DIY

Hi there you would need to get screws which are bigger than the insulation and the also need to add the thickness of the osb boards and a good length of thickness to the main roof under the insulation

The best solution to get a solid fixing is to purchase 200-300mm wood screws, identify each rafter inside and apply your measured sizes to the roof outside, this way the osb sheets are correctly installed to the existing frame work giving you a safe platform to work.

You can’t fix OSB boards directly into thick insulation — they need to be supported by a solid base. For a flat roof, you should either use long screws with large washers to reach through the insulation and anchor into the structural deck below, or install a mechanically fixed or bonded system designed for warm roofs. Most setups use insulation adhesive or insulation fixings that go right through to the joists or deck to hold both the insulation and OSB firmly in place. Never rely on just gluing the boards to insulation — it won’t be secure or meet building standards.

Not sure I follow this question . You really don't want to compress insulation it's air in it that gives the insulation u value. Compressing removes air and it becomes far less efficient. The other thing is you need rafters timbers to fix to