The greatest sin in science fiction is when your science isn’t believable. Yes, it’s science fiction, so it doesn’t have to be true, but you should at least make it believable. It doesn’t have to be possible, but your job is to make me think it’s possible.

My current example for this is Ashfall by Mike Mullen. A super volcano has erupted, covering Iowa in a foot or two of ash. It’s also been raining, turning the ash into a wet slurry. But suddenly that wet ash is causing buildings to collapse. Not one or two, but almost every building collapsing under the weight of a few feet of ash.

Huh? Is ash really that heavy? We easily had two feet of accumulated snow last year and roofs weren’t collapsing. I have a hard time believing that ash is that much heavier than snow.

Now I’m just a dumb reader, what do I know? I didn’t do the research: Apparently ash is heavier than snow. But you have to make it convincing. Give me reasons to believe the science (especially if your science is indeed fiction). In this case it could have been a simple comment comparing ash and snow. Or a little more variation on which buildings collapsed (my take on the research suggests that in an area with heavy snowfall like Iowa, more of the buildings would have survived).

In the end you want your reader thinking about your characters, in this case worrying how he’s going to make it and if he’ll be reunited with his family. You don’t want your reader focusing on something silly, like whether or not ash could collapse roofs.