Fitting vertical hanging tiles

[Ambergreen45]

Hi,

The roof is being replaced on the blocks of flats where we live. In addition all vertical tiles on the sides of the buildings are also being replaced. We noticed each concrete vertical tile has two holes but are being nailed once rather than twice onto each batten, apart the ones at the ends which are nailed twice. Should any vertical tile be nailed only once? I've seen it done on a building a few years ago, so is this normal practice?

Thank you.

 

[Ambergreen45]

I forgot to mention, they are plain tiles.

 

[palaceray]

Vertical plain tiles should have 2 nails in each, but probably 75% of the time they have only one nail in each and are fine.