Dome roofs are usually supported from the shell by framing consisting of rafters and girders pressed to the roof radius, Cone roofs are usually supported from the shell by rafters or trusses, Large cone roof tanks can also be built with columns and girders to support the rafters and roof plates, Due to environmental concerns and to reduce vapour losses there is tendency to install more and more internal fixed decks in fixed roof tanks either under construction or as a retrofit. This combines the advantage of both fixed and fixed roof design.