I have both replaced the entire rubber roof and plywood below, and used the liquid EPDM to re-coat a roof.
On the replacement I had damaged wood that was soft, so had no other choice but replace it. I think it took be about 40 hours, and it was a 21 ft trailer. I do not remember the cost, as the rubber was under warranty and Manufacture provided that and the cement. I did have to pay for wood, and any new roof accessories.
The liquid EPDM was on a 28 foot trailer and it cost about $400. Certainly a lot easier if roof deck is solid and rubber is in reasonable condition .
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