The ice chunks that landed inside a Calgary home came from an airplane lavatory, said transportation officials. ((CBC))

Chunks of ice that smashed through a Calgary woman's roof came from the lavatory of an airplane flying overhead, transportation investigators said Thursday.

Marian Liknes was sitting and talking on the phone in the bedroom of her southeast home Thursday morning when she heard what she thought was an explosion behind her, said her husband, Wade.

Firefighters found two to three 15-centimetre chunks of ice on a bed inside the home, along with pieces of shingles, plywood, drywall and insulation.

The falling ice broke a hole about 30 centimetres wide in the roof.

John Lee, a manager with the Transportation Safety Board, confirmed the ice — made up of toilet water, antifreeze, disinfectant and human waste —  came from an airplane bathroom.

He told CBC News planes can't evacuate the contents of a tank while in flight, but flying into warmer air can trigger some melting, which can cause a holding tank to malfunction.

Lee said the agency will track down the aircraft to notify its operator that its toilet system needs to be fixed.

Incidents like this are so rare that Transport Canada does not keep statistics on them.

Wade Liknes said he's not too worried about his roof, just glad that his wife wasn't hurt.  

"That's life. This is a one-in-a-million chance of something like this happening," he said. "I've heard of it in other places, but this isn't something that's going to happen often."