“Help! My house is subsiding!”

Is my house subsiding?

We can’t tell you how many times in a month we get a call from a homeowner expressing concern about cracks in their walls. In fact, people often tell us their house has cracked over the weekend. It hasn’t. What has happened is that you suddenly start noticing cracks because you’re looking for them and assume that they are all new. 

Here we explain what subsidence is and how you can identify if you should worry about cracks in your house.

Do cracks mean I have got subsidence?

More often than not no.  All houses crack sometimes – after all they are big solid boxes that get hot and cold, wet and dry and altered.  Very often cracks in old houses are due to regular thermal and moisture changes in the structure and brickwork and are not a cause for worry.

What is subsidence?

Subsidence occurs when one part of a building moves at a different rate to another. This is caused by a change in the ability of the ground below the foundations to support the weight of your house. Often this is because the ground shrinks. If your house is very old, its foundations could also be shallow and encourage subsidence.

Subsidence shouldn’t be confused with other ground movement types such as heave (upward movement of the ground beneath a building when soil expands); landslip (downward movement on sloping ground); and settlement (which usually happens within the first 10 years of a house or extension being built).

House Subsidence Diagram

What can cause subsidence?

The most common cause of subsidence is tree roots, even tiny ones, which take water from the soil and cause it to dry and shrink. Your home is more susceptible if it is built on land with a high water-bearing capacity, such as clay content, rather than well drained ground such as high chalk or gravel content. 

The other common cause of subsidence is a broken drain or other underground water leak which can cause soil to be washed away. This situation can be worsened if tree roots get into the broken drains, grow and the pipework cracks further. 

Prolonged dry weather can make the problem even worse. A long period of dry weather causes the soil to dry out while trees continue to extract water, so subsidence has a greater risk of developing in the summer than the winter. 

Can subsidence be prevented?

It can take many years for subsidence to show any effects on your house. Unfortunately, it is difficult to prevent subsidence, especially if it is based on the soil type beneath your house. 

However, ensuring trees are planted far enough away from your property so they cannot take all the water from the soil beneath it can help. If trees are already growing too close, you might consider removing them, or at the very least keeping them well pruned. Sometimes simply pollarding or a crown reduction is all that is needed.

If you do remove them, consider the impact of that too. You could inadvertently make the problem worse by disrupting the established root system. This could then cause the soil to become unstable or for water to build-up where plants are no longer using it. If you fear trees or shrubs are a causal factor, you should seek professional advice before removing them.

You should also do annual checks of your property, particularly the pipework, gutters and drainage systems to ensure there are no blockages or regular flood events.

What do subsidence cracks look like?

Bay Window Subsidence

 

Bay Window Subsidence
Subsidence to a front bay of an old Victorian house

Subsidence Cracks Near Window

Subsidence Cracks Near Window
Cracks above and around windows caused by subsidence

Cracks caused by subsidence will often be diagonal and wider at the top than the bottom. They will be visible both inside and out and are likely to be found around doors or windows, or where an extension joins the house.

Have a good look at the cracks and see if there is a pattern or system of cracking. Walls crack in their weakest places, usually at door and window openings. If there is a subsidence problem, these will often go out of square so check that they still open and close properly.

Cracks may also occur where walls meet ceilings and on internal walls above doors. Look outside. Are there trees close by? Lift the inspection chamber lid. Is the drain blocked or not draining quickly?

Single cracks in just one location, and fine cracks less than a couple of millimetres wide, are unlikely to be a cause for concern unless they worsen and get larger.

What size cracks in my house should cause concern?

Most surveyors and engineers don’t get too excited about small cracks. We use a crack size scale (below) when advising upon the extent and nature of cracking. This enables a consistent approach understood to other professionals.

  • 0 – Hairline cracks of less than about 0.1mm. These are classed as negligible. No action required.
  • 1 – Typical crack widths up to 1mm. Fine cracks that can be treated easily using normal decoration. Damage generally restricted to internal wall finishes. Cracks rarely visible in external brickwork.
  • 2 – Typical crack widths up to 5mm. Cracks easily filled. Recurrent cracks can be masked by suitable linings. Cracks not necessarily visible externally. Doors and windows may stick slightly and require easing and adjusting.
  • 3 – Typical crack widths are 5-15 mm, or there are several around 3 mm. These cracks require some opening up and can be patched by a mason. Repointing of external brickwork and possibly a small amount of brickwork to be replaced. Doors and windows sticking. Service pipes may fracture. Weather-tightness often impaired.
  • 4 – Typical crack widths are 15- 25mm, but it also depends on the number of cracks. Extensive damage which requires breaking-out and replacing sections of walls, especially over doors and windows. Windows and door frames distorted, floor sloping noticeably. Walls leaning or bulging noticeably and some loss of bearing in beams. Service pipes disrupted.
  • 5 – Typical crack widths are greater than 25mm, but it depends on the number of cracks. Structural damage that requires a major repair job, involving partial or complete rebuilding. Beams lose bearing, walls lean badly and require shoring. Windows broken with distortion. Danger of instability.

Most houses have small cracks, especially if they are more than 30 years old. After all a house is a solid box built on something soft that gets wet, dries out, gets hot and goes cold.

What if cracks appear to be getting bigger?

Before you contact your insurer or even a Chartered Surveyor like Collier Stevens, a simple marking on your wall will help you assess whether the cracks on your wall are increasing over time. With a pencil, draw a line across the crack and a line at the end of the crack, write the date next to these lines. It’s crude but you can then check if there is ongoing movement – after all, big cracks start as small ones. If there is deterioration over time, then you should consult a building surveyor or structural engineer.

Only then do we suggest notifying your insurance company. They will appoint a loss adjuster who may or may not visit and is likely to request a report from your own surveyor. They may then want to monitor the cracks for deterioration before undertaking investigation works. This process can take a long time. A year is not unusual. Insurers will not consider what remedial work may (or may not) be required until this process has completed.

How can I fix subsidence?

Getting to the cause of the problem early is the best solution and will not necessarily mean repairs to your house foundations are needed. If tree roots taking water from the soil are the cause of your subsidence, consider removal with the advice of a specialist.  If there are leaking drains, get them repaired.

Underpinning is often used to fix problems with subsidence where the cause cannot be eliminated this means building new foundations underneath the existing ones 

So, what else could the cracks be caused by?

Although subsidence is a risk for some properties, it may not always be the problem. There are many causes of cracked walls, and subsidence is often the least likely. 

Subsidence should not be confused with ‘settlement’ which causes the cracks you see in new builds, or new extensions. These cracks can occur for many years after work has been carried out and often where the new work meets the original building. This should always be considered before you cry ‘subsidence’ if you are worried about a new build, or your new extension or loft conversion.

Cracks can also be caused by changes in seasons. Dry weather in the summer and wet weather in the winter causes changes to the ground beneath your property and temperature changes cause woodwork, walls and ceilings to contract or swell. For these reasons, cracks often appear wider or narrower at different times of the year. 

Thermal cracking is extremely common and rarely a concern in modern buildings. It is now a Building regulations requirement to have movement joints built in to allow a building to move without causing damage.

So many different factors can affect the formation of new cracks or changes to them. It is always best to get them checked if you are still worried after reading this or associated articles. 

Steve Way, Practice Principal, Collier Stevens, says: “The effects of subsidence can be dramatic. All buildings move to some degree and all buildings will demonstrate some cracking. Many householders become alarmed about cracks and worry unnecessarily. That is why it is essential for a diagnosis to be made by experienced Building Surveyors or engineers. They will be able to review the pattern and location of cracking, consider external factors such as trees or drains and provide an accurate assessment.”

Further Reading

For further reading, there is a great article from the Home Owners Alliance which explains fully what subsidence is, how to spot it and what all the potential causes could be: https://hoa.org.uk/advice/guides-for-homeowners/for-owners/subsidence/.

This article from Hunker also explains what you can do to monitor cracks yourself, before seeking professional help: https://www.hunker.com/12610789/how-to-determine-if-the-cracks-in-walls-are-serious.

Full building surveys and single defect surveys

If you are looking to buy a new house, look out for the signs of subsidence and ask if the homeowner has had any problems in the past. Always have a full building survey undertaken if you see the signs of subsidence too.

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