Pigeons can carry fleas, ticks, mites and disease. Their droppings can dirty your home, porch, patio and even remove the paint from your car. So if you see them setting up house on your roof, or on homes nearby, you are wise to do all that you can to keep them at bay.
Install plastic or metal spikes on ledges, window sills, roof peaks, or other flat or curved surfaces that pigeons might land or nest on. These spikes are easy to install, and barely noticeable, but make it nearly impossible for pigeons to land where spikes are present.
Consider a chemical bird repellent if pigeons are already a nuisance on your home or in your yard. Some repellents are made from Methyl Anthranilate, which is applied with a fogger. It will not physically harm the birds, but causes a very unpleasant reaction. After a few applications, the birds will come to associate your home with this reaction and steer clear.
Place chicken wire around areas such as small attic windows where pigeons are prone to perch. You may also consider boarding up small cubby-type areas around your home to detract them.
Adopt a scarecrow. While the word "scarecrow" might conjure up thoughts of floppy hats, straw and stuffed pantlegs, when it comes to pigeons you will be better suited with an owl scarecrow. If you can find one that moves (like a weathervane, perhaps) that is even better.
Call a professional. If you have tried all of these steps and the pigeons still will not leave, contact a reputable pest control company in your area. Even if they don't specialize in ridding your roof of pigeons, they will be able to refer you to someone who does.