14

My house has a tin roof over a porch. The house is about one hundred years old, and the tin porch roof is original, as far as I know.

My question is whether the tin roof (which is currently in good shape -- no rusting or leaks) will last indefinitely provided that I periodically apply layers of roofing tar to maintain it, or if tin roofs only last so long.

Another way of asking this is: "Is roofing tar a temporary fix for tin roofs that need to be replaced, or will it keep the roof in serviceable shape indefinitely when applied properly?"

When I say "roofing tar," I mean a coating like this, not the "wet patch" temporary fix stuff:

Enter image description here

I ask because a contractor is trying to convince me to hire him to replace the tin roof, but it's currently fine and has apparently been there for a long time, so it's not clear why I'd need to rip it out and install a new one.

  • 5
    Your roof is almost certainly neither tin nor tin plated steel, but rather zinc coated steel, also known as galvanized steel. "Tin cans" are actually tin coated steel, although sometimes aluminum cans are (incorrectly) called tin cans. If you roof is not leaking, I would leave it alone. – Jim Stewart Mar 29 '18 at 14:44
  • I agree with blacksmith37's answer. However, the one reason I can think of to replace it would be appearance. If you don't like how it looks, it's time for it to go. If you're fine with the appearance, then it sounds like the roof is good to go as is. – FreeMan Mar 29 '18 at 14:55
  • The original pressed tin ceilings (Interior) were in fact made of tin or tin plated steel, and then painted. Some modern "tin ceilings" are simply painted steel or aluminum, but higher end tin ceiling panels are tin plated steel with a powder coat finish on top of that. – Jim Stewart Mar 29 '18 at 15:12
  • Is their any other reasons for the "replacement"? As in: better insulation, solar, matching the rest of the house (if its different), matching additions or other work being done (You have a contractor for a reason right?)? Or is this just a random suggestion? – WernerCD Mar 29 '18 at 17:15
  • Any flat roof can be kept in service almost indefinitely if you're willing to silver coat it once a year. – Mazura Mar 29 '18 at 18:48
17

I guess you mean a galvanized or otherwise coated steel. Life depends on weather, specifically rain. In a desert it will last forever. If the current coating is repaired at any damage spots, it will last almost forever. "Repair" could be anything from dabbing on roofing tar to sandblasting and coating with a zinc-rich epoxy (typically used to protect industrial steel surfaces). Even conventional paints could be used - paint on cars and houses lasts pretty well. It depends on how you want it to look and how often you are willing to "touch it up". For industrial steel tank coatings (aka oil tank farms), touch-up is part of the maintenance program. The contractor is just trying to make money. Addition : There are also "aluminum" coatings commonly sold for mobile home roofs. These may sort of match your galvanized in appearance.

  • 1
    I agree. The only issue if there's corrosion so severe that the structural integrity of the panels is in question. – isherwood Mar 29 '18 at 13:39
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    I also agree if it is leak free and no rust it may last longer than us with simple maintenance. Some folks think metal needs replacing when it is patched and looks older, I treat it like a car tire the patch is holding and still safe why throw $ away. – Ed Beal Mar 29 '18 at 14:30
  • As a point of interest, what is the profile of the galvanized steel roofing sheets over your porch? Corrugated or with flats and ribs? – Jim Stewart Mar 29 '18 at 17:10
  • Not just rain, but Acid Rain, which is more of an issue in some areas than others, and perhaps only an issue if the tar, paint or other non-metalic coating has been compromised. – mickeyf_supports_Monica Mar 29 '18 at 18:44
  • @Ed agree on the roof lasting, but a car tire is probably not the best comparison - even one that hasn't needed patching will eventually become brittle and need replacing while it may still look fine. – CactusCake Mar 30 '18 at 15:37
9

A galvanized roof that leaks may have leaky nails

Tin roofs are typically held down by nails, and over the years the nails loosen and leak. The new approach is to use nails or screws with an elastomeric rubber pad, and set their compression pressure carefully. I am super fond of roof screws of an appropriate size to not overlarge the old nail holes too badly, and a "speed wrench" with bit holder to drive them both quickly and precisely. These elastomeric rubber pads give out after 20-40 years and you need to replace the roof screws.

A galvanized roof can be refreshed in place

Normally I prefer heavy prep when painting, but in this case you want only the lightest scuffing - you need to remove dirt, dust and contaminants but don't remove the weathered patina from the zinc! Paint won't stick to shiny metallic zinc, you need the patina to give the surface "tooth".

Then I use "cold galvanizing compound", zinc-based "paint" which is specifically for galvanized roof repairs.

enter image description here

The paint is 90% zinc, and is HEAVY. I subdivide my gallons into 4 quarts so I can use the paint shaker without breaking it. You must frequently agitate your paint cup to keep the zinc in emulsion, as it wants to fall out. After about 6 months to a year, it will gain the same "patina" as old tin roof.

This stuff works direct on the metal, so if you do rust removal on a galvanized roof, you don't need another primer.

In my experience, tar is a one-way trip

Go ahead and tar a 3 inch by 3 inch square of your roof. I'm limiting it to 3" because I'm not that much of a sadist. Wait a year. Then buy a gallon of mineral spirits (don't use gas) and some nonmetallic scrapers and brushes, and remove the tar from the roof without wrecking the galvanized underneath. You will quickly (well, 20 minutes) reach the same conclusion I have: tar is non-removable in any sensible timeframe.

Once you tar a roof, it will fail again - that's why you hear people talking about having to re-tar "annually". (what kind of lousy roof needs annual attention?) Heat/cold and drying out starts introducing cracks and gaps. At first, slathering on more tar fixes that. But after a few years, the tar is so thick and congealed; the gaps are too wide; it is peeling up too wholesale - no amount of additional tar will fix it. Now you need a new roof.

Why not just remove the congealed tar? Tar is very sticky, and when the gooey fingernail-attacking, glove-ruining gunk is painstakingly removed, leaves a great deal of surface contamination that needs dozens of passes with mineral spirits. (don't use gas, or you'll forever have stink of gasoline additives wherever that roof leaks into). IME it takes about 2 hours per square foot, in my practical experience, oh, do I loathe that job.

Several of our docents advocate tar on every roof. None of the are interested in the "excess tar removal" job. Funny, that.

4

Please see https://www.nps.gov/tps/how-to-preserve/briefs/4-roofing.htm#historic

Most likely what you have is tin plated iron or some kind of galvanized metal. Please check with a historical roofing specialist before you do ANYTHING to your roof. Tar patching is generally for asphalt roofs only. People do put tar on other roof types, and they ruin them (especially slate). Keep in mind most (um, 99.9%) contractors haven't the faintest clue about historical materials and will cheerfully ruin them. Talk to a historical specialist. Copper is about the only roofing material that lasts indefinitely.

  • Copper does not last forever, it degrades to verdigris. en.wikipedia.org/wiki/Verdigris – Crowley Mar 29 '18 at 19:47
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    You're referring to the green patina that copper develops? This is entirely beneficial and doesn't degrade any function of the roof. That WHY it lasts so long. People who maintain the natural appearance of their copper and do not let it "green" are just ignorant of the material. It's also much more beautiful green. According to wikipedia, a copper roof can last 1000 years. Is that long enough for you? en.wikipedia.org/wiki/Copper_in_architecture#Durability/… – figtrap Mar 30 '18 at 20:28
  • In theory. it can last thousand years. If there is no mechanical load on the surface it can last long, If there is mechanical load, the protective layer, which is more brittle than the base material, may break and expose new copper to oxidize. – Crowley Apr 3 '18 at 16:26
1

I sit here under a tin roof (galvanised steel) that's 40years old in a semi-coastal location (next to an estuary, less than 1 km from the Pacific Ocean). The roof was painted with oil paint when new and has been repainted once since then and had a few nails replaced.

The sheet roofing is light-weight tough and inexpensive. If you can prevent it from rusting it will last forever. If it fails it's relatively cheap to replace.

  • I agree with forever because in my world a life time is forever just a little maintaniance it will last, If we want to get picky nothing last forever. – Ed Beal Mar 30 '18 at 20:36
0

I think roofs cannot last forever. The aspect I want to point out here is the galvanic corrosion, or marine corrosion.

Extracted from http://www.boatus.com/boattech/articles/marine-corrosion.asp:

Galvanic corrosion of the more chemically active metal can occur whenever two metals that are connected by actually touching each other, are immersed in a conductive solution (any liquid that can transfer electricity). Anything but pure water is conductive. Saltwater, freshwater with high mineral content, and polluted freshwater are very conductive, and conductivity goes up with water temperature.

Electrons flow from the anode (the metal that is more chemically active), to the cathode (the metal that is less chemically active). Anode's atoms become ions (an atom with one or more electrons either missing or added) and break away into the water. Your anode is literally dissolving through galvanic corrosion.

Please notice where the tin is situated in this table. It is likely to corrode if in contact with nails made of copper or stainless steel.

Corrosion can take place at the junction of roof to nails, or other metal components such as rain pipes. Of course, the key ingredient is humidity (needed to form a conductive solution); while roofing tar will keep your roof waterproof from the outside, it will not keep it safe from the humidity from withing the house.

Steam from bathrooms can rise to the roof. Water can combine with the carbon dioxide found in the air, to form carbonic acid - a weak acid, but enough for electrical conductivity.

So, as far as the nails are made from a different metal than the roof (which certainly is the case, because the roof is made of tin, while no nails can be made of tin), either your roof or the nails will corrode, depending which metal is more chemically stable.

Again, in order for corrosion to take place, there must exist an electrical conductive solution. This solution can be water combined with atmospheric CO2, or different pollution compounds such as sulphur oxides and smoke.

The source of the water can be even the condensation process - if condensation happens on windows, it can also happen on the inside part of a roof.

The key aspect here is that corrosion can attack the tin from INSIDE the house, not necessarily from the outside.

  • 1
    The Galvanic series is only pertinent when materials are submerged. – blacksmith37 Mar 30 '18 at 14:57
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    not true: materials don't have to be submerged; a wet atmosphere is enough. Vapors condense on the material, forming a thin layer of liquid, which is enough for electrolysis to take place – Newton fan 01 Mar 30 '18 at 15:02
  • googling for "atmospheric galvanic corrosion" gave: electrochemsci.org/papers/vol8/80607687.pdf . It is mentioned here that Bell Telephone Laboratories have studied "the galvanic behavior of different bimetal couples exposed to the atmosphere". A particular experiment was concerning with measuring "the resistance of aluminum steel reinforced cables in coastal zones and as a way to classify the corrosivity of marine and industrial atmospheres " – Newton fan 01 Mar 30 '18 at 15:07
  • i might have misinterpreted your comment. Initially, i understood "The Galvanic series" as "the series of your explanations regarding galvanic process". Now i read it as "the galvanic table" – Newton fan 01 Mar 30 '18 at 15:44
  • "Submerged" may be a few drops of water at the junction of 2 metals. Per NACE certified Corrosion Specialist # 1635 ( aka , me ) – blacksmith37 Mar 31 '18 at 15:11

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