There are cracks in all buildings, some minor and insignificant, some requiring expensive repairs and in some extreme cases the only solution is total demolition of the building.
This article is intended to put cracks into context: obviously one of the major concerns of house buyers. “How serious is it” is a question I am often asked.
Firstly I think it is important to understand why buildings move. The two main reasons are settlement and subsidence, and these can best be explained in the diagrams below.
Settlement occurs due to downward pressure. 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 can cause subsidence. Drains are connected to a building, and if these 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.
In this article I will deal with the five most common causes of cracks that I have encountered on my travels, this is not a definitive list and my advice, if you are concerned about cracking in your building, is to seek advice from a Chartered Surveyor or Chartered Structural Engineer.
For definition the diagrams are shown in brickwork; however cracks form in stonework in a similar fashion.
1. EXPANSION 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 cracks. The crack shown in the main 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 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.
Left alone, the 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 selastic 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
Four causes are:
(a) Removal of windows or doors with inadequate propping,
(b) Inadequate bearings,
(c) Loads applied directly over the opening,
(d) No lintels.
Cause (a) – Removal of windows or doors with inadequate propping
Whilst not wishing to get complaints from PVCu window installers, the most common reason for this type of cracking is the removal of existing window frames to install PVCu. On several occasions I have seen the total collapse of brickwork above bay windows! On older properties with brick on end lintels (soldier course), the lintel is formed in a similar fashion to firmly pushing several books together between your hands. It would be possible to place books on top of these without collapse, but collapse would happen instantly if you released one hand ever so slightly. This same principle applies to the soldier course lintels.
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, you should note that collapse of the brickwork above the opening will be likely when the window is next replaced.
Cause (b) – 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. However, on some older properties I have seen huge stone lintels with only 50mm (2 inches) bearings. “Passed the test of time” is a phrase I like to use in these instances.
Replacement of the lintel is recommended. However, once again repointing will suffice until the window or door is replaced.
Cause (c) – 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 (d) – No lintels
In some properties builders (bless them, they keep me in business) decided not to install lintels at all 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. WALL 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.
The worst and most serious type of cracking and consequently the most difficult to repair.
Subsidence can occur due to a variety of reasons:
1. Mining activity
2. Leaking underground drainage
3. Tree root activity
4. Peak subsoil
5. Clay subsoil
6. 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. (subsidence itself will be discussed in detail in a later article).
This will normally involve some form of underpinning. However, specialist advice from a structural engineer will be required.
The pattern is similar to subsidence cracks; 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 (the problem of trees and tree roots will also be dealt with in a later article).
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.