It sounds as through the sagging has been caused by moisture getting to the chipboard, but if it has been covered with felt properly we would expect this not to happen after only 3 years. The options for covering the roof are felt, rubber, tin or even shingles - up to you and your budget.
There's nothing that stipulates the 'unsupported dimension' for a shed/summer house - If it's sagging, I suspect you need more support?
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