Rivers burst their banks and submerged villages in floods up to "neck-deep" while hillsides crashed onto homes and highways, officials said.
Some residents in vulnerable areas ignored evacuation warnings, Stephany Uy-Tan, mayor of the town of Catbalogan in Samar province, told DZMM radio.
"The rains were really strong and people thought the storm won't be too strong based on the news," she said.
Twelve people were killed after a landslide buried two vans on a mountainside highway in Catbalogan, she said.
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Jangmi, which was forecast to bring up to 15 millimetres of rain per hour, barrelled through fishing and tourist areas today, a day after it smashed into the mountainous southeastern region on Mindanao island.
Thousands were evacuated ahead of the storm's arrival, with most expected to be sent home later Tuesday as floodwaters start to recede, officials said.
Jangmi's 65-kilometre per hour wind gusts were weak compared to the last storm to traverse the central region earlier this month, Hagupit, which had winds of up to 210 kilometres per hour.
"We are focused on floods and landslides because, while the storm's winds are weak, it will bring heavy rain," national civil defence chief Alexander Pama told DZMM radio.
Eight people including three eight-year-old children drowned after raging floodwaters washed away their shanty homes in the coastal town of Ronda in Cebu province, regional civil defence officer Lemuel Tabada told AFP.
Two teenage boys, meanwhile, died from electrocution while wading through floodwaters at Loon in Bohol province, said Allen Cabaron from the same civil defence office.
Yesterday at least three people were killed after Jangmi, known locally as Seniang, slammed into southeastern provinces, triggering floods and landslides.
Ten flights to and from the affected areas today were cancelled.