Soft spot on roof

[jdc0222]

The house is a 1960's ranch, fairly shallow pitch to the roof, with a "gable" facing the front. The gable pitches up in front, so there are 2 levels for the roof that have a seem, running towards the front corner of the house. The roof appears to be less than 10 years old.

Went out this weekend to clean out the gutters, and in the corner of the lower roof section where the seem channels water, I stepped down and felt the entire roof give in. I didn't step through, but it feels very mushy.

We have had a lot of rain in the past few weeks, so I'm sure it hasn't dried out yet. I haven't had a chance to look in the attic, but the bedroom under that corner of the house is dry. I usually clean out the gutters in the fall and spring, so at most this could be about a 6 month problem.

I realize a roofing company can do this a lot quicker than I can, so I intend to call in professionals for this project. Any suggestions on where to start diagnosing so I can have an intelligent conversation with a roofing contractor? Does it make sense to just fix that corner of the roof, or just replace the whole thing?

Thanks
Jon