We tackle several problems related to a finite irreducible crystallographic root system $Φ$ in the real vector space $\mathbb E$. In particular, we study the combinatorial structure of the subsets of $Φ$ cut by affine subspaces of $\mathbb E$ and their projections. As byproducts, we obtain easy algebraic combinatorial proofs of refinements of Oshima's Lemma and of a result by Kostant, a partial result towards the resolution of a problem by Hopkins and Postnikov, and new enumerative results on root systems.