The stain on the ceiling is frequently not directly below the leak in the roof. Water may run down the rafter or sheathing until it comes to a drip point and fall from there. I'd check for evidence of a failure of the flashing around that chimney.

Agree with Randy, you have to find out where this water is getting in and correct that.

"siding backer board" = sheathing? If there's vinyl behind it, this must be a vertical section of wall in the roof structure, such as a gable.

Maybe a picture would help. As Randy says, water can show up quite a ways from where it's getting in.

Thanks for your replies. As luck would have it the rain has been pouring down today so I checked the attic. It does indeed look like water is coming from above, running across 6-8 inches and dripping down. Will be getting chimney flashing checked as first step.

Thanks again.

Also check the cap of the chimney to make sure there are no gaps that let water run down around the liner and get between the brick facing and the liner. There is also a small possibility of water getting between the chimney and liner through mortar gaps/cracks on the brick facing; however, should be able to see those gaps.

Bruce