In most parts of W Africa, trees for long spanning beams are sparse, so roofs are supported in other ways. Buttresses, large posts (as in the drawing), and adobe reinforced with bound saplings or trees are all means by which roofs are supported in the dryer areas of western Africa.  Where trees are available along the coast and in Guinea, thatch roofs are made from palm fronds. The thatch is woven and stacked on itself to form layers, then connected to an inner “shell” sometimes made of bamboo, which in turn is supported by columns and a central pillar. One such mosque in Kébaly, Guinea has a thatch roof that starts 1.5 feet above the ground, and reaches 40 feet into the air.