Settlement and Subsidence Cracks: Fixing Cracks in Walls and Plaster

There can be a huge number of different reasons for cracks in the home. In the majority of cases small, hairline cracks in walls, ceilings and plaster can be a sign of simple expansion and contraction movement within your home.

This is mainly due to humidity, temperature and the way that 100+ different materials in your home react to this, all expanding and contracting at different times and different rates.

At the other end of the scale there could be bigger problems with foundations where they need underpinning in some way.

For example timber will expand and contract due to moisture levels and temperature.

Put this in the context of an exposed wooden window frame in a bathroom that has been plastered right up to it.

You can imagine in this environment, due to huge humidity and temperature fluctuations that the window frame will be exposed to, it will expand and contract significantly on a daily basis causing the plaster touches it to crack.

In most cases you wouldn’t have the above scenario, as the bathroom may well be tiled and you may have a UPVC window, but it’s just an example to demonstrate that your house is constantly on the move.

If your house is close to a busy road, the rumble of the passing traffic and vibration of the ground can also be the cause of cracks appearing in your home.

Additionally in newer properties there can be a certain period of “settlement” that the house has to go through whereby effectively everything "settles down".

This can in turn cause small cracks to appear. In terms of the size of these cracks, a settlement or shrinkage crack is really anything less than 0.5mm in width.

In light of the above points, these sorts of very small, hairline shrinkage-type cracks in wall, ceilings and plaster are fairly minor and more of an aesthetic issue, but at the same time should not be ignored as they may be a sign of the beginnings of something more serious. They should be kept an eye on and if necessary, investigated.

In this project we are more interested in examining the causes of larger cracks and splits in brickwork and blockwork, normally visible on the outer cavity wall’s. These can be the sign of something significantly more serious.

Types of Cracks

In general, there are two types of larger crack that can appear in the walls of your home that should cause concern. The direction of the crack should also be noted as this is significant:

• Horizontal cracks (including diagonal or stepped) – Usually appear along the mortar joints between brickwork or blockwork. They can follow the mortar joints in a line or diagonally upwards in a staircase pattern. If the crack follows a straight line it is normally caused through applied, sideways pressures e.g. a retaining wall where this may be holding back a bank of soil in the garden, or in a basement where the retaining walls provide the support for the upper floors and also hold back the soil in-fill around the sides. Additionally, poor drainage issues can also contribute heavily to these issues. If the crack is running in a staircase pattern across and up a wall then this is normally caused through subsidence or settlement issues causing the foundations to shift. Another possible cause for this is wall tie failure. Wall ties can corrode and expand forcing mortar joints apart and creating cracks. More about this can be read here. In some cases these cracks may also appear in the foundation itself. Obviously this may be hard to see due to the fact they may be below ground so other, visual effects may need to be adhered to



• Vertical cracks –Minor cracks of this type usually tend to appear on internal walls and are the result movement in materials such as plasterboard or brickwork causing the plaster to crack. In the case of major vertical cracks running on exterior walls of your property (minimum of 3mm in width) this can indicate serious settlement issues or foundation problems. These are normally caused by a drop in a section of the foundation that then causes a property or building to effectively pull away from the rest of the structure creating the crack

When any crack suddenly appears in a property, regardless of it’s size, it should be investigated immediately if just to add peace of mind that there is nothing structurally wrong.

In some cases wall coverings, paint or render may obscure the extent of the cracking and damage making any cracks appear to be less serious than they actually are.

In the case that large cracks suddenly appear you should contact a structural engineer or professional contractor right away as your property may not only be unsafe for you but also other properties and people around you.

How Settlement Damage to Foundations Can Happen and Cause Cracks

Poor Drainage, Flooding and Rain

Occasionally, if the soil surrounding a property is not properly graded to ensure that any water flows away from the property.

Guttering may not have been correctly installed or is in poor condition then water can be allowed to sit and overflow.

Once this happens it soaks down into the soil causing separation or voids in the ground and if it is particularly clay-heavy then it can become soft and cause foundations to sink.

Leaks

In many cases plumbing, wastes and pipework can run under the floor of a property.

If any of these develops a leak then in a very short space of time soil can become saturated around foundations and severely weaken it’s load bearing abilities.

Trees

If you have any large, aged trees in close proximity to your property then they can cause serious issues.

An aged tree’s root system can be twice as large as anything visible above ground and due to this root mass can absorb a considerable amount of moisture causing soil shrinkage.

Additionally as roots grow and move they can also cause loosening and movement in soil.

Prolonged Drought

When soil is subjected to lack of water and moisture over prolonged periods it can shrink and drop causing foundations to shift due to the lack of solid support.

Incorrectly Compacted Soil Fill

During the construction of a property and to ensure that the base of any footings are flat and level, the builder or ground-work team will bring in material to replace the excavated soil and line the base of trenches.

If this material is not correctly and suitably compacted then the weight of property above it will cause collapse and the foundations will fail.

Tell–Tale Signs of Serious Settlement Damage

Aside from the above types of visual cracking there are a few other, slightly more subtle signs that you could have problems:

• Internal Doors – When serious subsidence or foundation issues occur they can cause your property to effectively twist and bend. When this happens, items such as door frames that need to be square so that the actual door fits within it, are pulled out of shape making the door tight and stiff to open and close. Additionally you may also notice gaps around the door and frame
• Window Frames – Similar to the above, when a building shifts, things are bent and twisted and the same can be said for window frames. If this happens, windows become stiff to open and close and the glass has been known to crack
• Nail and Screw Heads – This particular occurrence is sometimes known as nail popping or nail pops. If you have visible cracks, no matter how big, check the area around the crack for any raised nail or screw heads. If you find any, this is a sure sign that you have some serious movement as, when a property begins to sag or drop due to subsidence or damaged foundations all it’s internals are pulled and stretched. Internal stud walls and the plasterboard coverings are pulled apart causing nail and screw heads to pop out. Do not be alarmed if you have a few popped nails round the home, in plasterboard ceilings or walls as this can happen with ordinary, minor movement caused by settlement. But where these popping nails are together with cracks appearing in the wall, it may be time to get the place checked over

Cracks in Walls – are They Still Active?

In respect to the above, it has also been known for houses to move suddenly and a few cracks to appear, then stay in that condition for donkeys years.

This may be due to a sudden shift in the ground below, or a void in the earth may have suddenly collapsed and filled with soil from above.

Any number of geological events could trigger earth movement, but it is the continuation of the movement which should cause concern.

Unbelievably the UK has several hundreds of minor earthquakes every year according to earthquake measurements taken by the British Geological Society and while most of these go unoticed by us ordinary human beings, they will obviously cause movements in the ground. These movements can translate into crack round the home.

From a foundation point of view, frequent home movements are associated with drainage pipe faults, water mains leaks and collapsing sewers, so always check these items first. You can find out more about finding and tracing leaks in your home in our project here.

Below you will see 3 ways of measuring the movement in a property against an existing crack. These are simple DIY methods of measurement.

However, you can also buy the Professional kit for this job – the Avongard Telltale. (http://www.avongard.co.uk) This clever piece of plastic is called a crack monitoring gauge.

It monitors horizontal and vertical movement across a crack on a flat surface to an accuracy of +/-1.0mm and by interpolation to +/-0.5mm. The opening or closing of the crack is then recorded on the monitoring card supplied with each Tell-Tale.

DIY Methods of Crack Monitoring

The above image demonstrates a crack in a wall. This can be either external or internal.

Point 1 – In the above image, 1 represents a piece of glass roughly 9 x 4 inches with a small hole drilled in each corner. This piece of glass is then attached over the crack using screws inserted into each hole (or you can use Araldite or a similar strong adhesive). Be careful not to tighten the screws too much and use some rubber bungs or washers if you can as this will avoid cracking the glass. If there is any further movement in the structure, the crack will widen and break the glass. You will need to monitor on a regular basis. If movement is evident you will need to contact a qualified structural engineer or building surveyor right away.

Point 2 – This represents a piece of strong tape such as Duct Tape or similar that will to a degree be resistant to elements such as rain etc…. and allow it to stick and hold to the walls surface. Once the tape is stuck you will then need to mark each end at the vertical with a pencil mark. If the gap opens and the crack becomes larger the tape will move. Again, this will need to be monitored regularly and the relevant professional contacted if movement is evident.

Point 3 – This represents a timber wedge that is knocked into the crack fairly tightly. Obviously do not hit it in too much if the crack is only small as this may cause more damage. You will also need to ensure that the wood is completely dry when you knock it in to the crack. If movement in the structure continues then the wedge will simply fall out.

The DIY methods above are ok to use as an initial indication but ultimately it is much better and in actuality better to monitor any movement as accurately as possible so a purpose made kit manufactured specifically for the job is the best thing to use.

Repairing Subsidence and Settlement Issues with Foundations

In the case of any settlement or subsidence issues this will need to be sorted as soon as possible to prevent any further damage to the property.

If these problems are not remedied then it may be the case that the property drops even more and cause very severe damage!

There are several methods of repairing foundations. Depending on the severity of damage or recommendation of repair from a structural engineer, a fix will more often than not be through one of the two following methods:

Slabjacking as an Underpinning Method

Sometimes this is referred to as mudjacking or grout pumping. This process involves pumping concrete into any voids below sunken driveways, foundations or ground slabs etc.

When the pumped substance fills and expands into any void areas and it then effectively lifts the sunken area back to it’s correct level.

This method is normally used where the area surrounding foundations is so unstable that piling cannot be used for fear of further disrupting any supporting base and causing more damage.

It is also important to note that slabjacking should not be used in any area where there is a possibility that the surrounding soil could expand as this could then end up pushing everything upwards and causing more issues.

Mass concrete Underpinning

Occasionally, when conditions are favourable, a huge holes is dug down alongside the existing foundation.

Sections of ground are dug away from under the concrete foundations and a mass fill of concrete is poured under the existing foundation.

The new concrete is given a day to dry, then the next section is underpinned.

Piling used as Underpinning

This can be done using either concrete or steel piles. In either case a hole will need to be drilled down into the ground to a depth or usually between ten and twenty feet (depending on how far down a stable rock base can be found) and around twelve inches in diameter.

With concrete, normally a set of reinforced steel bars are then lowered into the hole for additional support and bonding of the concrete and the concrete is then poured in and allowed to set.

Once cured a set or hydraulic lifts are then used to raise the level of property back to it’s correct height and then concrete or steel spacers are inserted on top the piers to support the foundation.

Normally in this case and once stable, the foundation will then be rebuilt back up to meet the property.

Pile and Beam Underpinning

Sometimes a pile and beam method is used where piles are drilled down into the ground either side of the foundation (very messy inside the home) and a concrete or steel beam is pushed under the foundation, or sometimes the brickwork on the foundation, each end of which sites on a pile.

The more beams inserted, the greater the load they will carry.

Screw (Auger) Pile and Bracket Underpinning

With steel piers these are slightly different. In some cases a steel pier will feature a screw (often called auger piles) at their base much the same as a corkscrew (Helical screw piers).

They are then screwed down into the soil and once in position special brackets are attached to support the base of the foundation.

The second type of steel pier is called a “push pier”. These are normally galvanised steel sections that are driven down into the soil using a hydraulic ram to provide the stable base for support.

As mentioned numerous times in this project, before you go about applying any repairs you should get an expert opinion and survey carried out by a structural engineer or specialist.

In most cases they will then carryout a full survey and then recommend the exact repair method that should be used.

Piled Raft Underpinning

If the entire building needs underpinning and the foundations are very deep, piles can be sunk in a grid, in and around the property.

After this "tunnels" are created between the piles and a concrete or steel "ring beam" system is connected to the piles.

The weight of the entire property is then carried by the ring beam and the load spread evenly amongst the piles.

Mini Piles Through Floor Slab

Sometimes the foundations themselves may stay relatively stable but (especially in clay soils) the compacted ground under the floor slab may drop when it is too dry or the slab has been laid on a sub-base which has not been properly compacted or contains organic material which has decomposed.

Remedial work here usually involves removing the existing floor screed, drilling down through the floor slab at intervals of between (usually) 600mm and 1,5m, in a grid shape, the depth of which is determined by a structural engineer but rarely less that 3m.

Grout is pumped into these holes and left to go hard. As the grout dries in the holes in the slab as well as in the ground, the slab and the piles become as one.

A new floor screed is then laid on top of the slab up to the correct level.

Large and Small Cracks in Walls and Foundations and How to Repair Them

Once you have repaired the cause of any settlement or subsidence issues using the above information and repair methods, the next job is to repair the visual damage such as cracks and splits in brickwork and blockwork.

If the repair of the cause was successful then this may have re-aligned any subsidence issues and actually closed up any splits or cracks but any remaining gaps will need to be strengthened and filled to prevent water ingress or instability:

Vertical Cracks

The best method for repair for vertical cracks in masonry is to effectively stitch the cracks back together using crack stitching bars.

These are usually either Helical-type bars that feature a course looking thread or an epoxy-glass type rod.

The normal method for fixing is to first cut out a bedding joint in the masonry spanning the vertical crack.

Specialist dust extraction equipment should be used to ensure that all dust and debris is removed from the joint eliminating the possibility or future failure.

Once the joints have been cut, the rods are inserted and a polyester or epoxy based resin is injected in to bond the rods to surrounding masonry.

Once the resin has set and the length of the gap has been stitched it can then be filled using a soft mortar mix.

When mixing the mortar you should also add a shrink proofing agent as this will then help to compensate for any further, small amounts of movement.

Horizontal Cracks in Foundations

Depending on the cause of the crack will depend on how it is fixed. Again, consult with a professional so that they can diagnose the specific cause of any cracking:

• Wall Tie Failure – As mentioned above, we have a full and detailed project on how to replace wall ties – this can be seen here
• Applied Pressures – As mentioned above, these cracks are normally caused through horizontal pressures pushing in on the outsides of walls causing them to bow and separate. The first type of fix for this issue involves bracing with steel columns or reinforced steel rods that can then be in-filled with concrete blocks that are inserted into the ground to apply a counter-force and stabilise the movement. Other fixes involve carbon fibre straps that are bonded into the walls that then increase the load strength of the wall and reinforce it. These will not repair the cause of the damage but will stabilise the walls. The final repair method and ultimately the best is to excavate around the outside of the property, removing the cause of the horizontal pressure. Once excavated, a new exterior wall is constructed against the existing foundation but this new wall will need to be an actual retaining wall with the ability to hold back any soil back-fill and prevent future the same issues happening in the future. In any case involving applied pressure this is unfortunately one for the professionals so you will need to consult with a structural engineer and then he/she will select the best and most cost effective fix.
• Poor internal Drainage – You will need to check all of your internal plumbing and wastes for leaks and them repaired as soon as possible. This may be a little trickier where pipework that is set in concrete etc. so in these cases it may be best to consult with a qualified plumber.
• Faulty external drainage – In terms of external drainage, check that anything that abuts the property (soil, pathways etc.) is correctly graded and directs water and moisture away from the walls and prevents it from sitting or resting in one place. It may be necessary to install a French Drain (see our project on French Drains) or other drainage method. Again, consult with reputable and qualified builder or building inspector. Finally, check all of your guttering for splits, leaks and cracks and get these repaired as this will also ensure that any water is dealt with correctly
• Settlement and Subsidence – In this case where settlement and subsidence occur, it may be the case the foundation has dropped down and split away from any brick courses above it. Repair would normally involve one of the above underpinning methods to stabilise the foundation and bring the property back up to the correct level

Once the above cause has been sorted and a fix put in place you will now need to repair the crack itself.

This is usually done by working in 600mm sections and removing one course of bricks above and below the crack

Any old mortar is removed and cleaned and the brickwork is then replaced with new mortar – NOTE: This can only be done 600mm at a time as any more may cause instability in the brick or blockwork above.

Filling Small and Minor Settlement Cracks in Interior Walls and Plasterwork

In the majority of cases these are pretty easy to sort. Firstly clear the crack and its surrounding area of any dust or debris using a small brush and then a vacuum cleaner if you have one.

Next, using Polyfilla or a similar crack repair filler, spread over the crack using a spatula, ensuring that you apply a little pressure to work it into the crack. Ensure that you smooth it out and get it as flat as possible.

Once complete, leave for a few days to ensure that it cures correctly and once it has you can then sand any bumps or raised edges until they are flat.

If there are any pits or dips remaining you may then need to fill these by repeating the above steps.

Cracks in Sills and Copings

As with the above, this will depend on the size of the crack and whether the sill or coping is load-bearing (a Structural Engineer will be able to tell you this).

In most cases you will need to widen the crack slightly to enable you to insert some mortar compound.

Once widened, mix up some mortar and insert into the crack, pushing the mortar in as far as you can.

Once the gap is filled up to the face level it off as flush as you can with the face of the sill or coping stone as this will ensure a natural looking finish.

If the crack is quite wide or it is indeed load bearing a Structural Engineer will be able to recommend the correct repair method.

In most cases this will involve stitching as in the repair method for vertical cracks.

Several channels will need to be cut at 90 degrees to the crack and around an inch deep.

An Angle Grinder with stone cutting disc will help you. Once channels are cut insert suitable threaded bar or Helical bar and fill with grout or resin.

Again, try and fill as close to the face of the sill or coping stone as possible to ensure a flush finish.