The most probable explanation for this is the presence of spores of  Trentepohlia, a lichen forming Algae on the roof. This algae can be found on tree barks, roofs, damp soil & even lamp posts.

Lichen is not a single organism, but the result of a partnership  between a fungus and an algae or bacteria, which is scientifically termed Symbiosis.

This type of phenomenon has been recorded before, one of the most noted one being the Red Rain of Kerala, India in 2001.From July 25 to September 23, 2001, red rain sporadically fell on the south Indian state of Kerala. Heavy downpours occurred in which the rain was colored red, staining clothes pink.

The colour would have been caused by the release of spores rather than by the  growth of the algae at this instant. The photograph shows a ladder,  trees in the vicinity as well as some dry leaves all of which could have favoured the growth of this lichen.

Laurens, you could probably look for growth of this 'orangish' lichen at some place out there - happy hunting!!

The image of this lichen forming algae which I found on Wikipedia,         at the following link

http://en.wikipedia.org/wiki/File:Trentepohlia_sp._on_Cryptomeria_japonica_bark.JPG is posted opposite.