roof lantern fitting to grp roof

Before, as the roof lantern should sit on top of the roof covering, as with all roofs you start from the bottom and work your way to the top, allowing the rain water to fall to the next step down.

Ideally before, although most roof lanterns now come with a capping that allows for the grp fibreglass to come to the top with out going over then the capping sits on top and overhangs the grp fibreglass. Done a few and never had any problems as the capping acts as an umbrella.