Underlay is generally held in place by the tile battens but where nailing is required nails should be extra large head felt nails of 3 35 mm shank diameter and 20 mm long.
How to felt and batten a hip roof.
A slight sag between rafters is ideal as it will allow any water to drain down the felt.
Work along the roof taking out any excess slack in the felt but do not pull it tight.
When the first length is complete lay the next layer of felt on top of the first so that it overlaps by at least 100mm 4 inches horizontal overlaps where one roll ends and the next one is started should be at least 150mm 6 inches.