Should I be worried about cracks in my concrete floors?

I had a slab concrete foundation issue in the garage area, which was repaired. Once I pulled my carpet up inside, however, I noticed superficial cracks on the concrete slab inside my home. How do I prevent these cracks from expanding and is it wise to cover them with hardwood floors?

The construction of the ground floor is most likely a ground-bearing concrete floor slab as you have correctly referenced: a “concrete slab” for short. A concrete slab is constructed on top of a compacted hardcore fill which is used to raise ground levels between the external rising walls.

It is difficult to produce a smooth finish on a concrete slab, so it is usual to create a non-structural finish, which is called the concrete screed (usually consisting of a sand/cement screed with no aggregate for older properties). The screed is what we see when there is no carpet or tiles in place.

To confirm the level of repair required, if any, an assessment needs to be carried out to determine the severity of the cracking. The Building Research Establishment (BRE) has categorised cracking size to assist inspection assessment. The classification that they have created extends from 0-5, 0 being defined as a hairline crack up to 0.1 millimetres and 5 being defined as structural damage cracks greater than 25 millimetres.

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Cracking between category 0 and 2 – that is, cracks up to 5 millimetres – are regarded as an aesthetic issue and do not require significant remediation other than general filling. Cracking in category 3 – that is, cracking between 5 and 15 millimetres – requires assessment, but is regarded as serviceable (minor repair). Cracking in categories 4 and 5 (greater than 15 millimetres) will require assessment and is defined as “extensive damage”, which likely will require significant repair.

It is unusual to experience cracking above category 3, however, if the cracking occurring is above this category, further assessment is required to determine if there is an underlying ground condition issue or if there is an issue with pyrite.

The presence of excessive pyrite in floor hardcore can lead to severe deterioration in construction materials as the pyrite oxidises. This results in cracks in floors that will need to be rectified by removing the floor hardcore and replacing it with material that conforms to the new building standards.

The cracking discovered when you lifted the carpet is likely occurring to the concrete screed which is a non-structural finish. Consideration needs to be given to the location of the crack as well as the crack size and extent. Cracking around the perimeter is less concerning, however, cracking of the floor within the middle of the room needs to be assessed differently.

Cracking to the concrete screed can occur because of poor strength mix, lack of depth, or underfloor heating pipes that are run too close to the surface. The cracking is likely to have been in place since construction so confirming the building age is important. It should be noted that cracks will expand and contract because of seasonal conditions.

You describe the cracks as superficial and you can confirm this by measuring them and using the BRE guide referenced above. If they are relatively minor, installing a timber floor should not cause any issue. The installer of the floor will complete an assessment of the floor prior to installation and will usually need to apply a liquid floor leveller to remove slight imperfections along with filling any cracks.

Depending on the method of installation and timber floor type, an allowance for expansion of the timber floor will be required around the perimeter. I would recommend that the new flooring is left in the room to acclimatise depending on timber species and product, but your provider can advise further on this.

Andrew Ramsey is a chartered building and project management surveyor and member of the Society of Chartered Surveyors Ireland scsi.ie