Welland's Fire Chief spending the day trying to pinpoint the cause of a serious apartment building blaze over the weekend.

Flames took over a second floor apartment of the building on Riverside drive just after 8am Sunday.

Denys Prevost says they spent quite some time looking for the person who lived in the apartment with her two kids before discovering she wasn't home when the fire started.

Prevost says the damage is quite substantial so a timeline as to when occupants will be able to return home is unknown at this point.

He says some of the residents in other areas of the building may be able to go into their apartments later Monday once they determine if the building is structurally safe.