Our chimney cap blew off during a windstorm. I went up there to secure it, but whenever I tightened the set screws, parts of the brick (circled in red) would break off. Now the set screws have nothing solid to drive into.

How can I rebuild that part of the chimney so I can finally secure this cap?

We do not have a fireplace. The chimney is hooked up to our oil burner.

 

I imagine the best solution would be to replace the tile flue. But depending how tight the cap fits (If you have room for it) you could maybe get someone to band it with some thin stainless steel plate... Assuming the cap would still fit if the flue was maybe 3/8" wider than it is now.

 

I think there is enough slack to widen the flue a little bit. Is there any way I could restore the flue with some sort of compound?

 

Maybe... If you could make a form for the inside you might be able to slather on some goop and make it work. Maybe PC7 2 part epoxy... Something like that. It would probably mean multiple trips up there... Maybe several coats, grinding and smoothing, etc. A steel band would be 1 trip, assuming it fit. I think if you tried any sort of concrete repair it would just break off because it won't bond and be strong.

 

Short term, you could pick up some flat stock at TSC or your local hardware or big box, bend it to the size you need, and drill the ends where they meet for a bolt and nut to make your own clamp. And I would say that short term is good enough for now as whether now or first thing in the spring, depending on where you are located, it looks like you have as big or probably bigger issue to look at. The crown appears in bad shape, and you don't want to ignore that because it looks like water can get into your bricks, which has the potential of damaging the bricks, as well as the ceilings and structure below. When you do it, the crown should be a minimum 2" thick, concrete, not mortar, sloped to the outside, overhang the bricks by at least an inch or two, and ideally have a detail to create a drip edge so that water does not migrate back on the bottom edge of it.

 

I didn't realize the chimney crown was in such bad shape. I've called a chimney repair company to get an estimate. Do you have any ballpark on how much this should cost so I know I'm not getting hosed? Thanks for the advice.

 

I don't make guesses on anything, too many variables, but, assuming there are no unique circumstances, and not to minimize their work, it's a pretty straightforward job; remove the old crown, form it, mix the 'crete, and you're there. I would ask them to inspect the mortar joints and repoint them if needed while they're at it, AND to replace that top section of flue. Have your cap handy and you shouldn't have to climb up there for that later on.