I have an existing felt roof that I want to overboard with OSB3 and apply GRP roofing materials over the top, can this be done?

While this is possible we really don't recommend it and doing so will void our Basecare Guarantee.

The main problem with doing something like this is you don't know what the condition of the decking is like underneath that felt and you could just be papering over the cracks so to speak.

If you are confident that the foundations of the roof are strong, then we would recommend you use a product like Fibrecoat, which can be applied directly over the existing felt surface without the need for boarding.