GARAGE ROOF

[greengrass]

I have a 32ft long x 10ft wide garage that needs re-roofing.
Existing contruction: 5" x 2" rafters runing the 32ft length supported by 3 cross beams of 6" x 2" @ approx 11ft intervals set on piers. Boarded with chipboard [cheap builders] parts need replacing due to ingress of water before we moved in and the areas are now like wet cardboard. My proposal is to clear off the stones cross batten with 2" x 1" then lay on battens OSB boarding and top off with the new type two layer hot laid felt.
Q: Will the existing structure need extra cross support to cater for extra weight?. I've had one estimate £2,000 to strip and re-cover but my finances don't stretch that far.
I am capable having worked on building work, but now retired, but never got to calculations of weight ratio I left that to profesionals and followed their instructions.
Thaks for your time reading this and potential replies.