On 28 Sep, 09:18, "N_Cook" wrote:
Or as tile and slate seems about the same weight per sq foot, what is the
amount of overlap for slates and for tiles?

I would have thought slate was lighter.
You don't say if the tiles are interlocking or not, or how big they
are or, the pitch (steepness) of the roof, or if you are in a very
exposed position. All these hae a bearing.
The tile manufacturers will tell you all their recommendations, it's
no use here anyone conjecturing.

For slates it depends also on whether they are top nailed or centre
nailed.
If centre nailed, the battens are the distance apart [centre to
centre] that the nail holes are from the top of the slate. (Assuming
they are pre-drilled)