Concrete Floors, Slabs

Ground floor slab types

1. Solid floating concrete ground floor slab

A solid floating concrete ground floor slab should only be used if the depth of granular fill is less than 900mm.

Diagram B51 - Typical detail of solid floating concrete ground floor slab

2. Cast in-situ suspended ground floor slab.

Where the depth of granular fill is greater than 900mm a solid floating concrete ground floor slab should be used. Cast in-situ suspended ground floor slabs should only be used in situations where settlement of ground under its own weight is unlikely. If a gap was to form it would create a risk of explosive gas mixtures accumulating under the floor.

Diagram B52 - Typical external wall detail for a cast insitu suspended floor slab

Cast in-situ suspended floor slab party wall detail

Where a suspended floor slab runs over a party wall, reinforcement should be placed in the top of the slab to reduce the risk of cracking.

Diagram B53 - Typical party wall detail for a cast insitu suspended floor slab

Explosive gas mixtures in underfloor voids:

In underfloor voids there is a risk of explosive gas mixtures accumulating.

Gas may occur where:

  1. The site on which building takes place is contaminated or reclaimed land, or

  2. Gas pipes are present in the vicinity of, or under the building area and the floor slabs are constructed clear of the ground, or where an underfloor void may occur due to settlement of ground.

For the cases outlined above, a ventilated air space no less than 150mm should be provided between the ground and the underside of the floor or insulation (if provided).

It is important to provide perimeter ventilation together with the ventilated air space detailed above to ensure adequate underfloor ventilation is achieved.

In all instances it is important to ensure that the ground floor level outside is lower than the top of the subfloor level to ensure a sump effect situation is avoided.

Problems associated with concrete ground floors

Failure in concrete floors has been identified as quite a large problem. Failure can generally be put down to one or more of the following: poor materials, poor workmanship or bad design. To prevent the need for expensive and possibly disruptive repairs at a later stage, care should be exercised at each of these stages. Good practice guidelines to avoid these problems are as follows:

1. Strip existing ground appropriately

Ensure existing ground is stripped to a suitable bearing. Before work starts it is critical that all top soil and vegetation is stripped from the entire area of the building. Leaving mounds of soft earth between the foundation trenches is not recommended as it increases the risk of it not being removed before filling the area with layers of granular fill begins. It is important that granular fill is placed on solid, clean and good bearing ground.

2. Granular fill appropriate for purpose should be used.

3. Use vibro roller to compact layers of granular fill.

Layers must be less than 225mm and greater than 150mm. Each layer should be compacted after being placed by a whacker plate or a vibro roller. If care is taken at this stage the risk of failure will be reduced.

4. Granular fill material should not be used if the depth to fill is more than 900mm.

A suspended concrete floor is less liable to fail and should be used in this case.

5. Blinding of granular fill material.

Blinding should not be required if each layer of granular fill has been well compacted. Blinding should be raked to ensure that sharp points left after the vibro roller are covered, once completed, DPM can be laid on the blinding.

6. Use 1200 gauge polythene as DPM

It is important that the DPM is not damaged by traffic when slab is being poured as the purpose of DPM is to stop rising damp. Virgin material should only be used and material described as ‘heavy duty’ or ‘C1200’ should be avoided. Virgin material used should comply with the requirements of EN 13967.

7. Make sure there is a lap between DPC and DPM

A continuous barrier must be formed by the DPC and DPM to prevent the penetration of moisture from the ground below. Construction of walls prior to laying polythene DPM and pouring the floor slab is incorrect as it does not allow a proper lap to be formed between the DPM and DPC.

Diagram B54 - Example of correct lap between DPC and DPM

8. Must not bear on the floor slab.

All blockwork or load bearing stud partitions and rising walls should have foundations as floor slabs are generally not designed or constructed to carry loads such as those from chimney breasts, piers, walls and load bearing stud partitions.

9. Be careful when providing insulation.

Insulation must be provided underneath the full floor area, the amount of insulation to be provided will be dependant on a number of project specific elements such as:

  • The type of insulation being used and its properties.

  • The required U-value that must be achieved.

  • The ratio between the area of the floor and its perimeter etc.

A thermal bridge can occur where two construction elements meet, for example where the floor and wall meet. Thermal bridging can have a large effect on the overall efficiency of a dwelling and as a result insulation must be carefully detailed and installed at such junctions. A number of methods on how compliance with this aspect of the regulations can be achieved are found in Technical Guidance Document L of the Building Regulations. One method outlined in the text is the use of Acceptable Construction Details (ACDs).

10. Care should be taken to ensure the risk of cracking is avoided.

In hot weather, it is necessary when laying concrete floor slabs to ensure that they are protected from direct sunlight and are kept cool. This will prevent cracking due to drying too quickly.

By including perimeter insulation at the floor/wall junction thermal bridging will be minimised.

Slab Failure Prevention

  • All top soil and vegetable matter should be removed from the site.

  • Granular fill material should be compacted in layers.

  • It is important to ensure an appropriate granular fill material is used.

  • Deep fill should not be used.

  • Blind the granular fill material.

  • A 1200 gauge DPM or radon barrier should be used where required.

  • The DPM should be places and the slab should be poured when the walls are at DPC height.

  • Walls should never be built directly off the slab; they must have foundations.

  • Insulation must be provided under their entire area to ensure compliance with the Building Regulations is achieved.

  • For rafts, DPM and screed must be on top of raft and screed must be at least 65 mm thick. It is advisable to incorporate a light mesh within the raft for reinforcement.

Reinforcement to suspended floor slabs

There is a range of reinforcement mesh available, mesh can either be prefixed by the letters A, B or C. For each prefix there are a range of diameters available.

Type A mesh consists of 200mm by 200mm squares, the standard range of type a meshes is A98 to A393. Type B mesh consists of 200mm by 100mm squares with the main reinforcement at 100mm centres and cross reinforcement at 200mm centres; the standard range of type a meshes is B196 to B1131. Type C mesh consists of 400mm by 100mm squares with the main reinforcement at 100mm centres and cross reinforcement at 400mm centres; the standard range of type a meshes is C283 to C785.

Due to the risk of explosive gas accumulating under a suspended floor due to settlement of ground under its own weight, Technical Guidance Document C of the Building Regulations recommends that suspended floors only be used in situations where it is unlikely that settlement will occur.

Diagram B55 - Typical suspended ground floor slab - reinforced concrete

Slab depths and reinforcement mesh types for typical domestic ground floor spans

The table below shows the appropriate slab depths and reinforcement mesh types for typical domestic ground floor spans. Assumption: no internal partitions bear on the slab. Main bars to run in direction of span and mesh should be placed with main bars below secondary bars. Nominal cover of 25mm should be provided between the underside of the slab and the main bars. Concrete mix should be 30N20. Supporting rising walls and slab bearing should be at least 100mm.

Table B5 - Appropriate slab depths and reinforcement mesh types for typical domestic ground floor slabs

Reinforcing steel bar equivalents

The table below lists the reinforcing steel bar equivalents which may be used as an alternative to B meshes. Main bars should be located below the secondary bars and run in direction of span. Bars should be tied at intersections. Assumption: no internal partitions bear on the slab. Concrete cover and strength is the same as above.

Table B6 - Reinforcing steel bar equivalents to be used as an alternative to meshes

T denotes high yield steel.

Alternative bearing detail for suspended floor slabs

Shown below is an alternative bearing detail for suspended floor slabs. Additional reinforcement can be placed in the top of the slab at the leading edges and especially in corners to reduce the risk of cracking.

Diagram B56 - Typical edge reinforcement

Continuous slabs over a wall should be provided with additional reinforcement to ensure a reduction in the risk of cracking occuring.

Diagram B57 - Typical slab detail continuous over a wall

It should be noted that it is only permitted to form pipe recesses in the perimeter of slabs.

Diagram B58 - Typical detail for pipe recesses

Precast Concrete Floors

A variety of precast concrete floor systems are available for use in suspended floor construction at ground level or above. These systems serve as an alternative to using suspended in-situ concrete floor slabs or suspended timber floors.

Precast concrete floors can be categorised, albeit generally, as follows:

  1. Hollow slab.

  2. Beam and block.

  3. Precast plank or pre-stressed plate with in-situ concrete topping (infill blocks or void formers may or may not be included).

Examples of precast concrete floor types are illustrated below. It should be noted that no particular manufacturer’s products are represented; illustrations are intended to give an indication as to the range available. Depending on the specific products to be used there may be variations to these details. Manufacturers details should be adhered to.

Diagram B59 - Block and beam floor typical detail

Diagram B60 - Proprietary T beam floor system typical detail

Diagram B61 - Hollow core slab typical detail

Diagram B62 - Block and plank floor typical detail

Key installation points

For systems of the types detailed above, close attention should be paid to the following aspects of installation when following the manufacturer’s detailed recommendations:

  • The span to be covered and the supports, propping may be required in some systems.

  • DPC location.

  • What happens when point loads are imposed such as partitions?

  • Is reinforcement required for screeds?

  • Screeding or grouting requirements.

  • Insulation. The location, amount and method used to install the insulation to ensure cold bridging is avoided and it meets Building Regulations requirements.

  • Where required, installation details of the radon membrane.

Additional details

If the span of suspended concrete ground floors is greater than 5m, it is a requirement of the Technical Guidance Document A of the Building Regulations that professional guidance is required regarding design of supporting walls. The requirement for upper timber floors with a span greater than 5m is the same.

Safety points

  • To avoid a sump situation arising, it is recommended that the level of the underfloor should be level or greater than the external ground level.

  • DPM or vapour control layer should be placed above the structural floor and beneath and screed or board finish in order to prevent water vapour from the ground damaging flooring or finishes.

  • Between the underside of the floor and the ground there should be a ventilated air space measuring at least 150mm.

  • Avoid overloading pre-stressed units with blockwork pallets during construction.

  • Concrete requires time to cure before it is capable of taking load, ensure adequate time is allowed.