1/ definition of (variable gauge): The “gauge” is the visible part of a tile on the roof and is also the part which is directly in contact with the elements.
The gauge corresponds to the gap between the battens/laths measured from top edge to top edge.
The manufacturer usually indicates the theoretic gauge which enables you to work out the length of battens needed to complete the roof.

2/ to determine the average gauge: The roofer selects at random a minimum of 10 tiles from the delivery. He places them upside down on a flat surface interlocking them and making sure that they are spread to their maximum length measuring the distance between the 1st and the 10 th tile.
The tiles are then pushed (butted) together into a position of minimum spread and the distance measured between 1st and last tile. These two distances are then added together and divided by 10 to establish the average gauge.

P = L + « 1 »
          20

P = Pureau
L =maximum length
« 1 » =minimum length
20 = 10 maxi pureau + 10 mini gauge

Example : useable gauge
P = 360 + 345 , soit 35,25 cm              L = 360 cm
            20                                         « 1 » = 345 cm

This formula will establish the nominal batten gauge required. Always refer the manufactures brochure for technical guidance or contact the Imerys technical department.