  To a homology theory one can associate an additive site and a new homological
functor with values in the category of additive sheaves on that site. If this
category of sheaves can be shown to be equivalent to a category of comodules of
a Hopf algebroid, then we obtain a new homology theory by composing with the
underlying module functor. This new homology theory is always flat and we call
it a flat replacement of the original theory. For example, Pstr\k{a}gowski has
shown that complex cobordism is a flat replacement of singular homology. In
this article we study the basic properties of the sites associated to homology
theories and we prove an existence theorem for flat replacements.
