Dear Judith: I bought a row house on Capitol Hill three years ago. It has an original tin roof and I was told by the inspector that if I painted it every two years it would continue to last indefinitely. I know they use a fibrous plasticized paint. My brother, who used to be a contractor, asked: When will the accumulated weight of the paint do-in the roof? That was a question that got me thinking: Why isn't the weight a problem? Let's begin with tin.

Our old roofs are not tin. Tin is an element long used in plating and may well be the plating on your metal roof. The sheets, however, are probably steel, with a tin or Terne (an alloy of zinc and tin) coating to allow the steel pans to be soldered. Metal roofs can also be sheet copper or lead, copper with lead coating, stainless steel with lead or Terne coating, and various other combinations. Most of our roofs, which are not at all exotic, are steel with a plated coating. You can recognize original metal roofs because the sheets are very short. While today a metal roof is likely to be sheet steel with a Terne coating that comes on a roll many feet long, in the late 19th century roofing came as pieces about 16-inches wide and only a couple of feet long.

Indeed, your inspector was right. If you keep your old metal roof coated properly it should last more or less indefinitely. The row of six houses, built in 1877, of which my house is one, includes one intact, apparently original roof. A hundred and twenty three years and counting seems like a pretty good life to me. (Most other roofing choices have life expectancies of about twenty years. Of the thirteen houses on our side of the street, five have residents who have been there twenty years or longer, so it is not so unusual to stay in a house long enough to look at several replacements of those twenty-year roofs. I always recommend metal roofs to homeowners so they will never have to reroof again though, naturally, installing a standing seam metal roof is more expensive than the twenty-year choices)

What constitutes a proper coating? First, what does not constitute a proper coating: roofing cement or any asphalted or bituminous coating. These are usually black, are the stuff your basic roof guy has in his truck, and are death to metal roofs. They trap moisture and the steel roof rusts out below them. When you have a leak, do not allow some guy up there with his bucket of black goo and a brush! Call a trustworthy roofer who will repair the roof with solder, metal patches, or other material appropriate to a metal roof. Follansbee Steel, the pre-eminent manufacturer of steel roofs, recommends painting with a waterbased acrylic metal roof paint that can be applied with brush, roller or power spray. We have used a linseed oil based paint called Tin-o- lin. This stuff dries very slowly, seeming to consist almost entirely of linseed oil.

Whichever you choose, this is work that should be performed when the surface and air temperature is between 50 and 90 degrees, which usually means not in the dead of winter or the middle of summer. Now would be good. Regardless of coating material, your roof must be clean and free of chalk; the powdery surface deposit paint gets as it weathers. Since so many of our roofs have low slopes, cleaning the roof is pretty easy. It can be done with a power wash or a stiff bristle brush and a TSP (Tri-sodium phosphate) and water solution. After cleaning, the roof needs a good rinsing. Prior to coating, check for rust. Anywhere you see rust on your roof, or the steel beams supporting your central air conditioner condensing unit, remove the loose stuff with a wire brush then coat all of those spots with a rust converter.

Clearly, none of this is rocket science, so if you like to court skin cancer, do this roof work yourself its a great way to get a jump on your summer tan or extend the summer tan into the fall. If you hire someone to do the job, insist on good preparation. How often should you coat your roof? I disagree with your inspector about coating every two years. Follansbee Steel suggests recoating when the finish shows wear or every eight years. So recoat your roof when it needs it. How will you know? Naturally, you will go up there every once in a while and take a look. If you can see areas where the coating has worn thin and the roof looks a little rusty, you need to recoat. Or, here the brilliant solution that Gregory Cavanaugh used for our roof: every other time you coat, change color. Tin-o-lin is available in red and green and 5 other colors. So, if your roof is red, when it seems to be wearing thin, coat it in green. Then, when you start to see the red through the green, you know it is time to recoat, this time in red.

I wouldn't worry about the build-up of weight of these coatings. As you can see from the above, the paint is basically wearing or washing away between coatings. Consider yourself fortunate to have your original roof. Treasure it, care for it, and feel superior knowing a piece of history is keeping the rain off!