I have obviously misunderstood the original problem! When the OP says he wants to "clad the underside of a pitched ... roof", I understand that to mean the two rectangular areas that make up the two halves of the roof. So I don't understand these comments about the boards "...getting shorter as they go up". Are we actually talking about the underside of the roof or the 'gable ends'?

Working on my original premise, if the roof centre height is around 1.7m then the slant height - ie the length of each of the two halves of the roof - is 2.27m. This gives a total area of 3m x 4.54m or 13.6mÂ². As T&G measurements are normally stated as the fitted width (ie 150mm T&G means the pitch between adjoining slats is 150mm) then for 150mm T&G you would need 13.6 / 0.15 = 91 lengths or for 120mm T&G it's 13.6 / 0/12 = 114 lengths - a 'length' being 3m, of course.

Naturally, I have been talking about a gabled roof. If the OP actually meant a tented or pavilion roof, where the centre goes to a point with four triangular sections making up the roof, then that's a different story! Full information helps!