Sizes and Material for Joist, Beams and Rafter for new Roof

[KVT]

Hi can anyone please advise me size, material and design of Beams/ rafter /joist for following slanting roof with high load because of sand filling.
We are doing for a sound proof room.

Measurement :
Length 6.6 M
Width 3,6 M
Height 3 M at one end and 2,6 meter at another end

Roofing layers
1) Bitumen or fibreglass waterproof membrane.
2) Marine ply or OSB 12-15mm
3) 50x100mm or greater timbers as builder decides is needed for the span.
4) 25x25mm battens
5) 12mm OSB
6) 50mm approx kiln dried sand
7) 25-50mm RW3 or RW 45 rockwool (60KG/M2)
8) 15mm plasterboard
9) 12mm OSB

Thank you in advance