The department said 105-degree heat caused the compost to burst into flame at a residence on the 17300 block of Marlin Place at 3:05 p.m. Wednesday.
An ember from the fire floated at least 200 feet away and ignited a wood-shingle roof on a separate residence. That fire spread to two more houses, causing damage totaling $76,000.
More than 36 firefighters extinguished the fires.
No injuries were reported.
City News Service contributed to this report.