A two-year-old girl and a 42-year-old woman were fighting for their lives in hospital after the accident, at an improvised venue used by an evangelical church popular with Haitian immigrants.
Emergency services said another 28 people had been less seriously injured when the first floor of the recently built building collapsed.
Christian Lambert, governor of the Seine-Saint-Denis region, north of Paris, said the toll could have been much higher if anyone had been on the ground floor. Officials had launched an inquiry into safety standards there in the building.
Stains mayor Michel Beaumale told AFP the structure was in good condition but "not made for such an event".
The worshippers plunged 2.50 metres (8.2 feet) to the ground while dancing rhythmically during the service.
"I heard a scream, I turned around and saw nothing but dust," said a man who introduced himself as Father Patrick. "I saw the floor had collapsed, it was awful."
The house was rented by the Church which has no official ties with established evangelical churches.
France's National Evangelical Council (CNEF) said they had not allowed the group to join as its venue was "not at all appropriate for more than 100 people" and did not meet safety standards.
"We are like a big happy family which meets every Sunday," said Father Patrick, one of the few people who would be interviewed after the disaster, as clergymen and psychological counsellors cared for survivors.
Lambert said between 100 and 150 people were celebrating on the first floor. "People came down with the floor, through "a hole".
Neighbours said the group had been meeting there for more than a year but according to Father Patrick the so-called Armed Fighters for Christ New Jerusalem had used the house for four years.
An official for a neighbouring evangelical community meanwhile said the house which has a cross on its front was "basically a ballroom".
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