Repairs: walls

Staining from acid rain or from bird droppings can cause erosion as well as staining. Limestones can be particularly badly affected in this way.

Solution

Washing down and fitting protective measures such as bird spikes/wires may be the only solution, though you might consider some extra flashing material to protect moulding details from guano or rainwater run-off where feasible.

Vegetation and lichens can also cause deterioration to some stone materials. Insecticides might help, but use of chemical treatments would need to be carefully considered and specialist advice taken, especially if the building is historic.