We propose a method for determining the flavor charge lattice of the continuous flavor symmetry of rank-1 4d N = 2 superconformal field theories (SCFTs) and IR free gauge theories from topological invariants of the special Kähler structure of the mass-deformed Coulomb branches (CBs) of the theories. The method is based on the middle homology of the total space of the elliptic fibration over the CB, and is a generalization of the F-theory string web description of flavor charge lattices. The resulting lattices, which we call "string web lattices", contain not only information about the flavor symmetry of the SCFT but also additional information encoded in the lattice metric derived from the middle homology intersection form. This additional information clearly reflects the low energy electric and magnetic charges of BPS states on the CB, but there are other properties of the string web lattice metric which we have not been able to understand in terms of properties of the BPS spectrum. We compute the string web lattices of all rank-1 SCFTs and IR free gauge theories. We find agreement with results obtained by other methods, and find in a few cases that the string web lattice gives additional information on the flavor symmetry. 7 Generalization to higher ranks 63 A Root systems and their lattices 65 B Curves and 1-forms of IR free U(1) gauge theories 67 B.1 Maximally deformed I n singularity. 68 B.2 Submaximal deformations of I n singularities 73 C Curves and 1-forms of IR free SU(2) gauge theories 75-1 -3 We use Dynkin notation for the simple Lie algebras, where A n = SU(n + 1), B n = SO(2n + 1), C n = Sp(2n), and D n = SO(2n), as well as the exceptional E 6,7,8 , F 4 and G 2 algebras. Also, we use a notation where U 1 = U(1). Finally, it is useful to keep in mind the following low-rank algebra equivalences: