Even the most bare-bones renovations to a vacant house - something as simple as replacing a gaping doorway or a broken window - could have an impact on violence in Philadelphia neighborhoods, a study coauthored by University of Pennsylvania researchers has found.

With two years' worth of data from the Police Department and the Department of Licenses and Inspections, the researchers looked at the rates of violent and nuisance crimes around houses where property owners replaced doors and windows to comply with a city ordinance to combat blight.

They said that on average, within a year of the repairs, the area around those houses saw an estimated 19 percent reduction in assaults and a 39 percent reduction in gun assaults.

And the blocks around a single vacant house that gets those mandated repairs could see, on average, eight fewer assaults and 10 fewer gun assaults, and five fewer nuisance crimes in the year after the renovations, the study said. Nuisance crimes include infractions such as vandalism and disorderly conduct.

"This is some hopeful news," said John McDonald, chair of the criminology department at Penn and one of the study's coauthors. He said the study was one of the first to observe the effectiveness of a city ordinance on vacant and deteriorated properties.

"We haven't had studies that looked at what happens when a city passes an ordinance on abandoned housing that's practical," he said.

Between 2011 and 2013, L&I issued more than 2,000 violations under the Doors and Windows Ordinance - a statute passed in 2011 and aimed at the property owners who left empty houses to languish in disrepair on so many city blocks.

Under the ordinance, owners of vacant houses on blocks with an occupancy rate higher than 80 percent are required to maintain functional doors and windows instead of simply boarding them up. Otherwise, they could face fines from the city.

L&I Commissioner Carlton Williams said the city was concerned that houses that did not look "lived in" would deter prospective homeowners from certain blocks or persuade them to leave the city altogether.

"There were national studies done that showed that where there were vacant properties, people had difficulties obtaining insurance - that neighborhoods decayed and crime increased," he said.

From 2011 to 2013, 29 percent of the houses cited under the ordinance complied with the city's request for repairs, the study authors wrote. Crime rates around those houses were compared with crime rates near houses that did not comply with the ordinance.

Charles Branas of the Department of Biostatistics and Epidemiology at Penn's Perelman School of Medicine said his group of researchers had been studying methods to decrease violence and improve health in urban settings for years, focusing mostly on programs that work with individual residents.

"We found them to be successful," he said, but "sometimes minimally so. We wanted to think more about changing the environments that might be affecting poor health and violence."

It's unclear whether repairing a vacant house directly causes a drop in crime - that, Branas said, requires more research that the study authors hope to conduct. McDonald said the crime drops around those houses, though, are cause for "cautious optimism."

Not all crimes dropped around the houses observed in the study. Notably, drug sales and possession were not affected by a renovated house on the block. Branas said that because abandoned houses offer dealers and drug users a secluded place to sell or to get high, those activities may have been pushed to the street, prompting more calls to police.

Some areas of the city saw larger decreases in crime than others - Northwest Philadelphia and West Philadelphia had bigger drops than South Philadelphia and North Philadelphia, McDonald said.

That may be because more homeowners in Northwest and West Philadelphia complied with the ordinance, he said.

"If you think about it as a [medical] treatment, there's slightly greater dosage" in those areas, he said.

Williams said his department has identified abandoned houses as a priority issue, and are considering investing in more technology to better address the thousands of abandoned homes in the city.

He said he was pleased to hear of the results of the study.

"It reinforces what we've always believed, that strategic enforcement solves problems before they become worse," he said.