When the white clapboard house in the tiny northern Colorado town of Carr went kaboom, the roof lifted off the top.

The chimney fell over. Windows blew out. Three people went to a hospital.

Rarely before has Colorado’s explosion of interest in marijuana been demonstrated so literally.

A boy injured in the explosion said two men in the house were attempting to make “oil that you use to smoke weed,” according to investigators. That makes the blast one of several to have occurred around the state in recent years by people manufacturing marijuana hash oil at home. The process often involves highly flammable chemicals, considerable risk and, as the Carr explosion shows, uncertain legality. Three people have been charged with felonies.

Fire officials across Colorado say they are concerned the state’s new laws on marijuana will lead to more incidents.

“We don’t encourage such a process,” said Patrick Love, a spokesman for the Poudre Fire Authority, which has handled at least two explosions or fires related to hash oil production. “It not only endangers the person who’s making it, but it could also endanger their neighbors.”

Even marijuana advocates say at-home production of hash oil should be approached cautiously — though they believe authorities’ fears of a house-explosion epidemic are overstated.

“You’re using potentially dangerous chemicals,” said Brian Vicente, one of the authors of Amendment 64, Colorado’s marijuana legalization law.

Hash oil is gloopy, concentrated marijuana — think of it like pot jelly — and its main appeal is its potency. Concoctions can be more than 75 percent THC, the psychoactive chemical in marijuana, and users describe single hits of hash oil like smoking an entire joint all at once.

To make it, do-it-yourselfers typically pack marijuana into a slender pipe, then blow compressed butane gas through the pipe. The danger comes from the resulting butane fumes that float around the room. The less ventilated the space, the more dangerous it is.

“If there’s any ignition source anywhere near it, like a pilot light, then you have problems,” said Bill Maron, an investigator with West Metro Fire Rescue.

Last month, the Colorado Information Analysis Center — part of the state homeland security office — put out a bulletin warning officials about the dangers of home hash oil production. The bulletin noted two cases, a February home explosion in Lakewood and a July incident in Colorado Springs. News reports show there have been at least five more spanning several years, including the explosion in Carr and one each in Steamboat Springs and Breckenridge. Two incidents have occurred in Fort Collins — including one, in 2009, in which a man died from burns.

Marijuana businesses in Colorado are allowed to manufacture hash oil, but with strict requirements — such as ventilation hoods and a “professional grade, closed-loop extraction system,” according to proposed new rules.

Though marijuana use and limited cultivation of marijuana is now legal for people over 21 in Colorado, there is considerable confusion about whether home hash oil production is legal, too.

In connection with the Carr explosion, three people have been charged with felonies under a state law that bans the production of marijuana concentrates. Vicente, though, said he believes Amendment 64’s protection for “processing” of marijuana plants covers home hash oil production. A separate law allows local governments to ban the use of flammable chemicals in at-home marijuana cultivation.

Vicente said courts will likely have to settle the debate.

John Ingold: 303-954-1068, jingold@denverpost.com or twitter.com/john_ingold