The owner of an office property on South Beretania Street in lower Makiki could face a stiff fine after a portion of a roof coating being applied to the building wound up in the gutter earlier this week.

Property owner Sanford Sato said a contractor's crew was applying the coating to the roof of the SJS Enterprises building at 1313 Beretania St. when unexpected rain showers fell on the coating before it had a chance to dry.

"The contractor told me it was water soluble, but once it dries, it's supposed to turn hard and not be affected by the rain," Sato said.

He said rain on Tuesday and Wednesday washed some of the roof coating into a system of gutters and downspouts that empty into drainage pipes under the sidewalk in front of the building. From there, the coating found its way into the gutter along the makai side of Beretania Street.

Martha Sanchez, who runs Mercado De La Raza next door, said she first noticed "a milky white substance" in the gutter in front of her shop Tuesday.

"Then on Wednesday, it was really spurting out from under the sidewalk into the gutter," Sanchez said. "The puddle went almost all the way down to Ke'eaumoku Street."

Concerned the contaminated runoff would empty into the ocean through storm drains, Sanchez called the city to report the problem. A crew showed up that evening to investigate, she said.

Marcus Owens, a spokesman for the city Department of Environmental Services, said inspectors from the department's Storm Water Quality Branch went back out Thursday morning to assess the situation.

He said the incident is being investigated as a possible illegal discharge of a chemical substance into the city's storm water collection system. Fines for such violations run as much as $25,000 per incident, Owens said.

Sato, meanwhile, said the contractor was planning to apply an additional layer of the roof coating but will wait until it is clear the weather will not pose a problem.