flat garage roof

[greengrass]

Garage roof 32x10 feet, flat, covering is pitch impregnated hessian hot laid on chipboard with grano chip finish. It's past its sell by date leaks like a seive.

Repair procedure: Strip stones off melt on hot lay felt as it goes on spread stones back on topping up stones as necessary. Can anyone see any fault in doing this.
Thanks in anticipation.