Do i need a new roof?

When I was up in the attic recently to get something I noticed some wooden sheathing looked dark. On touching it it felt damp. I subsequently felt many other damp parts on other parts of the wooden sheathing. I might add I have had no major leaks meaning no wet patches on the ceilings. I have been told it is welsh slate on roof. THe house was built in the 1950s. I had a roofer out and he felt it was a ventilation problem and suggested putting 6 skate vents on the side of the roof where most wet timbers are. There is insulation in the loft and no visible signs of vents in eaves or anywhere else. I had another roofer out and he thinks it's a new roof I need ie the slates are breaking down. This roofer then suggested I could buy some time by adding vents 3 each side would help dry the wood and give ventilation. But on the whole he thinks roof is needing replaced. The house is an ex council property, on no other houses in street have been re-roofed. I did ask one of my neighbors if they had noticed this problem in their roof and they hadn't. I don't know what to do.