Whilst discovering that your home is suffering from subsidence can be unsettling and leave you panicking about how much underpinning costs, it doesn't always spell disaster. In fact, many subsidence issues can be solved fairly easily.
This guide will not only explain how much underpinning costs, but also what causes the structural movement in the first place.
What Does Structural Movement Mean?
It is a fact that all buildings move. It is also a fact that many older properties were built with shallower foundations meaning they tend to move more than modern buildings in response to factors such as seasonal changes in ground conditions or new loadings — particularly in regions with clay subsoils.
Whilst structural movement might cause the odd door or window to stick in its frame, a little judicious planning could soon put right such minor niggles. So, for homebuyers with a cool head, concerns about structural movement and underpinning costs can sometimes be turned to an advantage by securing a bargain.
Is Underpinning Necessary?
Although much advice might say that the solution to subsidence is underpinning, which involves excavation below the sunken wall and pumping in large volumes of concrete, underpinning costs money and should only be carried out as a last resort.
This is particularly true of older properties because the part that hasn’t been underpinned will continue to naturally move in tune with ground conditions, setting up stresses with the newly rock solid underpinned length of wall.
Underpinning also tends put off future buyers and insurers.
Before you begin to think about underpinning costs, make sure the cause of the subsidence is thoroughly investigated.
What Causes Subsidence?
Some cases of subsidence can occur if the ground under part of the foundations shrinks, robbing the wall of support (commonly caused by leaking drains, or long periods of drought exacerbated by trees extracting moisture). The unsupported part of the wall immediately above can then suddenly drop, with this movement causing cracking.
This is very different from ‘settlement’, where the ground is slowly compressed over time by the loading placed upon it by the building. All buildings settle after construction or in response to major new structural change such as extensions and loft conversions – as the ground adjusts to the new weight imposed upon it.
Interestingly, most insurance claims for alleged subsidence turn out only to be cosmetic damage, and the majority are rejected as invalid because the cause lies elsewhere, such as differential movement cracks between lightweight conservatories and the main house. Of the valid claims, nearly three quarters are tree-related.
Regardless of the precise cause of movement, the big question to answer in all cases is whether the movement is historic or ‘progressive’, and likely to move more in future.
Tree-related subsidence leaves you with two basic options:
- remove the offending tree
- manage it (usually by pruning).
But if you simply cut down a large, thirsty tree, the ground may then react by swelling with moisture which is no longer being absorbed — causing ‘heave’. Be aware too that cutting down a tree protected by a Tree Preservation Order (TPO) or in a Conservation Area without consent can lead to prosecution.
One remedy is ‘pollarding’ (severe pruning), but not all trees will tolerate heavy crown reductions. For pruning to be effective, reductions of around 90% are required, and some species respond to pruning with vigorous new growth which quickly restores the tree’s moisture uptake. This is why specialist advice should be sought before deciding whether to prune, pollard, or remove a tree.
Installing root barriers can provide an alternative method of tree management. This involves excavating a trench around 4m deep between the offending tree and the building and inserting large sheets made of rigid plastic to protect the whole building from the influence of the vegetation.
This method can have considerable advantages compared to underpinning, plus there’s the benefit of retaining trees. However, there can be practical limitations; barriers, for example, can’t be inserted so close to the tree that the main roots are damaged, risking instability. It’s also important that they are sufficiently robust and designed to accommodate any drainage pipes and underground services.
What Causes Bulging Walls?
Solid walls that bulge outwards or lean, with resulting cracking internally to plasterwork and sticking windows are not unusual and there are a number of possible causes. Leaning tends to be most visible at window and door reveals near roof level.
Bowing or bulging in old solid walls is often due to ‘jerry building’, whereby the walls were never properly tied into the floor or roof structure in the first place, or were just cheaply built. This is sometimes evident in the side walls of Victorian houses and end terraces, for example.
Solid walls were sometimes built with small embedded ‘bonding timbers’ to tie them together. These timbers can, over time, be prone to rotting and expanding, causing the wall to bulge.
Walls can also bow outwards because they are overloaded, having been built too thin or as a result of botched structural alterations. The most common cause of leaning is from ‘roof spread’ where the rafters push the tops of the main walls outwards if they’re not restrained by collars or ceiling joists. This may have happened because the joists were poorly nailed to the rafters or have rotted.
Remedial Action for Bulging Walls
Before the affected wall section(s) can be repaired – or indeed cut out and rebuilt –the cause of the movement must first be identified and eliminated.
- Bulging walls caused by excessive loadings need some form of additional support and a structural engineer will need to be consulted. A thin, 115mm single-leaf wall, for example, may need to be entirely rebuilt, or the loadings could be diverted or relieved by installing a steel beam
- Walls suffering from a lack of restraint can be secured to the upper floor joists or roof structure by building in metal ties — a modern variation on traditional S or X iron tie bars (below) that can be seen adorning the walls of many an old cottage
- Sometimes a wall will have ‘come loose’ because of rotten floor joist ends that used to hold it in place. The rotten timbers must be cut out, the cause of the damp remedied, and the ventilation improved, before defective lengths can be replaced with new pre-treated timber protected by a damp-proof course
- Similarly, where the roof has pushed out the walls causing them to spread, the solution is to provide additional restraint such as new ceiling joists or collars, and, in serious cases, to rebuild the upper wall
What Causes External Cracks in Masonry?
Potentially problematic external cracks in masonry, which could allow damp to penetrate, should be addressed before doing anything else.
There are two main kinds of vertical cracking in masonry: those that run straight down through the bricks or stones, and stepped cracks that follow the mortar joints in a zigzag pattern (either tapered or uniform in width). Less commonly, cracks may also run horizontally. Internally, cracks to plasterwork and sticking doors and windows are signs that something is not right, too.
Identifying the Cause of the Movement
Before carrying out repairs, the first step is to identify the cause of the movement. Diagnosing the cause can be difficult, so insurance companies often want to monitor and measure the rate of cracking for at least 12 months in order to diagnose the risk of future movement. This typically involves fitting ‘tell tales’ across the crack. Tell tales are best described as graduated glass microscope slides, screwed in place or secured with epoxy resin adhesive.
One of the most common causes of cracking is thermal movement due to adjacent materials expanding and contracting at different rates. This is usually found at the juncture between different materials, such as where old solid brick walls meet blockwork walls. But large stretches of wall made from a single material can also develop thin, vertical thermal movement cracks (hence the need to incorporate vertical movement joints every few metres when building).
Certain materials are especially prone to this form of cracking, such as white calcium silicate bricks (commonly used in the 1960s and ’70s) and reconstituted stone blocks.
Differential movement is commonly found where new extensions with deeper foundations abut an old house, or when bay windows and porches were built with shallower footings than the main property. Different foundation depths and loadings mean that these will move at a different rate, setting up stresses which in turn lead to cracks.
Water and Rust
If water penetrates a wall and then freezes, it can, over time, cause metal components to rust and expand. If rainwater gets into a crack and there’s an old steel lintel in the wall which begins to rust, for example, the expansion can force the surrounding brickwork to move.
Similarly, iron cramps embedded within traditional stonework can rust where the protective lead coating has cracked. The rust expansion will ultimately crack the stone it was meant to restrain.
Rust too can be the cause of cavity wall tie failure where ungalvanised ties have corroded and expanded. (This is more common in exposed walls in coastal areas.) The expanded ties can jack up the wall, resulting in horizontal cracks at periodic mortar joints corresponding to the tie positions. This is usually more evident to upper walls.
Doors and Windows
It is very common to find stepped cracks above openings in cavity walls where the doors or windows have been replaced, too. This is fairly common in properties built from the 1940s to ’70s with no lintels in the outer leaf; the original steel windows were often designed to support the wall loadings. A lack of support during replacement work can cause the wall to drop slightly.
Subsidence is more likely to occur in older properties with shallow foundations on clay subsoil, on slopes, and with trees nearby. Alarm bells should start to ring where cracks extend below damp-proof course level down to the foundations — this may well be the time to begin thinking about underpinning costs and how they may impact your budget.
Other clues are where tapered diagonal cracks increase in width as they go up the building (typically widening in summer and closing during winter), and where you can see ‘mirror image’ cracking visible both inside and out.
‘Heave’ is the opposite of subsidence: the ground swells up rather than sinks. It can often occur on clay subsoils typically after the removal of a large nearby tree. The water that would have been absorbed by the roots swamps the ground, and this can then freeze and expand further, pushing the shallow foundations upwards.
Bad Building Work
Finally, we mustn’t overlook the power of botching. Incompetent builders or faulty materials can cause structural defects — steel beams installed with inadequate support at either end being a prime example.
Repairing Wall Cracks
In any case, both thermal and differential movement need to be addressed. This can be done by forming an expansion joint with a flexible mastic sealant. You must notify your local Building Control department in advance of all structural works.
- Where the cause of the cracking has been stabilised, stepped cracks can be made good simply by repointing with a suitable mortar or using a clear flexible mastic
- With vertical cracks, on the other hand, the damaged bricks or stone will normally need to be cut out and replaced
- Another remedial technique for areas of substantial cracking, is to drill a series of holes along the crack and inject them with thixotropic resin grout. The face of the brick can then be repaired with colour-matched mortar which should be undetectable
- Alternatively, engineers sometimes recommend that cracks are stitched up using stainless Helifix small diameter rods cut into the masonry and bedded in epoxy mortar
- To prevent unsightly cracks reforming in rendered surfaces, a fresh coat of render can be carried across the hidden joint on stainless steel lathing; this gives flexibility to accommodate future movement
- Original lintels that were too thin, missing or have become overloaded will need cutting out and replacing
- Rusted metal components can be cut out and replaced too with more durable new components, such as stainless steel cramps
- Where there’s horizontal cracking along mortar beds, specialist firms can carry out cavity wall tie replacement
- Movement due to poorly executed past structural alterations may require a structural engineer to design a new method of additional strengthening.
Underpinning Cost Table
If all of your investigations leave you no choice but to rely on underpinning to resolve the problem, then it pays to understand how much underpinning costs before you begin.
As always, it's good to get at least three quotes and make sure you know what's included so you can compare like for like.
|Works||Labour & Material (incl VAT)|
|Repairing Cracking||Repair vertical cracks in brickwork to prevent further cracking Reinforcing rods to extend 500mm each side of crack. Rake out mortar joints and insert 6mm helical reinforcing rods, bonded with masonry grout; repoint to match existing||£400/m length|
|Repair cracks in brickwork (one brick thick) including cutting out bricks and replacing with new, and pointing to match existing||£87/m length|
|Cut out defective facing bricks and replace with matching new bricks, including pointing||£17 (single brick)|
|Pointing up a crack or replacing a few bricks||Typical costs around £300|
|Lintels and Beams||Renew defective 1,200mm-long timber lintel, remove old lintel and one course of bricks, fix new steel lintel and brick courses and make good||£358 (in 265mm-wide cavity wall)|
|Fix new steel lintel 2,100mm long in prepared opening and make good||£185 (in 265mm-wide cavity wall)|
|Remove defective brick arch, clean off bricks, prepare and rebuild arch, and point in mortar||£163 (up to 1m long and half brick wide)|
|Cavity wall tie replacement||Removal of say 200 defective wall ties, replacement and additioal ties, and allowing for scaffolding. (Add for repointing brickwork or re-rendering and making good disturbed plaster finishes)||£4,500 (total)|
|Underpinning Costs||First excavation of trenches down to base of existing foundations, say 1m deep||£112/m²|
|Support sides of trench||35/m length|
|Extra for breaking up 150mm-thick concrete bed||£20/m²|
|Second excavation below level of existing foundations (say 1m deep)||£109/m³|
|Support to sides of trench (second excavation)||£37/m length|
|Cutting away existing foundations (projecting concrete 300mm wide x 300mm deep)||£57/m length|
|Concrete underpinning: plain concrete (1:3:6 with 20mm aggregate) filled into form work in foundation trenches, 150–300mm thick||£335/m³|
|Typical ballpark cost for underpinning an average house||£10,000 to £15,000|
For more information head over to our guide on repairing brickwork and render.
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