Also unlike Telford and other road builders, McAdam laid his roads as level as possible. His road required only a rise of three inches from the edges to the center. Cambering and elevation of the road above the water table enabled rainwater to run off into ditches on either side. Size of stones was central to the McAdam's road building theory. The lower road thickness was restricted to stones no larger than . The upper layer of stones was limited to size and stones were checked by supervisors who carried scales. A workman could check the stone size himself by seeing if the stone would fit into his mouth. The importance of the 20 mm stone size was that the stones needed to be much smaller than the 100 mm width of the iron carriage tyres that traveled on the road. Macadam roads were being built widely in the United States and Australia in the 1820s and in Europe in the 1830s and 1840s.