I dont think it is a must. Garage floors dont have another layer, so I guess alfresco areas dont need them either. That being said, it does provide additional protection for the slab which Im guessing could crumble slightly on the edges if there is nothing over the top. We are having stencil concrete on the top of ours. (For looks and extra protection on the alresco), but I understand the need to save $$$. We will be tiling over the top of ours later when funds are more plentiful.