In accordance with BS EN771-1, manufacturers have a duty to declare length, width and height dimensions of a clay masonry unit in millimetres, in that order. The manufacturer shall also declare which of the tolerance categories the mean size values fulfil against the test method detailed in BS EN 772-16.
For testing purposes, 10 bricks are randomly selected across several packs, superfluous material removed, and measurements of length, width and height are taken for each individual brick.
For length and width sizes, all 10 individual figures are added, divided by 10, then rounded to the nearest whole number to produce a mean value.
For height size, the measurement is taken at 2 points (as shown in the above diagram) and averaged for each brick. The resulting 10 individual figures are then added and divided by 10 to product the mean value.
Mean values are applied to tolerance categories shown below to determine tolerance.
|T1 lower & upper
|T2 lower & upper
Deviation in mm declared by the manufacturer. Can be wider or closer than the other categories. Refer to product data sheet for quoted figure.
Range tolerance covers the overall difference within a given sample between the largest and smallest bricks. This can be called upon to resolve on site problems with size variation.
For the 10 bricks measured, the length, width and height measurements must not exceed the range as stated in the following table.
|RM||Range in mm declared by the manufacturer. Can be wider or closer than than the other categories, refer to product data sheet for quoted figure|
In case of size tolerance concerns on site, the following steps should be taken to assess whether the bricks in question conform to the manufacturers declared tolerance.
Sample 10 bricks randomly across several packs (a minimum of 6 packs is recommended) and remove any superfluous material.
For each measurement in question, lay the bricks out on a flat surface in contact with each other, as demonstrated in the below diagrams
Measure the overall dimension of the bricks using a retracting steel rule, then divide the figure by 10 to ascertain the mean value to the nearest whole mm. Compare the resulting figure against the manufacturer’s stated product tolerance.
All information provided in this document is correct at the time of publication and should be used for guidance only.
Revision: April 2019