The picture attached is a cement vent pipe I found on the roof for water heater venting. Looks like the cap has been broken off, but I have no idea what was the original looks like. The diameter of the cement pipe is 4.75". The closest Lowes vent cap I found is either a 4in or a 6in Selkirk cap. But Im not sure if i get the 6 inch, are the screws going to penetrate the cement enough to get a good grip? I dont want it to be blown off in the future. What is the proper way to do this job?
That flue pipe looks like "Transite" pipe which was used on some gas only appliances before about 1960. When I got into the heating business in the late 1960's nobody was still using it. There were better alternatives. If it is transite, it probably contains asbestos. I don't know of any caps available for use on this product. You could take a standard 6" cap and slit the bottom connection in about 6 sections and bend the slit pieces inward and use a large hose clamp to hold the cap to the old pipe. You could "GOOGLE" transite to learn about this pipe. Hope this helps.