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  • Movement, Subsidence and Settlement

    6th December 2017

    Movement, Subsidence and Settlement

     

     

    Movement and subsidence are dreaded words when looking at a property. But what actually are they and are they as bad as they sound? Like anything, it’s fine in moderation. All properties are likely to be affected in some way by these issues – properties and the ground around them move. It just depends to what extent.

    Before we start, what is the difference between movement, subsidence and settlement?

    1. Movement is generally where the property moves but it is not caused by shifts in the ground and so directly affects the property above the foundations.

    2. Settlement can be defined as: downward movement as a result of the soil being compressed by the weight of a building. Settlement and subsidence directly affect the foundations and indirectly the property above.

    3. Subsidence can be defined as: downward movement of the ground beneath a building (other than by settlement).

    4. Cracks to the property can be caused by all three – every property will suffer from them at some point

     

    Movement

    All properties move a bit over time. It’s an issue when it is moving too much. Often the cause is quite simple, as is the remedy.

    Examples of movement

    1. Thermal movement

    Materials expand and contract as they heat up or cool down. Changes in temperature occur daily as well as seasonally. Masonry (bricks or stone) expands during the hotter summer months and contracts during the cooler winter months. Cracks can occur as a result where the other materials aren’t able to flex enough. This doesn’t affect the structure of the building but can look unsightly but it can be filled. Different materials in the property will expand or contract different amounts with temperature change.

     

    1. No lintel or an inadequate lintel

    Where openings are made for a window or door, a lintel should be in place to support the wall above and spread the weight over the wall either side of the door/window. Lintels are generally made of concrete or steel. In some cases a lintel may not be installed or an inadequate lintel is used that is not strong enough to support the weight of the wall above. Where the window or door is not designed to be weight bearing it starts to give way and the masonry above drops. In this situation a new lintel needs to be installed to prevent further movement or collapse.

    Bowing lintel, movement to wall
    Bowing lintel, movement to wall. Source: JSA Specialists

     

    Why might there be no lintel?

    One example is where the property was built with wooden windows. In some cases lintels weren’t used as the wooden frames were strong enough to support the opening. uPVC replacement windows are generally not designed to be weight bearing but your contractor may not put a lintel in where one wasn’t installed before – either to keep their costs down or because they aren’t aware. The result is movement to the property as the weight of the wall above incorrectly distributes through the walls below.

     

    1. Failing or insufficient cavity wall ties

    A cavity wall is made up of an inner wall and an outer wall held together by wall ties to stop them moving apart. A cavity wall has been standard construction in houses since the early 1900s but has been known to be used in some properties pre-1900. Over time the wall ties can corrode. The corroding metal expands which forces the wall out causing it to bulge or crack.

    Cavity Wall Ties diagram
    Cavity Wall Ties. Source: Ancon

     

    1. Poor workmanship or alterations

    An example of this is roof spread – caused where the roof causes the walls to push outwards. This occurs where either the roof structure is poor or a roof is retiled with a heavier tile it was not designed to take (e.g. slate replaced with concrete tiles).

    Further examples are removal of internal walls or chimney breasts and failure to provide the correct support. Unsupported the weight is incorrectly distributed and causes cracking (and can collapse).

    Supported chimney breast. Source: Wayswithwood

     

    1. Sulphate attack

    This occurs as a result of a chemical reaction between cement paste and sulphates in the filling under a concrete slab. It is usually set off by water. A common example is concrete floor slabs. As the reaction occurs the slab expands and cracks (often upwards) and can push the outer walls outwards. It has similar symptoms to ‘heave’.

    Concrete floor sulphate attack
    Concrete floor sulphate attack. Source: 3C Drawing Services

     

    Settlement

    Settlement is very common and in most cases is not an issue. It is defined as the downward movement of the site on which the building stands. It is also known as consolidation or compaction and is not an issue where the building moves uniformly (usually only giving rise to small cracks).

    Settlement is most relevant to new buildings or extensions in the first years after they are built. Settlement is is an issue where there is differential movement – different parts of the building move at different rates. This most likely happens where new additions (like an extension) settle differently to the original structure.

    Source: TheCivilBuilders

     

    Subsidence

    The ground moves downwards but not as a result of settlement. There are many things that can cause the ground to move downwards over time.

    Examples of subsidence

    1. Seasonal changes – Clay Shrinkage / Expansion 

    One of the most common causes which most often occurs in hot weather where there is also lots of vegetation. In summer months clay, which is about 1/3 water, dries out and the volume of the soil beneath the property decreases – as the soil drops, the property subsides. On the other side of the coin, the soil can expand or swell as it gets wetter (winter months) and push upwards (heave).

    Common cause: trees – a regular factor in shrinkage because of the water they draw away. Solutions are removing, thinning, topping and pollarding (cutting the tree’s crown back to the main trunk) to reduce their water consumption. However, you should be careful not to remove too much vegetation resulting in heave.

    clay shrinkage
    Source: Wikipedia

     

    1. Escape of Water

    This can cause the soil to wash away, reducing the volume of soil supporting the house. Alternatively, it can cause the soil to soften. The soil loses it’s ability to hold the mass of the property above and subsides.

    Common cause: broken or cracked drainage pipes / leaking water pipes

    drainage leak
    Source: British Geological Survey

     

    1. Mining / Landfill

    There are many old mines around the country. These mines can collapse many years since they stopped being used. You can check if your property is in a current or previous mining area and ask the owner if there have been any previous pay-outs from the mining company.

    With landfill the ground can compact as the material in the ground decomposes causing subsidence.

    subsidence from mining
    Source: RWTHAACHEN University

     

    1. Poor workmanship

    In the same way the property can be built badly, the ground can be poorly prepared before the property is built which can result in far greater ‘settlement’ than there should be. Examples of this are poor quality ground in general, using inappropriate materials for the site or failing to compact the material correctly before building.

     

    Cracks

    There is no hard and fast rule to guarantee a crack is safe or not and if in any doubt get a professional to check it out properly. Monitor cracks and how quickly they grow and expand and if they are getting large at a fast rate then get them checked out. However, some examples are below.

    Vertical or horizontal cracks in plaster on ceilings or walls that are hairline cracks usually indicate the contraction of materials where they are drying out and are nothing to worry about. They just aren’t aesthetically pleasing.

    hairline cracks plasterboard ceiling
    Source: Homebuilding.co.uk

     

    Stepped cracks on wall are usually a sign of movement. A large crack (0.5cm as a guide) is a sign of something more serious and you should get checked out. A small crack (0.25cm and below) is generally just a sign of settlement or movement that is nothing to worry about.

    Step crack wall
    Source: Stablwall

     

    Seen cracks on a property you want to buy? Get Building Survey quotes now.