Building Degradation

Condition Formula

$$ Condition = \frac{0.67}{1+e^{(1789/25000) * (DaysSinceLastRepair - C)}} + 0.33 $$

$C$ is either $100.87$ or $107.87$.

There is speculation that the additional 7 days for $C$ is a bug as it happens inconsistently.

Repair cost is determined by:

$$ RepairCost = BuildingCost - ReclaimableCost $$

ReclaimableCost is determined by:

$$ ReclaimableCost = floor(BuildingCost * \frac{180 - min(DaysSinceLastRepair, 180)}{180}) $$

All of the above is applied on a per material basis.
If your building is affected by the 7 day bug, subtract 7 from DaysSinceLastRepair. If DaysSinceLastRepair goes negative, set it to $0$.

For a great walkthrough of building degradation mechanics, see this Google Sheet by MoonSugarTravels