Across Kent
Why identical houses behave differently a few miles apart
Find out what the ground beneath your home is made of and why that matters when cracks appear. Asking costs nothing and takes a minute.
Weald Clay across the south and west of the county
The heavy clay running through Sevenoaks, Tonbridge and Tunbridge Wells shrinks when it dries and swells when it gets wet again. Trees pull moisture from this soil in summer, so foundations move as the ground contracts. A house on Weald clay can sit perfectly still for years and then start cracking during a dry spell.
The movement is seasonal. Cracks tend to open in late summer and close slightly over winter. Because of this pattern, specialists look at root samples and soil shrinkage tests before deciding whether underpinning or resin injection is needed. The clay itself does not wash away, but it changes volume enough to tilt walls and jam doors.
Homes built before deep foundations became standard are the ones most likely to show the strain. Read more about subsidence help in Royal Tunbridge Wells or cracks and clay movement in Tonbridge to see how this plays out street by street.
Greensand Ridge and the higher ground
Between the clay vale and the chalk Downs runs a band of sandstone known as the Greensand Ridge. It passes near Maidstone and Ashford. The rock here is firmer than clay, but the sand above it can shift if water runs through it.
Houses on the ridge rarely suffer the same seasonal heave as those on clay. Instead, problems come from drainage. If a soakaway or broken pipe washes sand out from under a footing, the ground loses support locally. A specialist will dig trial pits to see whether the sand has been disturbed or whether the bedrock is doing its job.
Because the ridge is narrow, two homes a mile apart can sit on entirely different geology. One might be on solid greensand while the other reaches down into clay. That is why the town pages for Maidstone and Ashford describe ground conditions block by block rather than treating the whole area as one thing.
Chalk Downs and the old workings beneath them
The North Downs cut across Kent from Dover to the Surrey border. Chalk is strong when it stays dry, but it dissolves slowly in acidic rainwater. Over centuries this has carved hollows and swallow holes beneath towns like Gravesend, Rochester and Canterbury.
Builders also dug into the chalk. Dene holes and old mining shafts honeycomb parts of north Kent. A house can stand safely above one for decades until the roof of the cavity thins and collapses. When that happens, the movement is sudden rather than seasonal.
Specialists use boreholes and ground radar to find voids before they fail. Underpinning through chalk often means going deeper than expected because the stable layer sits below the worked zone. The pages on Gravesend, Rochester and Canterbury go into what living above chalk actually means for homeowners.
Thanet Sands and brickearth near the coast
Around Margate, Ramsgate and Herne Bay the chalk is capped by a layer called Thanet Sand. It drains quickly and holds together well unless water flows through it fast enough to carry grains away. Brickearth, a wind-blown silt found nearby, behaves similarly.
These soils do not shrink like clay. The risk is erosion. Heavy rain or a leaking main can wash material from under a slab or footing, leaving a gap. Houses on Thanet sands sometimes develop cracks after storms rather than droughts.
Coastal weather adds another factor. Salt spray and wind-driven rain penetrate masonry faster here, so surface damage can look like structural movement even when the ground is sound. A specialist separates the two before recommending work. See the pages on Margate, Ramsgate and Herne Bay for local detail.
Alluvium along the Medway and the northern marshes
The Medway valley and the Thames marshes around Gillingham, Chatham, Sittingbourne and Sheerness sit on alluvium. This is soft sediment laid down by rivers and tides over thousands of years. It compresses under weight, so buildings settle into it gradually.
Settlement on alluvium is usually slow and even, which most houses tolerate without cracking. Trouble starts when one part of a building bears more load than another, or when the water table drops and the soft ground dries out unevenly. Extensions are particularly vulnerable because they add new weight to old, already-compressed soil.
Screw piling is common here because driven piles reach past the soft layers to something firm. Specialists working in Gillingham, Chatham and Sheerness know exactly how deep the good ground sits. The town pages explain what to expect if your home is built on river mud rather than rock.
What the signs point to on different ground
A crack in plaster means different things depending on whether the house stands on clay, chalk or sand. The soil decides the cause, and the cause decides the fix.
- Cracks that open in summer and close in winter
- Clay shrinking and swelling with the seasons. Root surveys and soil tests confirm whether a tree is driving the cycle.
- A sudden dip in the floor or garden
- Possible void collapse in chalk or washed-out sand. Boreholes and ground radar locate the gap before any repair begins.
- Doors sticking after heavy rain
- Water moving through sand or silt and carrying support away. Drainage checks and trial pits show where the loss happened.
- An extension pulling away from the main wall
- New weight settling faster than the old house on soft ground. Level monitoring tells a specialist whether movement has stopped.
Typical costs for ground stabilisation work
Prices vary because every site has different access, soil depth and damage extent. A small bay porch needs far less material than a full terrace wall. The ranges below reflect what similar jobs have cost recently across Kent.
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Resin injection for a single footing
£1,500 to £4,000
Depth of weak soil and how many injection points the specialist needs to reach it.
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Screw piles for a porch or bay
£2,500 to £6,000
How far down firm ground sits and whether machinery can reach the spot.
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Repairing a cracked concrete slab
£3,000 to £8,000
Slab size, whether the sub-base has washed out and the finish required.
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Underpinning a section of wall
£8,000 to £20,000
Length of wall, depth to stable ground and access for excavation plant.
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Full side-of-house underpinning
£15,000 to £40,000
Number of bays affected, soil type and whether drains need rerouting.
These figures indicate typical costs for this type of work. Nothing here is a quotation. Only an inspection of your home can settle a price.
Tell us what you have noticed
Send a short description of the problem and your location. A specialist who works on that type of ground in your part of Kent will review it and reply. There is no charge and no obligation to proceed.
- No fee to ask
- Local ground knowledge
- No obligation