The solid-on-solid model is a model of height functions, introduced to study the interface separating the + and − phase in the Ising model. The planar solidon-solid model thus corresponds to the three-dimensional Ising model. Delocalisation of this model at high temperature and at zero slope was first derived by Fröhlich and Spencer, in parallel to proving the Kosterlitz-Thouless phase transition. The first main result of this article consists of a simple, alternative proof for delocalisation of the solid-on-solid model. In fact, the argument is more general: it works on any planar graph-not just the square lattice-and implies that the interface delocalises at any slope rather than exclusively at the zero slope. The second main result is that the absolute value of the height function in this model satisfies the FKG lattice condition. This property is believed to be intimately linked to the (quantitative) understanding of delocalisation, given the recent successes in the context of the square ice and (more generally) the six-vertex model. The new FKG inequality is shown to hold true for both the solid-on-solid model as well as for the discrete Gaussian model, which implies immediately that the two notions of delocalisation, namely delocalisation in finite volume and delocalisation of shift-invariant Gibbs measures, coincide.