Stains down an outside wall of a house are usually a sign that a gutter is leaking – either because debris has accumulated in it causing a blockage or because the putty in the joint between two lengths of guttering has deteriorated. This is a common fault with cast-iron gutters.
If faulty putty is the cause, repair the gutter by unscrewing the nut and bolt at the joint and raising the top gutter section enough for the old putty to be scraped away. Spread a layer of metal casement putty, about 6mm thick, within the joint. Bolt the two sections together again and. After cleaning off the excess putty squeezed from the joint, apply a coat of bitumous paint to both the inside and outside of the joint.