[shoshone (OP)]

Hi there, I'm starting with a brief introduction.
Geology is not my area of expertise, and to be honest I'm not even sure this is related to the board, but I'm taking a shot anyway. Apologize if this is out of topic.

Field is architectural, for a university course of building restoration and consolidation: part of the course is about Lithology and Geology.
I'm supposed to make a survey of the facades of my building, and map all the damages due to weathering, etc.

I've encountered something peculiar on one of the facades, and I haven't seen anything similar on books and manuals. Can anyone help me identify this kind of damage?
It's the 'tiger stripes' on the plaster between the two columns on the left. Just under it there is an irrigation ditch.

The building is a mill built in masonry (brick units) and plaster. Location is the Po Valley in the north of Italy.

Thanks!

[RD]

Ventifacts on stone can have a sand-dune-like parallel ridges ...


https://commons.wikimedia.org/wiki/File:Ventifact_at_Ventifact_Ridge_in_Death_Valley_NP%283%29.JPG

[shoshone (OP)]

Thanks! I will inquiry on that!

[Bass]

Are the stripes raised? or just discolored?

[shoshone (OP)]

They are just colored!

[chintan]

Some kinda fungus or algae from plants and the atmosphere also looks humid to so have a piece of it and take it to chemist or biologist

[RD]

... Some kinda fungus or algae...

We've had stripey-fungus on this forum previously , see ...
http://www.thenakedscientists.com/forum/index.php?topic=49171.0;nowap

[puppypower]

Some kinda fungus or algae from plants and the atmosphere also looks humid to so have a peace of it and take it to chemist or biologist

Stucco and other forms of plaster contain lime; CaO, as one of the main ingredients. Lime, is often used on lawns to lower the pH of the soil, since it is alkaline or has high pH. If the plaster is not mixed properly, such as not enough water or too much lime, then the finished stucco will be hot; high pH. The unreacted lime will also leach out over time, after the surface gets wet and dries during rain cycles. The answer could be one bad batch of plaster under the overhang.

Another possibility is the White House in the USA is white because the siding was lime washed. In this process, they will mix a slurry of water and lime, and this will be painted onto the surface to give a white color. If that was the case, under the overhang will not dry the same way as outside the overhang. The striping could also be an artifact of how it was applied, since the lime wash is thick. In painting terms he did not keep a wet edge, but had double coat overlap. Often the apprentice plasterer would be given a small job like, with the larger expanses left for the experts. 

[shoshone (OP)]

Thanks everyone for your inputs.
Will check with teacher at the end of the week and let you know what she thinks!