This roof deck is only a few years old. It is home to a happy new population of black mold thanks to four factors.

There is no ventilation path in the eaves of this roof. True, the roof has a few box vents up high. But roofs need cross ventilation or bad things will happen.

This bath vent hole is completely unsealed from the bathroom below. Huge amounts of moist, hot air pour out of this hole into the attic.

This is a very cheap, uninsulated and poorly sealed vent pipe. It allows moisture and warmth to escape into the attic all winter long.

The floor system in this attic has very little insulation, which allows heat to escape from the living space below.