Repairs: floors and flooring

Problem 4: Setting of concrete floor slab

Once settling starts to occur as ground conditions consolidate or due to seasonal changes in soil volume (particularly on clay soils), it can be very difficult to consider an appropriate and effective remedial solution.

Solution 1

In particularly bad cases (such as on a sloping site with poorly consolidated hard core under the floor, and where the sloping site means there is more hardcore on one side than another) considerable floor movements can occur.

In the worst cases it may be necessary to grub out the floor entirely, and consolidate the below ground area using, for example, micro-piles on which to support a new cast floor slab. This is an expensive solution, but the problem should not reoccur.

Solution 2 - patch repair

Sometimes where the floor is settling or moving out of true it may only be necessary to apply a new layer of screed to re-level the floor surface every 20 years or so.

Vacuum cleaners give bad vibrations

An engineer of the author's acquaintance has accounted for many of these floor slabs dropping because of the vibrating rotary action of upright vacuum cleaners. These can set up a resonance through the floor, encouraging it to slide down inside the comparatively low-friction bitumen damp course painted onto the wall within which the floor has been cast.

Note that if the DPC is not operating properly – either because the bitumen has not been properly applied, or due to a lack of sand blinding to cover over any sharp edges in the hardcore (any plastic membrane having been punctured as a result) – moisture penetrating from below can cause the concrete to deteriorate under certain circumstances.

This is sometimes due to contamination from ground water, e.g. if there is soil contamination causing chloride attack on the concrete or other chemical salt attack, perhaps lack of sulphate-resisting cement within the mix and so on. All these contaminants can cause significant breakdown in the floor.

In these situations it may be easier to break out the floor and replace it with a properly detailed and specified modern equivalent rather than to repair the damage. Consider the value and use of the building in arriving at a recommendation.

Typical ground-supported slab, with older bitumen DPM detail

Cost sequence - Defective and settled concrete ground floor (ground supported) slab

  1. Assumed ground floor area for typical semi-detatched house (i) 15m by 10m elevation dimensions, and (ii) 5m by 10m.
  2. Remove all skirting boards, floor coverings and cart away.
  3. Carefully breakout existing floor slab and remove hard core.
  4. No allowance has been made for finding any contaminative amterials within the hard core, e.g. old asbestos or other old building products.
  5. Set aside hard core for reuse.
  6. Using transportable rig set micro-piles - nominal 100mm diameter steel tube piles to a depth of 2m from pile cap to assumed load bearing/friction position. At 2m centres.
  7. Set pile caps.
  8. Reinstate hard core to form suitable surface for casting of new floor.
  9. Pour in new floor in concrete mix to top of pile cap.
  10. New floor assumed minimum 80mm polystyrene insulation and 60mm screed with 2000 gauge polythene DPM inserted and terminated at main DPC alignment.
  11. Install new skirting boards and allow for 1 coat primer, 2 undercoat and 1 top coat.

Defective and settled concrete ground floor (ground supported) slab - (i) Cost £28,000; and (ii) £9,500.

Small works pricing

Since the original price estimating in 2008 the economic turmoil of the last few years has caused some disparities in pricing - if you are reviewing these figures in the London area they may look laughable - if you are located in Hull they might still look a bit expensive. Please treat them with caution and of course always consider obtaining more than one quote against which to test prices. Reference to the BCIS or a small works pricing book such as Spons or Laxton's will help assess costs and the hourly labour rates upon which they are based.