I have looked at the options for strengthening the joists in our loft
before boarding it and have decided that for our situation gluing and
screwing new 4"x2" over the existing is the way to go. My one one
concern is if the new wood shrinks (as I believe it is likely to) it
will cause the whole new joists to bend, I don't think it would have
to shrink very much for the change of shape to be quite significant.
Has anyone done this? Do I have some way of ensuring that the wood
that I buy is fully seasoned?

Thanks

Brendan.