Hi Peter not a clue when it was built! I found a newspaper beneath one of the board from the 1990s so I guess it's at least as old as that ? it is about 3m X 2m in size.
The joists are 350 centres, and we only store a tent and some camping chairs up there. From what I can see there are 2 of the joists been cut where the opening was made. The two joists that have been cut have then had some kind of reinforcement put in, in the form of a few pieces of 6x2 connecting the cut joists and then also attached to the casing for the opening
Hope that makes sense.
Danny