From the street, 6139 Irving St. looked like a time-worn house that needed some cosmetic fixing - a new porch roof and a good scrubbing - but nothing so serious as to merit a visit by the city Department of Licenses and Inspections.

That is until the department took to the air and used its new laser-based technology to discover that the house might have a collapsed roof.

Within two months, the house was demolished.

"From the front, it looks completely intact," Rebecca Swanson, planning director at L&I, said. "This is a property we would've never known about if we had not done all this work and . . . gotten an inspector back out."

L&I has developed a high-tech model to determine which properties might be vacant and imminently dangerous before an inspector sets foot on the property.

The model was developed by a GIS and data team of Office of Information Technology employees who have been assigned to L&I. The GIS and data team, led by Andrew Newell, has been testing out the model since August and fixing any glitches.

So far, several properties, such as 6139 Irving St., have been identified that might not have otherwise surfaced as problematic.

Officials estimate that there are about 13,000 vacant residential structures in the city. Many have been long abandoned by their owners, some of whom have died.

L&I officials believe that about 4,800 vacant homes are unsafe and 230 are imminently dangerous, a term for properties that are in such bad shape that usually the only way to render them safe is to knock them down.

The number of imminently dangerous is fluid. As one is demolished, another might deteriorate that wasn't previously on the list. L&I's $33.8 million budget provides nearly $10 million for the demolition of about 500 vacant properties this year.

The new computer mode, Swanson said, "allows us to be more proactive about it" and target the properties that are a real danger to the community.

Since August, Newell and his team have been running daily data sheets that score properties from from 1 to 49, with the highest number being the most dangerous.

The score is based on various indicators such as whether the property has had previous violations or the water bill has not been paid in more than four months. It also takes into account whether the property is an "urban single," a freestanding rowhouse that used to have adjacent properties.

"Those are most likely to collapse," Swanson said.

The GIS and data team also looks at whether the property is next to a school or bus stop or somewhere it could be a public safety threat. Then the team overlays mapped crime data that shows properties that tend to be hotspots for crime such as drug activity.

The crime data helps determine the order of demolitions. When properties are in equally bad shape, crime hotspots go first.

The city also has new light detection and ranging (LIDAR) technology, which allows inspectors and L&I staff to look at aerial shots of homes and determine which might have collapsing roofs. The system, using laser imaging, shows a color range to signal height disparities across a property's roof.

Properties normally show green or yellow, but orange blurbs indicate holes, Newell said.

"With some, the trees cover the entire roof, and you wouldn't normally be able to see with normal imagery," Newell said. "This goes through that and you are able to identify holes."

That's how the Irving Street property was found to be in danger.

L&I's computer model first flagged that the property had a series of unpaid water bills, and the most recent property assessment had noted that the exterior of the property was concerning.

Once it was flagged, GIS analysts used the LIDAR system to look at the roof and found that it had a big, gaping hole.

"We didn't even know it was vacant until this model said we should go out there and look," Swanson said.

The cost to the city to purchase and use the technology was $467,000, L&I spokeswoman Karen Guss said.

Property records show that John D. Veal and his wife, Mamie M. Veal, purchased the house in 1954. After the couple died in the 1990s, the ownership of the house was transferred to their son Larry K. Veal.

Veal, who records show is still the owner of the property, could not be reached for comment.

When the city demolishes a private property, the owner is billed. If the debt is not paid, a lien is attached to the property.

A typical rowhouse demolition costs between $15,000 and $17,000 each, Swanson said.

The city is looking to create a model similar to the residential structural assessment for commercial properties. Officials estimate that there are about 1,000 large, commercial vacant buildings throughout the city that also need to be monitored.

"It is such a high-stakes issue, even though the whole thing is pretty under the radar. It's a real public safety thing," L&I's Guss said. "Every day that goes by where nothing falls, we feel like that's our job - to then get to the next day when nothing falls."

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