Warning: don't trust the 'clean' look of a cap after a storm, found this out on a 1923 bungalow in Toledo
Honestly I almost skipped the second pass on this job and it would have bit me bad. The cap looked fine from the ground, no rust, no gaps, so I told the homeowner it was just a quick sweep and I'd be done in under an hour. When I got up there and pulled the screen off the top, the whole thing was packed with wet leaves and a mouse nest the size of a softball. Turns out the last big windstorm pushed debris up under the cap and it was sitting right on top of the flue, blocking maybe 70 percent of the opening. I ran a camera down and you could see creosote already building up way faster than it should have for a house that gets burned maybe twice a week. What saved me was just taking a five dollar hardware store mirror and angling it down into the cap gap before I even climbed up. Now I do that on every job over 15 years old and I've caught 6 blocked caps this season alone in the Toledo area. If you're not checking under the cap on every sweep, what's your go to move for spotting hidden blockages without tearing the whole thing apart?