Is it leaking mainly along the overlap joint.?
This was my case for years on my shabby old Big6 profile cement/asbestos? roof. Huge amounts of rain dripping into the garage along the overlap joint at the mid span across the roof. Drops forming on the uphill edge of the under lying sheet. The uphill sheets largely dry (except for condensation). How is the water running uphill.???
Aaaaahh, my roof has sagged at the overlap enough to let rain run back up under the overlap.
Cut the J bolts holding the roof to the midway support beam and hammer timber batten(s) shims (25mm in my case) between beam and roof sheet to restore the slope. Also lift the top lying sheet at the overlap and squirt plenty mastic/sealant into the valleys and let the weight of the top sheet squish it into place. This cured all the drips except in 2 place where the underneath sheet was also cracked at the overlap but secondary gunk/sealant cured that.
It took me years to figure it out and ££££s wasted on useless gunk/paint/sealants.
