The pattern behind the plaster is meant to represent a slightly organised effort at opus craticium, a light, cheap method of building used in Herculaneum and Pompeii as people added additional floors and overhanging balconies. Pompeii had a lot of rebuilding to do after the previous big earthquake. Unlike the brickwork (at least for the first few floors) we can still see in the skyscrapers of an ancient city such as Ostia, opus craticium was fragile, not particularly fireproof, and not much of it would survive in the archaeological record if not for Vesuvius.
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