Dealing with Subsidence: A Beginner’s Guide

Most movement in the ground beneath your home is harmless, but more acute shifts can cause damage. Tim Doherty explains how best to tackle this issue
by Tim Doherty
1st August 2018

All buildings are subject to fractional but constant movement: bricks, timber and concrete will expand and contract based upon changing environmental conditions over monthly and seasonal cycles.

Normally, a building’s superstructure can comfortably accommodate this, but more acute movement may lead to evidential cracks in walls, floors and ceilings. These are normally categorised as either a failure in the initial design, building settlement or subsidence.

Settlement versus subsidence

Issues with the initial design would be channelled straight back to the architects and engineers, but what about settlement and subsidence? The former describes the compaction of a building’s individual material components under its own weight.

Most settlement occurs shortly after it has first been constructed, for example, as materials like timber joists lose a fraction of their moisture content and become squeezed under the weight of the roof structure. A typical building will settle into position over one to 10 years and cracks may manifest where stiffer elements are adjacent to others that are less so.

Subsidence, by comparison, is a change in the volume of the supporting soils under the building’s foundations and structural floors, causing downward movement. When this occurs, the key need is to identify the individual and specific cause for this volumetric change.

Common signs of subsidence

Most of the tell-tale signs of subsidence in an existing house are to do with cracks – but not every crack is as serious as it might look. Examples caused by subsidence are likely to have some or all of the following features:

  • They’re thick cracks (more than 3mm) and are forming in the walls, ceilings and outside brickwork.
  • Cracks are diagonal and wider at the top than at the bottom.
  • The cracks are visible both internally and externally.
  • They’re located close to doors or windows (usually at the corners), or where an extension joins the main house.

Other potential indications of subsidence include cracks appearing after a long phase of dry weather; rippling wallpaper that’s not being caused by damp; and sticking of doors and windows.

It’s important to note even if some of these issues are present, they may not have been caused by subsidence. Minor cracks are fairly common, for example, and some big cracks could be triggered by other issues. If you’ve cranked the radiators up high during a cold snap, for instance, the temperature difference can cause cracks that are easy to repair.

What causes subsidence?

The first consideration when looking into the risk of subsidence and settlement is ground conditions and the make-up of the soil. Granular earth, which normally contains larger particle sizes of sand or stone, is more porous and free-draining, whereas clay is much more likely to shrink or swell depending upon the prevailing moisture conditions.

With clay soils, ground moisture may bond to the surface of its fine particles, which will form a jelly-like consistency, rather than filling the spaces between the particles. This type of soil stays wet, and may potentially continue getting wetter than granular versions.

Clive and Susan Andrews had to tackle to tackle severe subsidence to build their Art Deco-inspired home
The plot's soft sand meant they had to opt for piled foundations. Click to read the full story

Water retention in clay ground is known as plasticity. A high level of this suggests an ability for the soil to both shrink and swell. If the soil shrinks, this will cause it to subside; whereas if it swells then it will heave. Both of these issues can affect buildings.

The second consideration will be any site conditions that could affect ground moisture levels, such as a leaking drain or a large tree that may be absorbing vast amounts of moisture. The NHBC (National Housebuilding Council) has published some useful information online about building near trees and how that impacts upon foundation design and depths.

Read more: Trees, TPOs and Planning Permission

The third consideration is the type of structure. Older housing stock featured lime mortar for the brick beds. This is much more flexible than cement, so it’s more forgiving when it comes to building movement. Conversely, foundation depths in older buildings are not usually particularly deep, whereas now they are almost always a minimum of 1m.

Subsidence and insurance issues

Subsidence was only really first introduced into typical insurance policies in the 1970s. But cover has changed over the years and some providers now publish their own definition of qualifying subsidence/heave.

The two building elements that will suffer first from volumetric change in the ground are the loadbearing wall foundations and, separately, a ground-bearing slab. The slab is detached from the external walls and the perimeter foundations, and is supported by its own compacted hardcore underneath.

Subsidence could affect the supporting materials underneath a slab, causing this to crack, heave or sink; or it might create movement in the loadbearing walls. Either one could be classified as subsidence. However, many policies will only cover subsidence underneath a ground-bearing slab where the problem has also affected the perimeter loadbearing walls and their foundations.

An insurance company’s initial response to a potential claim would be to instruct a loss adjuster to visit and inspect the property. If qualifying movement is noted, the usual stance is to identify the underlying cause and then, following corrective treatment (for example, drain repairs or tree removal), monitoring the building to determine whether the initial cracks are still live.

Read more: Choosing the right site insurance for your build

A few decades ago, most loss adjusters moved straight to underpinning as the solution, but in reality this is not always necessary.

Subsidence caused by a neighbouring property

An interesting case for my firm recently involved subsidence in a ground floor slab where there was no movement identified in the external walls.

The slab had shrunk unevenly by up to 40mm and an examination of the subsoils confirmed that there was a likelihood of further movement. A void had been created through shrinkable clay becoming desiccated by tree roots and a poorly compacted sub-base above.

Tests by an arboriculturalist identified the culprit as a large coniferous tree on the neighbouring property’s land. The insurance company refused cover for removing and relaying a new ground-floor slab on the basis that their definition of subsidence had to include active movement in the foundations as well.

However, the neighbouring tree’s roots are deemed to be trespassing on the property and the homeowner is currently embroiled in a civil action with their neighbour (and most likely their insurance company) concerning liability for consequential damage arising from that trespass.

Underpinning and/or slab replacement

The Building Research Establishment (BRE) has produced a paper, The Assessment of Damage in Low-Rise Buildings, which classifies visible damage to walls. Crack widths are classified in categories of 0-5, with Cat-0 being hairline (less than 0.1mm), Cat-1 up to 1mm, Cat-2 up to 5mm and so on.

Most professionals refer to these guidelines to help determine necessary remedial works as part of a wider assessment. If there are substantial cracks in the external walls, then reduction/removal of the trees may not be enough. In fact, it may even be counterproductive, with reactionary over-rehydration of the subsoils leading to soil expansion and potential heave.

Where volatile conditions prevail, underpinning may be necessary to help stabilise the building. The subsoils below existing foundations are excavated in manageable bays (usually 1m wide) and to a depth identified by the engineer and building control team, which are subsequently filled with concrete.

The final connection to the underside of the existing foundations is then dry-packed and grouted. It’s expensive because the building must be temporarily supported at all times during the works.

If the problem is a failure of the ground-bearing slab, the ultimate solution may involve the excavation and replacement of the slab, its sub-base hardcore and a good amount of the desiccated shrinkable material beneath. In some cases a suspended floor might be introduced instead, which can be more cost effective.

But both options are incredibly expensive because of the internal disruption, and will be resisted by those having to fund the work (which is why determining an underlying cause, with lengthy monitoring periods, make sense).

Can subsidence affect property value?

One of the surveyor’s tasks on a pre-acquisition house survey is to look for signs of building movement – and in particular subsidence – but without being able to peel back wall or floor coverings. Lightweight external cladding (such as tiling or timber) makes this especially difficult and shrewd homeowners may fill and redecorate walls and ceiling surfaces internally before going to market.

This means the signs of movement might be confined to mortar repairs in brickwork, uneven floors, sloping window sills, sticking doors etc. Tree proximity, the condition of the surrounding drains and the contours of the site will be noted, as will the likely soil conditions. In densely populated areas, the surveyor will also look for evidence of movement on neighbouring properties.

Any reference to subsidence in a report can affect a building’s appeal, and thus its value. If the issue has been identified by your surveyor then the most sensible solution is for the vendor to register a claim with their insurance company.

Once this is accepted and validated, it can be passed over to you, so you will also inherit the existing insurer. If you’re thinking long-term about a property it’s an attractive solution, but if you plan to move on in a few years you may find that a recent reference to subsidence (regardless of the corrective works) diminishes the property’s appeal for prospective buyers.

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