How to clean roof dryer vent?

Apparently whomever designed my house has never had to clean out a dryer vent. Dryer vent goes about 25' or 30' straight up and vents to the roof. The roof is too steep for me to access the vent - I almost slid off the roof when I attempted it. The laundry is in the middle of the house so I don't know if I can re-route it. Any thoughts/suggestions?

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