The most typical and oldest kind of Norman house is the half-timbered house, recognisable at first glance by its wooden frame visible on the outside. Half-timbered houses are built around thick beams that form tiles, an architectural technique inherited from the Middle Ages. The tiles are filled with a cob made of clay, water, and hay; once dry, the walls are plastered with a lime made from limestone extracted in Normandy. In the countryside, the roofs are often thatched, but are increasingly covered slate and tile in the city.