In the first instance one should consult a structural engineer. This will ensure a certified answer to what can and can’t be constructed, it will also indemnify you against any future issues if anything should go wrong with the roof or indeed the deck structure.
In short, to keep the point loading of the decking structure to an absolute minimum and to reduce the impact of the structure and how it interfaces with the roof materials you should use pedestals to spread the load.
Pedestals every 500mm and joists every 400mmÂ centreÂ will suffice for most applications, this way every square metre of decking including the structure will have the dead load divided my 6.5, thus a 45kg/sq/m load will be adequately Â spread.
Be aware that should there be any issues with the roof one must remove the decking or at least part of it to gain access to the roof. Also consider creating access panels for maintenance near water outlets.
look forward to your thoughts…