garage roofing?

Brick built garage, Flat roof fall on the 32ft run, £1,200 roofers estimate to overlay existing chipboard roof which is topped with hessian based felt and hot lay mastic leaking like a sieve and £2,000 to remove it.

So can't afford either price so am going to OSB it over existing after removing the pea shingle. seal joints and use Wickes traditional Underlay felt being cheaper than the green and will be recovered in shingle. Any reason why I shouldn't just use the underlay felt.

If so will have to temp cover and save for the green stuff. rafters 5"x 2" with 3 support beams spaced every 10ft size 6" x 2.5".
Many thanks