Many gull species carry off mollusks and shellfish and then drop them to try and break them open for food. They do this on all kinds of surfaces, even soft sand (which obviously doesn’t work as well as pavement or maybe your roof). Apparently your local gulls see your roof as a good place to do this. Perhaps with some experimentation you can make your roof less attractive as a drop spot. You might try putting up some gull decoys—as gulls flying over may be less likely to drop food where they think another gull might steal it.  However, since gulls are quite intelligent and may not go for the ruse. If gull decoys attract other loafing gulls to your roof, you may be able to install some attractive flags up there to displace the real birds, as given a choice gulls usually prefer to hang out in less distracting environments.