Hi @Latzi,
A warm welcome to the Bunnings Workshop community and thank you for your question.
Unfortunately, there is no way to screw panels onto a roof from the underside. This is because the screws used for this kind of installation, like these Suntuf Clearfix Roofing Screws, have a rubber washer that compresses against the top of the sheet to create a waterproof seal.
Is there just a frame currently? Or is there already sheet material on the shed?
If there is just a frame, you can often work from a ladder placed on the inside of the shed and work across to the last panel which can be screwed off by placing your ladder close on the outside of the side of the shed.
If there is already roofing material in place, you may need to remove some panels and follow the above process to reinstate them.
Alternatively, contacting a handyman to assist might be a good option.
Let me know if you have any further questions.
Jacob
