Sulphate Floor Testing

The problem of sulphate affected floors is caused by the use of industrial waste materials used as hardcore infill under concrete floors in domestic properties built between 1940s and 1972.

The material is classified in 4 main types – Red Ash (Shale), Black Ash, Slag and Grey Fly Ash although other industrial materials and building rubble etc. could also be a potential problem.

This material originated from various industrial sources ie. Coal Mines, Steelworks, Foundries, Power Stations etc. all of which were present in the North Midlands. As this material was widely available and was virtually free, builders used the material extensively as it’s use at that time did not contravene any Building Regulations.

It became apparent however that this infill material could cause damage to the concrete floors and brickwork below DPC level of the property due to the sulphate present in these materials. The sulphate in the hardcore infill when combined with moisture (water) can cause the sulphate to migrate into the concrete floor slab causing cracking and possible upward movement of the floor slab and, in extreme cases, cracking and lateral movement of the brickwork below Damp Course level.

The Building Research Establishment (BRE), based in Watford, started issuing information on this problem in the mid 1950s and in 1967 the Building Regulations changed to restrict the use of industrial materials as hardcore infill. At the same time a 500g polythene membrane was introduced to provide a barrier between the concrete floor slab and the hardcore infill. This membrane was increased in thickness to l000g in 1970 and to 1200g in the early 1980s.

The BRE regard the 500g membrane as an acceptable membrane barrier between the existing concrete floor slab and the hardcore infill. The minimum requirement to comply with current Building Regulations for new build or replacement floor construction is a 300 microns (1200g) polythene. Although the use of industrial waste materials which had high sulphate levels was outlawed in the 1967 Building Regulations, it’s use can still be found in properties built up to circa 1970. Surveyors may therefore request tests for properties built up to circa 1972.

Sulphate floor tests are usually requested by Surveyors when a property is being sold or remortgaged etc. although the request for a test is at the discretion of the individual Surveyor.

The Surveyor may not see any evidence of possible damage caused by sulphate action but will still ask for a test to be carried out due to the age of the property.

The level of sulphate in the hardcore infill is determined by taking a sample of the hardcore infill from under a floor in the property. As well as taking a sample of the hardcore infill we also determine whether there is any cracking or movement to the floors or walls below Damp Course level, whether the hardcore is dry or damp and whether there is a polythene membrane present between the concrete floor slab and the hardcore infill. The hardcore sample is then sent to an independent laboratory approved by the Building Research Establishment for analysis using a 2:1 water/soil extract test and the result of that, together with the information taken from site, is incorporated into our report.

This report is then read by the Surveyor and/or Mortgage lender and the potential property buyer or home owner is advised accordingly.

Sulphate classification used by Surveyors for floor assessment in the North Midlands are usually based on the levels in a recent document issued by the Communities and Local Government Department. This can be viewed and downloaded from

This document has been prepared by the Building Research Establishment.

The four Classes generally considered relevant to hardcore used in house construction are as follows, expressed in both gramme and milligramme per litre of sulphate determined as SO4 :-

  • Class 1 <.10g SO4/Litre to <0.50g SO4/Litre (<100mg SO4/Litre to 500mg SO4/Litre)
  • Class 2 0.50g SO4/Litre to 1.50g SO4/Litre (500mg SO4/Litre to 1500mg SO4/Litre)
  • Class 3 1.60g SO4/Litre to 3.00g SO4/Litre (1600mg SO4/Litre to 3000mg SO4/Litre)
  • Class 4 3.10g SO4/Litre to 6.00g SO4/Litre (3100mg SO4/Litre to 6000mg SO4/Litre)

In most properties tested the result will fall into Classes 1,2 or 3. It is not the type of hardcore infill that is a potential problem but the level of sulphate found in that material.

There are many properties therefore built with industrial fill under the floors that are not potentially a problem due to the low sulphate level in the material.

A Class 1 result is generally acceptable to Surveyors and mortgage lenders in the North Midlands as the level of sulphate is not considered to be high enough to cause damage to the floors and walls of the property in the future.

It is the Class 2 grading that poses the most problems for Surveyors and Mortgage Lenders. Generally a low Class 2 result is acceptable especially if there is no evidence of damage and the hardcore is dry. The presence of a polythene membrane will provide additional protection. A high Class 2 result increases the potential for damage although if the hardcore sample is dry and there is no evidence of damage due to sulphate action then the chances of any damage occurring in the future are still very small. Any decision however concerning the removal of the floors and hardcore infill are at the discretion of the Surveyor and/or Mortgage Lender.

A Class 3 result is generally not acceptable especially if there is no polythene membrane present between the hardcore infill and the concrete floor slab.

This method will apply to all properties built with traditional foundations. The property may however have been built on a raft foundation due to poor ground conditions, mining subsidence etc.. Where a raft foundation is found it is usually not necessary to take and test a sample of the hardcore infill as the amount of hardcore infill under the raft is likely to be very small, the raft will be usually minimum 200mm thick, will be a stronger concrete mix than in a normal traditional floor and will be reinforced with steel mesh reinforcement. Taking all these factors into consideration we interpret current BRE guidance as indicating the Building Research Establishment say that even if sulphate containing industrial fill is present under the raft foundation, the rate of attack on the concrete is unlikely to exceed 1mm per year making the risk of damage minimal.