Trees planted in lawn areas often develop surface roots because of the shallow watering that the lawns are given. Some trees are more prone to this than others and Ficus is notorious for having surface roots. This is one reason why planting trees in lawn areas are often discouraged - they have distinctly different watering needs. Trees need water to soak down to a depth of three feet while grass only needs moisture to a depth of 6 - 8 inches.

If you cut roots you run the risk of severing important arteries that are supplying water and nutrients to your tree. If you try to bring in fill dirt you will be suffocating the roots of your tree. A term in the tree care industry is "fill kills" because roots deprived of oxygen will eventually die. Root death will affect the health of your tree.

You could try to increase the height of you mower blades so they will not hit the roots.