The roads in Fountaingrove and Oakmont, two of the wealthiest neighborhoods in Santa Rosa, are in nice shape, with most ranked by the city as either "good" or "very good."

So why is the city spending nearly $400,000 to resurface them this summer while streets plagued with potholes in other parts of town are crumbling like granola?

It's a question the city gets a lot, but the answer is pretty simple: It's easier and cheaper to keep good roads in good shape with a coat of sealant than it is to rebuild roads that are beyond repair.

"What you see is pavement preservation," said Clay Thistle, an associate civil engineer in the city's Transportation and Public Works Department. "It's like the preventative maintenance you would do on your car to avoid that big repair."

A contractor under the direction of city road crews began covering residential roads in Fountaingrove this week with a coat of black goop known as slurry seal, a mixture of oil, water and small rocks spread to a depth of about 3/16 of an inch.

The mixture dries in a just a few hours, leaving a road surface that looks as good as new, is more skid-resistant and, most importantly, protects the underlying road by sealing out water, Thistle explained.

It's a cost-effective way to prolong the life of roads and prevent them from becoming prematurely potholed or severely cracked, sometimes described as alligatored, Thistle said.

Once that happens, a road has to be either resurfaced with new asphalt or rebuilt completely, both of which are far more expensive than slurry sealing, he said.

Crews from Central Valley Engineering & Asphalt of Roseville started Monday at Lake Park Drive and will work their way up the parkway over seven days. Most of those streets are about 15years old. Then they'll switch to Oakmont, where the streets are about 20 years old, for three days starting July 31

The roads are closed to traffic during the work but reopen at the end of each day. The sealant takes up to a week to cure completely, depending on the weather, so drivers are asked to tread lightly on the fresh surface for the first few days.

The city has programs for repairing potholes and rebuilding streets that are too far gone to continue patching. But they're severely underfunded, according to Rick Moshier, the director of the city's Transportation and Public Works Department.

It would take an investment of about $15 million per year just to maintain city streets at their current average condition of "good," Moshier told the City Council in May. The city is now spending about a third of that amount. It budgeted $5.6 million last year and $5.5 million this year for road repairs.

That has resulted in city streets deteriorating rapidly. Brand new roads have a Pavement Condition Index of 100. Anything down to 70 is considered "very good." Fifty-one to 70 are considered "good," 26 to 50 is considered "poor," and zero to 25 is considered "very poor."

Last year, the condition of the average city street was a 66. This year, that figure dropped to 62. "We used to always try for 70," Moshier said.

At current levels of maintenance, the average road condition is expected to drop to 57 by 2018 and 51 by 2023. The longer the city keeps spending less than $6 million a year, the more it will cost the city in street repairs, Moshier told the council.

If the city waits until 2018 to increase funding for roads, it'll take $22 million per year for a decade just to return to present conditions. If it waits until 2023, that figure increases to $26 million per year. By 2032, if nothing changes, the streets on average will be considered "poor" and it will take $42 million per year for a decade just to get back to where the roads are today, Moshier said.

"Every year that goes by it gets a little worse," Moshier told the council. "The later you wait to start, the worse it is."

You can reach Staff Writer Kevin McCallum at 521-5207 or kevin.mccallum@pressdemocrat.com. On Twitter @citybeater.