Roof gaugeing?

ok i need to know how to “gauge” a roof.
i’ve done roofing before but when fixing batons i only got told how far apart to put the batons not how to “gauge”.
any help or advice would be great
pompey roofer

I assume you are putting down tile or slate?
If so then take the length of the tile, say 16", subtract four inches for the “waterline” this leaves 12" halve that, 6". So 6" is the distance BETWEEN the battens.
When laying the battens. instead of using a tape and snapping lines like a shingler does, cut a 6" block and use it as a template. It is a quicker method and ensures all your battens are equidistant.
(my brothers a Southsea boy)

Another way, if it is for a tile roof, is to mark a tape measure at the increment of optimal exposure with a marker at every xx" mark.

Go from the eave to the ridge with it. Now pivot the tape measure at the ridge line until the marked spot on the tape is in alignment with the location of the very top batten position.

Now, without moving the marked tape from THAT position, make your marks on the felt at every marked spot on the tape measure from the eave to the ridge.

Repeat on the opposite side of the roof.

Now, snap your chalk lines.

Every course will be exactly even.

You have to take in consideration how much tile overhang you will have at the eave edge and adjust from that point, rather than the actual eave edge though, because your first batten wil be a shorter distance from the eave to the batten that the rest of the measurements between battens.