My house is under construction now, how could I avoid seeing these black marks on the roof around the roof exits because my roof is very light in color?
It all depends on what you call "roof exits".
If it is the chimney stub, it must extend at least 40 cm above the ridge of the roof (Arrêté 22.10.1969, art 18), which normally prevents the fallout on the roof. As for its location, it must be located outside the pressure zone created by the prevailing winds in contact with the slope of a roof (in clear opposite the prevailing winds). Thus, the fumes will naturally be "sucked up" and directly evacuated which avoids the phenomenon of blackening around roof exits.
In the case of ventilation outlets including VMC, make sure that a range hood is not connected...
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