Summary: In this paper we develop the mathematics required in order to provide a description of the observables for quantum fields on low-regularity spacetimes. In particular we consider the case of a massless scalar field \(\phi\) on a globally hyperbolic spacetime \(M\) with \(C^{1,1}\) metric \(g\). This first entails showing that the (classical) Cauchy problem for the wave equation is well-posed for initial data and sources in Sobolev spaces and then constructing low-regularity advanced and retarded Green operators as maps between suitable function spaces. In specifying the relevant function spaces we need to control the norms of both \(\phi\) and \(\square_g \phi\) in order to ensure that \(\square_g\circ G^\pm\) and \(G^\pm \circ \square_g\) are the identity maps on those spaces. The causal propagator \(G = G^+ - G^-\) is then used to define a symplectic form \(\omega\) on a normed space \(V(M)\) which is shown to be isomorphic to \(\mathrm{ker}(\square_g)\). This enables one to provide a locally covariant description of the quantum fields in terms of the elements of quasi-local \(C^*\)-algebras.