In article
Dave Page wrote:
snip
As it was, I'm not planning on putting too much weight up there, and was
considering only raising every other joist to save on timber.

That means the load on the loft floor would only be supported by half of
the joists, and the boards would be spanning a considerable gap -
neither of these is a good thing.

On the
other hand, the loft hatch is only 750mm wide, so if I want the boards
to be supported by three joists, I'll need to space the joists at
roughly 350mm anyway

The boards go across the joists, not along them.