Suspicious blazes in Adelaide's north

Two separate fires at unit blocks in Adelaide's north are considered suspicious.

Residents of a Salisbury Downs complex were forced to evacuate the building just before midnight on Tuesday after a mattress was set alight in a stairwell, police say.

And at about 5:30am on Wednesday, occupants of a block of units at Salisbury North had to flee a blaze that has caused at least $150,000 in damage.