caused in the building above.

Heave is where the clay expands and pushes the foundation up and shrinkage is where clays below contract and cause the foundation to drop.

Evaporation can change moisture content in clay but this usually is restricted to the top 1 metre.

Trees and shrubs close to a building can remove water from the ground and cause the clay to shrink. The removal of water by shrubs and trees depends on the species, for example a Willow tree can take water from the ground 40 metres away.

Broken drains or water running in the ground from soak-aways, ground water or water in service trenches can remove granular soils from below foundation causing subsidence, this is known as washout.
Where minerals or fines are removed from below foundations voids are created causing the material below the foundation to drop and as a consequence of this subsidence occurs, this is known as leaching.

Ground below foundation can slip where this is a fault plane, for example houses built on rock close to a cliff edge.

Cracks from Foundation Problems

Cracks caused by foundation movement tell their own story by the line they follow and by the size and direction of the cracks.

When an area of a building starts to move away from the main structure and crack, it is said to be rotating.

Where a surveyor considers the movement to be an ongoing foundation problem trial holes are usually formed and a structural engineer becomes involved.

Remedial Works to Foundations

This can involve the removal of trees and shrubs. Repair of defective drainage and underpinning/wall stabilization works. An example of this would be mini piles such as the Shire Piles system.

The works will depend on whats found when opening up works are carried out. Access and cost considerations and structural engineer specifications will all have a bearing on these works.

Superstructure

This is the area of the building constructed above ground.

In the British Isles most buildings are constructed like boxes with masonry and timber framing containing window and door openings. Floors and roofs are component parts of the building which help tie it together and give lateral restraint.

Walls

Walls can be solid stone or brickwork tied together with through stones or brick headers, rubble fill – where two leaves of masonry are filled with rubble or in cavity wall construction.

Cavity wall construction can be stone, brick, block or timber frame in two leaves separated by a cavity and tied together with steel wall ties.

Cracking Caused by Superstructure Defects

Where wall ties have failed a cavity wall can lose two thirds of its strength which in extreme situations can cause an outer wall of masonry to fall away from the building.

Some large section steel wall ties such as fishtail ties will expand as they corrode causing severe bed joint cracking every 6 to 8 courses of brick or stone.

Corroding steel can expand over seven times its original size and will easily lift and crack masonry.

Cracking can also be caused by corroded steel beams or fixings embedded in the walls of buildings.

Arches

Arches usually crack because of lack of buttressing, this often occurs at the corners of buildings.

Creep

Creep is the ongoing movement of a building component under a loading.

Timber Decay

Where timbers used in the construction of buildings become damp they can rot leading to failure of floor and roof members and beams over openings.

Cracking over Openings

When cracking appears as a triangle over an opening the lintel or beam has usually failed. A triangle of cracked masonry can be seen above the opening which was once supported by the lintel. The brickwork either side of the triangle is supported by the corbelling effect of adjacent masonry as long as there is an adequate bond to the walling construction. This type of cracking can also occur when a structural timber window frame has been replaced by a non structural uPVC window frame without making lintel provision.

Bowing/Bulging and Leaning of Walls

A pitched roof frame should be triangulated by cross ties to stop roof spread. When cross ties have been removed or failure of a mechanical fixing has occurred the roof will spread under the load of the roof covering and push the tops of the supporting walls out.

Where floor membranes are not tied into the walls adequately there is a lack of lateral restraint and the wall can bulge or bow.

Frost Action

Water can expand by 9% when it freezes and this can cause the face of masonry to spall. This type of damage is unsightly but usually only aesthetic.

Chemical Action

Clays used in bricks contain sulphates which often appear on wall surfaces in Spring as efflorescence with the lower air pressures that occur at this time of year. These salts can usually be brushed off without damage.

In extreme cases, sulphates and other aggressive salts contained in the ground or structure and the presence of moisture can cause attack and expansion of Portland Cement, causing bowing and leaning of walls, this can often be seen on chimney stacks and parapet walls (especially where one side of the wall gets wetter for prolonged periods than the other).

Remedial Action

Traditionally heavy steel tie rods and plates were used to restrain buildings and corroding steel was exposed in localised areas of cracking, primed and re-pointed in.

These systems are not always ideal as corroding steel tends to keep expanding and heavy steel bars do not allow for much movement and can cause additional cracking in the building themselves.

A solution often used today is the formation of masonry beams using flexible austenitic stainless steel with an upper and lower cord to give strength in compression and tension.

These systems can also be used as a remedial wall tie solution, for crack stitching and for barrel wrapping of a building to improve buttressing at corners.

An example of such a system would be Thor Helical, part of the Wykamol group who have a range of engineered structural solutions backed up by an engineering department who will assist in the design of a scheme. Guarantees issued on such structural works depend on their approval.