Im planning on training 2 grapevines to grow ontop a pergola but im a little confused as to far they'll span (shoots) without dropping so that i can space my rafters accordingly. Plus, do i need slats ontop the rafters? If yes then again, how far should they be spaced? And should the vines run parallel or perpendicular to the rafters?
For the last question: the usual method is one year parallel, and one year perpendicular (because it is the direction it will take naturally: new branches perpendicular to previous branches).
About the distances: they are vines, they can reach few meters distance without problem (but with risk some go down, and so you need to move them up). For real distances, you need to estimate static load. Calculate few kilograms per square meter on center (where you will have most o grapes), and estimate winds. I would have some kind of rafters, or lines every 50 to 100 cm. On my place, we usually prefer larger (and more distant) rafters with some wires between them (but I think mostly because of snow). You can use thinner rafters, but do not forget that plants (any plant) will weight much (and grape vine produce grapes: very watery, so also heavy).
I've trained a vine (cultivar - Phoenix) up a pergola in my garden, so I can tell you what works for me. There's a permanent "T" shaped framework consisting of a single main stem going up one vertical face of the pergola, which divides into two horizontal arms perpendicular to the roof batons and tied in at the top. The roof batons are 450mm apart. Each year I let one new shoot grow along each baton, tying them in as necessary and removing all side shoots. These are the fruiting canes. Each winter I prune these back to a couple of buds of the horizontal "T".