Flue gaps at cap

[Dave Rose]

What is the proper way to seal this gap at the top of a lined chimney on a 1950s house?

[Eric Barker]

I expect that the gap may be too wide to be effectively sealed. The condition of the flue liner appears to be another issue.

[Markus Keller]

you could get by with backer rod and mortar but I would prefer backer rod and solar seal
some might tell you to use fire caulk but I wouldn't get picky about it at that location
the key for good life span would be backer rod and a nice sloped joint to promote water run off

[Dave Rose]

Thanks for the replies. Yes, I noted the cracks also and recommended a chimney contractor evaluate the flu more thoroughly.

I'm not familiar with solar seal...does it have some characteristic over and above silicone caulk? Thanks for the input.

[Ken Rowe]

From my past continuing ed courses, that seam should never be mortar but should be backer rod and urethane caulk.

[Eric Barker]

I though of the backer rod but then you'd be putting a combustible material against the liner.

[Mark Reinmiller]

True but you will hardly ever see it. Anyways that needs a chimney liner as the flue is cracked.

Even if only the top section is cracked? I have seen that many times. The top section can be replaced if the flue is otherwise ok.

[Bob Harper]

The codes have never figured this one out. Things that do well against heat don't do well keeping out water and vice versa. I have used fiberglass rope gasketing with red RTV silicone on gaps up to about 1/4". Gaps wider than that should trigger a crown rebuild. The codes only recognize carrying the flue tile up before the surrounding wythes. Yes, you 'can' butter the joint, lower a tile, strike the joint on the side and call it a day. Not what the code allows but it is a practical repair IF it is only that one tile. However, this should trigger a Level II and I seriously doubt a proper inspection would reveal the rest of this chimney does not warrant a liner. Whether this damage was from a water penetration, a chimney fire, mis-handling at construction or whatever, we know it needs to be replaced, we need to perform a level II and most likely, there will be evidence of significant water penetration and damage below. You can make chimneys with simply crummy mortar crown washes or you can go for the gusto with a bond break under a concrete crown with a drip edge or even do a full sheet lead through-pan with weep holes like they do in the UK.