How bad is the rot?
If the wood is literally falling apart then replace.
If its only got some soft spots I'd probably just remove the soft bits with a wire brush (chisel?), treat the rest with Metalex, fill any holes and then repaint. Especially if the roof has been replaced.
https://www.bunnings.co.nz/metalex-500ml-green-concentrated-timber-preservative_p0570965
