On Thursday, 6 March 2014 13:28:34 UTC, Martin Bonner wrote:

Why? Rafters are expensive. Don't make them bigger than you need
(unless you want them big for visual appearance).

The intention was to have them covered so it was more to accomodate the necessary level of insulation to satisfy U-value requirements and it being between-the-rafters as I figured that would make things easier. Whilst I shall be getting a builder to build the shell I will be doing the inside, including insulation (or at least boarding once I've sorted the wiring), myself over a longer period.

Much better to have
a solid layer of insulation above them.

Ah, I hadn't considered that an option. Will go and find out more.

Mathew