Several types of cracks occur in masonry walls in a building which can be minor and insignificant, some requiring expensive repairs and in some extreme cases the only solution is total demolition of the wall. Causes of these cracks in masonry walls can be movement of building causing settlement and subsidence.
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Common Causes of Cracks in Masonry Walls
Settlement and subsidence of building walls is shown in figure below.
Settlement of masonry wall occurs due to downward pressure from the loads on wall and subsidence occurs due to the removal of earth beneath the foundations. Settlement is usually easily dealt with via cosmetic repair, whereas subsidence can prove difficult and costly to repair.
There is a situation where settlement of walls can cause subsidence. If the drains connected to a building shear or crack due to settlement, the subsequent leakage can cause subsidence by washing away the subsoil.
All buildings settle when they are constructed; the trick is to keep the settlement to an infinitesimally low level. The other causes of movement and cracking are due to poor design, poor construction methods, or poor maintenance.
For definition the diagrams are shown in brickwork; however cracks form in stonework in a similar fashion.
Types of Cracks in Masonry Walls and Causes
1. Expansion Cracks in Masonry Wall
Causes of Expansive Cracks
Walls are affected by temperature and moisture change. Materials can suffer from initial shrinkage and/or subsequent expansion and contraction. This movement gives rise to the expansion cracks in masonry walls.
The crack shown in the picture is shown as vertical, which is often the case. However, the crack sometimes follows the line of least resistance and can end up stepped.
The expansive cracks are often seen above window and door openings where the opening itself relieves the crack. This type of crack has a consistent width and it is this that distinguishes from other more serious cracks.
Repair of Expansive Cracks
Expansive crack is of no real structural significance, although it may allow water into the cavity in brick built houses, and subsequence cause deterioration of the wall ties. Therefore filling the crack with a mastic or sealastic compound is recommended.
However, for more severe cracking it is advisable to form an expansion joint. This would be cut into the wall, filled with a compressible material with a waterproof stopper to the outside.
On some modern buildings these are formed at construction stage and then hidden behind rainwater downpipes.
2. Cracks Above Openings in Masonry Walls
Four causes of cracks above openings in masonry walls are:
- Removal of windows or doors with inadequate propping,
- Inadequate bearings,
- Loads applied directly over the opening,
- No lintels.
Cause-1: Removal of windows or doors with inadequate propping
The most common reason for this type of cracks in wall is the removal of existing window frames to install PVCu.
Repair of Cracks
The best repair is to reset the lintel and repoint or rebuild the brickwork above, and refit the window. The poor repair is to do nothing more than repoint the cracks, as the brickwork is now resting on the new frame. However, collapse of the brickwork above the opening will be likely when the window is next replaced.
Cause-2: Cracks due to Inadequate Bearings
The correct overhang (bearing) of the lintels above openings is 150mm (6 inches) each side. If the bearings are insufficient the lintel will drop and the cracks will appear.
Repair of Cracks due to Inadequate Bearing
Replacement of the lintel is recommended. However, once again repointing will suffice until the window or door is replaced.
Cause-3:Cracks due to loads applied above the opening
This often occurs above first floor lintels where the roof purlins have been installed directly above the window openings. The load imposed is too great for the lintel to cope with and the downward pressure causes the cracking.
Once again replacement of the lintel is recommended. The severity and age of these cracks would decide whether simple repointing would suffice until the window is replaced.
Cause-4: Cracks in Masonry Wall due to No Lintels
In some properties no lintels are provided relying on the timber frame of the window to support the masonry above, but once the window is replaced the cracks occur.
New lintels need to be installed and the cracks repaired.
3. Cracks in Masonry Wall due to Tie Failure
Wall ties are metal ties that are built into both solid and cavity walls built in stretcher bond to hold the outside skin of brickwork to the inside. Failure normally occurs when the ties rust. When the metal ties rust they expand causing the cracking normally seen every sixth course horizontally in the mortar joints.
Replacement wall ties are essential. The cracking is an early indication of failure. Without replacement, collapse of the wall could occur. Repointing and removal of the existing ties is recommended.
4. Cracks in Masonry Wall due to Subsidence
This is the worst and most serious type of cracks in masonry walls and consequently the most difficult to repair. Subsidence can occur due to a variety of reasons:
- Mining activity
- Leaking underground drainage
- Tree root activity
- Peak subsoil
- Clay subsoil
- Running sand
The list is endless; however, the basic problem is the same; the foundations of the house are moving. The cracks are normally the first indication of a problem; often they are raking cracks (widest at the top) and can occur to corners of the building or from the top to the bottom of the walls.
Repair of Cracks due to Subsidence
This will normally involve some form of underpinning. However, specialist advice from a structural engineer will be required.
5. Wall Cracks due to Ground Heave
The pattern of crack is similar to subsidence crack, however, the crack will be widest at the base of the wall. The most common cause of ground heave is expansion of clay subsoils.
On older properties with shallow foundations the clay can expand and contract dependent upon the weather conditions. If the clay becomes waterlogged it can expand and push the foundations upwards causing the cracks.
The removal of trees can also cause ground heave, which is why trees that are too close to the property should be taken down in stages, slowly over a number of years to allow gradual ground movement.
Repair of Cracks due to Ground Heave
In extreme cases underpinning and/or deeper foundations will be the only solution. This is however, a drastic measure. In cases of ground heave problems the solution will be to remove as much of the clay from around the foundation as possible and to replace it with hardcore.