We generalize the almost positive roots model for cluster algebras from finite type to a uniform finite/affine type model. We define a subset φc of the root system and a compatibility degree on φc, given by a formula that is new even in finite type. The clusters (maximal pairwise compatible sets of roots) define a complete fan Fanc(φ). Equivalently, every vector has a unique cluster expansion. We give a piecewise linear isomorphism from the subfan of Fanc (φ) induced by real roots to the g-vector fan of the associated cluster algebra. We show that φcis the set of denominator vectors of the associated acyclic cluster algebra and conjecture that the compatibility degree also describes denominator vectors for non-acyclic initial seeds. We extend results on exchangeability of roots to the affine case.
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