A sinking floor is usually a ground-floor concrete slab that has dropped because the fill or ground beneath it has compressed, been washed out by leaking water or was never properly compacted. Sometimes it is a sign of wider subsidence affecting the walls too. The repair depends on the cause: it may be resin injection to fill voids, breaking out and relaying the slab, replacing it with a suspended floor, or dealing with movement under the walls. A chartered structural engineer decides which, and we carry out the work to that design.
How do you know a floor is sinking?
The signs tend to be gradual, which is why they are easy to put down to an old house being old:
- A visible slope across a room, or a marble that rolls towards one corner.
- A gap opening between the bottom of the skirting board and the floor.
- Internal doors that start catching on the floor, or door frames that are no longer square.
- Cracks in floor tiles or a cracked screed, especially in a line across the room.
- Kitchen units or worktops pulling away from the wall.
- Cracks in walls or ceilings at the same time, which suggests the walls may be moving too.
The RICS consumer guide lists new sloping in solid floors among the common signs of subsidence. It is a reason to get the floor looked at, not proof of any particular cause.
What kind of floor do you have?
| Floor type | Typical in | Common reasons it drops or sags |
|---|---|---|
| Ground-bearing concrete slab | Many post-war and later houses, extensions, kitchens and rear additions of older houses | Poorly compacted hardcore fill, fill washed out by a leaking pipe or drain, soft or made ground beneath, clay shrinkage near trees |
| Suspended timber floor | Many Victorian, Edwardian and inter-war houses, particularly front rooms | Decayed joists or wall plates, failed sleeper walls, poor underfloor ventilation, damp |
| Suspended concrete (beam and block) | More recent houses and extensions | Less often affected; problems usually relate to the supporting walls |
In practice, the solid slabs are the ones we are most often asked about. In homes with rear extensions, common across places like Maghull and Childwall, the extension floor sometimes drops relative to the original house, and the problem can be the slab, the extension's foundations, or both. Our extension subsidence repair page covers that situation.
What causes a solid floor to sink?
The slab itself is rarely the problem. What matters is what it sits on. If the hardcore beneath was laid too deep or not compacted properly when the house or extension was built, it can keep consolidating for years. If a water pipe or drain under or beside the floor is leaking, water can wash fine material out of the fill or soften the ground beneath. In general terms, parts of Merseyside have compressible ground, including some made ground and backfilled former clay pits, and coastal areas have sandy soils that a leak can wash away.
Leaks deserve a special mention. A slab that has dropped next to a kitchen, bathroom or gully often has a drain or supply pipe nearby. Repairing the floor without finding and repairing that leak usually means doing the job twice. See drainage-related subsidence for more on how drains are investigated.
How is a sinking floor investigated?
- Talk it through. We ask where the floor has dropped, how long it has been happening, whether walls are cracking, and where pipes, drains and any extension are.
- Look and measure. We visit, check levels across the floor and look at walls, doors and skirtings for related movement.
- Find the cause. Where needed, the engineer may ask for small cores or a trial hole through the slab to see the fill and ground, a CCTV drain survey, or leak testing of supply pipes. Our soil and site investigation page explains these.
- Decide whether the walls are involved. If the walls are moving as well, this becomes a wider subsidence question and the engineer will advise on monitoring and repair.
- Design the repair. The engineer specifies the method that suits the floor, the ground and the cause.
What repairs might the engineer specify?
There is no single fix, which is why the cause comes first.
- Resin injectionExpanding resin injected through small holes to fill voids and stiffen the ground beneath the slab. Quick and tidy where suitable; see resin injection underpinning.
- Break out and relayRemoving the slab, replacing poor fill with properly compacted material and casting a new slab with the right membranes and insulation.
- New suspended floorReplacing a slab on deep or poor fill with a floor that spans between supports, so it no longer relies on the ground below.
- Repair the causeFixing a leaking drain or pipe, then giving the ground time to settle before the floor is put right.
- Wider subsidence repairWhere the walls are moving too, the floor is part of a bigger scheme; see subsidence repairs.
Please do not drill, lift or self-level a sunken floor before the cause is known. Screeding over a dropping slab hides the evidence and can add weight to fill that is already failing. If movement may be subsidence, contact your buildings insurer before any work is done.
What affects the cost?
The area of floor affected, the method chosen, what has to be removed and refitted (kitchens, bathrooms, flooring), drain or pipe repairs, the depth of poor fill, and whether Building Control is involved. Replacing a floor or underpinning walls is structural work, and some floor replacements also bring in insulation requirements. Our cost guide explains the wider factors. We quote in writing once the engineer has specified the repair.
Sinking floors are one of the less dramatic jobs within our subsidence and underpinning work in Liverpool, but getting the diagnosis right saves a great deal of disruption.
Floor started to slope?
Call 07777 659786 or fill in the form. Tell us which room, roughly how old the house or extension is, and whether walls or doors are showing changes too, and we will call you back to talk through the next steps for ground-floor and foundation repairs across Merseyside.
Frequently asked questions
Why is my floor sinking but the walls look fine?
Ground-bearing concrete floors usually sit on a layer of compacted fill, separate from the wall foundations. If that fill was poorly compacted, has been washed out by a leaking pipe or is resting on soft ground, the slab can drop while the walls stay put. A gap under the skirting is a common sign. An engineer needs to confirm the cause before a repair is chosen.
Is a sinking floor the same as subsidence?
Not necessarily. Subsidence describes the ground under the foundations moving down. A floor slab can settle on its own fill without the walls moving at all. Sometimes both happen, for example where a leaking drain affects the ground under a wall and the floor. The RICS lists new sloping in solid floors as one sign of subsidence, so it should be checked properly.
Can resin injection fix a sunken floor?
Sometimes. Expanding resin injected through small holes can fill voids beneath a slab and stiffen the ground, and it often takes only days on site. It is not suitable for every soil or cause, and it will not help if a leak is still washing the ground away. The structural engineer decides whether it is appropriate after looking at the floor and the ground.
Should I tell my insurer about a sloping floor?
If you think the floor movement may be linked to subsidence, for example because walls are also cracking or doors are sticking, contact your buildings insurer first. The insurer decides on investigation and repair. Policies differ on whether floor slab settlement on its own is covered, so check your wording or ask the insurer directly.
My suspended timber floor feels bouncy. Is that subsidence?
A springy or sagging timber floor is more often caused by decayed joists, rotten wall plates, failed sleeper walls or poor ventilation than by foundation movement. It still deserves a proper inspection, because damp under a timber floor can point to drainage problems. We can look at it and recommend whether a structural engineer is needed.