Vegetation is often present at varying heights, similar to the rungs of a ladder.
Under these conditions, flames from fuels burning at ground level, such as a thick layer of pine needles, can be carried to shrubs which can ignite still higher fuels like tree branches. Vegetation that allows a fire to move from lower growing plants to taller ones is referred to as ladder fuel. The ladder fuel problem can be corrected by providing a separation between the vegetation layers. Within the defensible space area, a vertical separation of three times the height of the lower fuel layer is recommended. This could be accomplished by removing the lower tree branches, reducing the height of the shrub, or both.