Wet shed - but how?

[Nicos]

Nice shed by the way!

Looking at BM's piccie of the back of the shed, it looks like heavy rain (coming from the left )hitting the roof would run down and fly off the edge like a skier and completely miss the guttering.
If the wind direction were such,( think of it coming from the left of the piccie) it might be blowing the shoot of rain back against the side of the shed.

Looks to me that you need to do the finishing touches with the roofing felt...ie cut to size, and nail down and sort out the guttering?

[Potstubsdustbins]

As above I would get the bits sorted out before doing anything else. The design of the shed should make it waterproof when rain/water drops form the sky if it is bouncing back up it can penetrate the joints.

[Nicos]

Hang on a sec!
Just been looking again at the first piccie.
What's the building next to the leaky side?
The one with the steep wriggly tin roof?
Has that got adequate guttering or could the rain have been blown off there directly onto the side of your shed?
That might be part of the problem?
Maybe another piccie?