PORTLAND — New census data shows a spike in the number of Oregon homes without complete plumbing facilities.

Oregon’s five most populous counties accounted for the entire 33 percent increase in the number of homes without plumbing from 2000 to 2009, the Oregonian reported.

States and local officials can’t explain the increase. Oregon law requires all residential construction to have indoor plumbing, and the census doesn’t count outhouses.

“I have absolutely not a clue,” Terry Swisher, Oregon’s chief plumbing inspector told the newspaper.

His boss, Chris Huntington, manager of policy and technical services for the state’s building-codes division, also can’t explain why such an increase occurred.

The data comes from the latest release of the American Community Survey. Complete plumbing facilities refers to hot and cold piped water, a flush toilet and a bathtub or shower.

The 2000 census found 7,025 of the state’s 1,452,709 homes or apartments were without plumbing. In 2009, however, 9,397 of 1,464,196 occupied housing units did not have plumbing a 33 percent increase, according to the American Community Survey.

While the 2000 census tries to count every American, the survey works from questionnaires filled out by about one-tenth of the population at any given time.

Multnomah County held onto its top ranking among the 36 counties, with 2,355 houses without plumbing in 2009, up from 2,252 in 2000.

Even the wealthiest census tracts were affected, including a tract just west of Tryon Creek State Park and south of Southwest Taylors Ferry Road. It went from zero homes without plumbing to 16, even as the median home value went up 35 percent to more than $421,000.

Clackamas, Deschutes, Marion and Washington counties also showed increased in housing units without complete plumbing.

The state’s 29 other counties showed significant decreases: Jefferson County, for example, dropped from 834 to 85; Baker County went from having 368 houses without plumbing to 65, and Wheeler went from 29 to two.