Thanks for your replys.
I've got building inspectors in because we are getting planning permission for this to be an official annex to the house.
We are looking at stabilising the underneath of the structure and they want to see how the present arrangement is....boards will have to lifted in order to do this and as I'm doing it I want to treat them. Bear in mind these will be facing outside on to rafters and once they are done, they will not be re lifted!!! So want a long lasting treatment
Smell will not be a consideration as we will be re covering the top of the boards.
Hope this clears things up.