underpinning

Written by

Emma Myrie

Insurance Underwriting Expert

Reviewed by

David Joyson

Home Insurance Expert & Customer Champion

What is underpinning?

Underpinning is a process whereby additional support is laid below ground level to strengthen a building’s existing foundations.  

To underpin a property, you need to extend foundations in depth and/or breadth so that they either rest on soil that can provide better support or simply redistribute the weight of the building over a larger surface area. 

Though underpinning house foundations is often seen as a last resort when tackling subsidence these days, there are occasions where you have no other option. The technologies involved in the underpinning process have developed a great deal in recent years, and there are now several different underpinning methods and techniques available. 

This article helps you understand what the various options for underpinning foundations methods are so that you can discuss them with your surveyor or structural engineer before making a choice. 

types of underpinning

Soil strengthening

Where soils are weakened, or extracted, a process of soil strengthening may be used instead of underpinning, which involves injecting a special type of grouting or structural resin foam into the earth. Foam underpinning foundations can usually be done quite quickly and without you needing to move out of your home.

Mass concrete underpinning

The traditional type of underpinning has historically been the mass concrete or mass pour method, which dates back a hundred years and has barely changed since. It involves digging box-shaped voids to a specified depth beneath a structure’s existing foundations and filling them with concrete in a specified order for a foundation-below-the-foundations effect.

This process is usually used when the property’s existing foundations are shallow but can be implemented even when they are particularly deep. Heavy machinery is not usually needed, labour costs can often be kept down and sometimes the structure can continue to be used while the work is carried out.

Beam and base underpinning

The beam and base method is a refinement of the mass concrete method and involves a reinforced concrete beam being built either above, below or to replace the current footing. This beam redirects the weight of the structure onto several mass concrete bases placed at certain strategic positions, the placement and size of which depend on the ground conditions and the size of the affected property.

The type of construction used to build the beam will depend on the type of building and where loads are applied. Where there is a danger of clay soils expanding after the measures are implemented, anti-heave precautions will usually be incorporated into the design of the underpinning works.

Screw piles and brackets

Underpinning with screw piles and brackets can be employed where traditional underpinning is too impractical, owing to the need to excavate to an excessive depth, or where there’s no space to get a piling rig close to the property. This underpinning method can be carried out much more quickly than the traditional mass pour techniques, so costs and inconvenience can be reduced.

Mini-pile underpinning

Mini-pile underpinning methods are useful where ground conditions are variable or where access to the foundations is limited, and allows the loads to be transferred down to stable grounds at depths usually greater than five metres. The technical installation of mini-piles is more involved than mass concrete, requiring specialist machinery and skills.

Pile and beam underpinning

Pile and beam underpinning can be carried out in conjunction with mini-piling processes. It involves the installation of mini-piles to either side of an affected wall, then a subterranean pocket of brickwork is removed so a pre-fabricated steel cage can be installed to span across the two piles. Once the cage is inspected by Building Control, it can be concreted to complete the underpinning.

Cantilever pile and beam underpinning uses a similar approach to pile and beam underpinning, only all of the works are undertaken from the property’s exterior. In this way, the removal of internal floors and fixtures is not required so the costs and inconvenience can be substantially reduced. This method can also be carried out in conjunction with screw piling.

Piled raft underpinning is a system used when the whole property has been affected by subsidence. All floors need to be removed and mini-piles need to be constructed, before pockets of brickwork can be taken out for the reinforcement to be placed under an engineer’s instruction. After inspection, the modifications are concreted to complete the underpinning.

How do i know if my home has subsidence?

When you spot cracks appearing in your home, it can be a worrying discovery indeed. No cracks should ever be ignored, but they are not necessarily indications of a subsidence issue. Typically, subsidence cracks are tapered in appearance and spread diagonally across your walls, often originating from door or window frames.  

Subsidence cracks will open quite wide and will not close naturally on their own. Cracks that close up on their own could be forming as part of your home’s seasonal expansion and contraction and may not be anything to worry about. This said, you should always seek professional advice when you notice any cracks appear. 

What causes subsidence and when is underpinning needed?

There are a number of common causes of subsidence, and there could be additional exacerbating factors. In the South East of England, the clay-rich soils mean that clay shrinkage is a likely candidate after extended hot dry periods. This can also be the case elsewhere, but it is particularly rife in areas around the Thames (where it is now and where it used to be historically). Settlement of soft ground can cause subsidence issues, particularly where leaking drains or pipes are an issue. No matter what type of subsidence you encounter, if underpinning is required then you will have to seek out underpinned insurance in the future.  

can subsidence be fixed without underpinning?

Underpinning is not always necessary to rectify a subsidence issue, and is increasingly being used only as a last resort. There are several other options that might be open to you, depending on the individual circumstances of your subsidence issue. Usually, once you have contacted your underpinned house insurance provider, a period of observation and investigation will commence to diagnose the issue properly. This monitoring may need to be done over long periods to make sure that the movement is ongoing, so getting a quick-fix might be out of the question. Consulting engineers may be called in to help determine the best course of action to take in order to rectify any structural issues. 

Where subsidence issues are being caused or aggravated by tree root incursion, instead of immediate underpinning you may be able to mitigate further damage with careful tree management. Removal of trees completely is not always a good idea, as this can lead to other severe ground movement problems down the line, so you will need to consult a professional arboriculturist or similar before taking action. Sometimes tree pollarding is used to control tree growth, and this might be preferable to the invasive and expensive practice of underpinning. 

when shouldn’t underpinning work be carried out?

There are two particular situations where underpinning work should not be carried out. The first of these is if the ground movement that caused any damage has stopped and is unlikely to reoccur. Underpinning is also inappropriate when the rate and final extent of any damage is not likely to impair structural strength or integrity and will not threaten stability during the building’s expected life-span. 

In both cases, repairing and redecorating after damage has occurred less expensive and less traumatic for the building’s occupants.