The gap between tenancies is the most under-used opportunity in a landlord's calendar. For roofing especially, an empty property is worth its weight — and a small amount of planned work at the void prevents the expensive, disruptive emergencies that happen when a roof fails mid-tenancy.
Keeping the structure and exterior — including the roof, gutters and external pipes — in repair is a continuing obligation; planned void-period maintenance is how prudent landlords meet it.— Landlord and Tenant Act 1985 s.11 & Renting Homes (Wales) Act 2016 fitness duty
With no tenant in place you get full scaffold access, no access-notice choreography, no disruption to anyone's home, and the freedom to do the job properly rather than patch around occupation. And the property starts its next occupation contract watertight, which is exactly where the Welsh fitness duty wants it.