Some ten years ago i lashed out on wooden fence. Although the upright
panes have weather well (something like a cedar wood i guess) the
horizontal rails (25 x 40 mm cross section ) are completely rotten and
breaking up.
I've bought a lot of treated roof battens 25cm x38cm to replace the
horizontal rails.
Since there are literally hundreds and hundreds of fixing each pane to
the rails and feel that staples are likely too easy to come adrift?

What is the best way of fixing them? small nails? if so, what kind
should i get, or small screws of some kind of weatherproof ones ?

Thanks for any advice on the most painless way of doing so very many
fixings.