Quoting an Elevated Concrete Floor – tag out the flatness risk!

The tolerances called up in table 5.2 NZS3109 “Tolerances for in-situ construction” are not achievable on elevated concrete floors. The relationship to datum is specified as +/-10mm. With  deflection and ongoing creep this is not going to be acheived. Then for flatness it references NZS3114 requirements. 5mm in 3m is also not going to be achieved 100% of occasions and if you have thin tiles such that the specification is 3mm over 3m then you are in trouble if the client measures the tolerances. Don’t expose your firm to the risk.

What we recommend is that for the relationship to datum you tag to +/-20mm from datum. All measurements to be taken within 72 hours and if propped the measurements must be taken prior to removal of propping.

For the flatness criteria we recommend using the British standard SR2 although the UK Concrete Society guide on elevated floors notes that SR2 will not be achievable everywhere. Thus we recommend changing this to 90% of readings meeting SR2. SR2 is 5mm measured over 2m.

Be up front with the engineer so that he is involved in the conversation early. Don’t wait until post construction when you may discover you have a very expensive bill for levelling screed.


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