This may sound so basic that there is little to no reason for me to mention it here. It is a very simple procedure. You buy the appropriately sized chimney cap and install it using the screws and hardware included. Unless you get an adjustable cap, everything is set for you and all you have to do is place the cap on and drill the screws, through the brackets, into the chimney. (A side note on drilling into concrete and other masonry surfaces, a good practice is to always pre drill holes before installing screws. Use a masonry bit at least one size smaller than the diameter of the actual screw to be used. Ex. For a 1/4” masonry screw, use a 3/16” masonry drill bit. This will reduce the likelihood of you splitting or chipping the masonry surface when driving your screws).
Yet, even as simple a job can be, there are always those outliers.
A short while ago I was approached by a man who was tired of having bird’s nests in his chimney, but had no idea what to do about a chimney cap. As it happened, his house was well over 50 years old and the portion of the flue, which extends through the crown (sloped area around flue to divert rain water away from hole), had been broken off along with the crown (don’t ask me how) and now he had a flat surface he needed to cover from the elements. I explained to him that the best thing to do would be to repair the flue and crown, but after a few minutes he decided that that was above his experience level and his wife had no desire of him being on the roof. He explained to me that he had decided to cover the hole with a sheet of roof flashing, screw in masonry screws around the edges into the brick beneath, and then weather proof the edges with roofing tar.
He stuck his chest out and I could see the proud look on his face, so I waited for him to finish and after a few moments of thinking I said, “What type of furnace do you have?” To which he responded, “Gas.” I then asked him how it vented. He did not know. I then informed him that in order for a chimney to work safely/properly the flue passage could not be obstructed, but if he could determine that his flue was not being used by his fireplaces, water heater, or furnace, then he could continue with his plan.
As it turns out, I saw him a few days later and yes, his furnace was being vented out of that chimney flue. Had he not asked and simply went ahead with what he had been planning, he would have been placing his family in risk of Carbon Monoxide poisoning or death. He ended up having a friend of the family rework his chimney for him and all went well. I wanted to relay this story just to remind you that when making home improvements or repairs, you should always plan them out carefully and why not ask an expert? Think Safety.