Hi, i wouldn't lay your boards directly on to the rockwool, take the 2nd rock wool out and relay all insulation between joists, then fit a 35mm x 45mm batten to the top of the existing ceiling joists to take the loft boarding, make sure you pre drill the timber and use 2 1/2 inch, 8 screws. you don't need many screws into the battens, then lay the boards , t&g loft boards would be best for your project, b&q sell them and they are about 450mm wide by 1200mm long so you can get through a loft hatch, make sure you put a few screws in the boards so they don't move. i recommend screws as this wont disturb the plastered ceiling below. Thanks Brett Rhodes

I cant see how you are going to board over a double layer of insulation, I take it one layer is laid inbetween the ceiling joists and the 2nd layer over.
Theres going to be some distance between the joist and top layer, and you cant compress it to fix boards to joists.
Its going to be costly to put in extra joist just for the storage of a few xmas decorations, if that is all you have then just loose lay a couple of loft boards on top, you wont get 8x4 sheets of ply through your hatchway.
If it was forseen, you could have used some rigid insulation, that would have been ok for light storage.

Yeah I agree with Rhodes joinery and tm property as far as using battening, as compressing Rockwool insulation removes the air from it and therefore reduces its effectiveness.

I would recommend using 3" x 2" CLS as it’s quite suitable in strength, size and gives you that bit of extra depth for the insulation it’s also about a pound a meter at the moment in Wickes. In contrast though I would recomend laying your batons at 90 degrees to your original ceiling joists this has 2 advantages it allows you to get the joist spacing’s right for the boards you decide to use, so that the boards start and finish on a baton which saves you cutting boards unnecessarily and secondly it allows you to lay one layer of insulation within your original joists and the second at 90 degrees to that within the battening giving overlap of the insulation and total coverage. Standard practice with 18 mm t & g floorboards is a joist every 400mm and I would recommend at least three 4 or 5 by 80 to 90 mm screws through the batten into your old ceiling joists where they cross for every 2.4 meter length.

Kind Regards
Tom
Di4y Ltd

rhodes joinery is right, 1 extra reason people forget is that in most roof spaces/lofts the electric cables run directly over the joists preventing you from direct boarding,regards,Terry

hi sir roger hill here sorry for not contacting you sooner iv had bother with my laptop,if you are just planning on basic storage and you have a few supporting walls you should be fine but if you plan to make it into a room you would be better getting a structural report done.
thanks
roger