The primary factor in the service life of any metal roof is how well it has been maintained. Often the metal roofs I inspect have only been neglected recently. Small areas of peeling paint and minor surface corrosion can be easily addressed, even by some non-professionals.
Even more extensive problems in metal roofs often can be repaired. But your biggest challenge may be finding a roofer with the equipment, skills and knowledge to correctly repair and maintain these historic roofs. I often see tar slopped into valleys, chimney flashings and plumbing vents on metal roofs. Roofing tar is an unsightly and temporary fix for a problem that should be repaired properly and permanently.
Common problem areas
Sheets of metal are joined by standing or soldered seams to form roofs. Significant leaks are rare at these joints. It's also unusual for leaks to originate on the surface of the metal.
As in most types of roofing systems, leaks are more common at transition points, such as:
- roof-to-wall joints,
- changes in slope (the problem you describe),
- and penetrations, like plumbing vents and chimneys.
New coatings for old roofsDon't despair if your roof has been less than meticulously maintained. Even if your roof has fairly widespread surface corrosion and a few there is still hope, thanks to recent improvements in elastomeric coating systems. This acrylic sealing system (not to be confused with the silver stuff you get from a home center) is brushed on in layers. When it cures, it forms an seamless membrane on the roof. (Here's a link to an OHW story about one such elastomeric coating product.)
At flashings, tar patches and spots of more severe corrosion, a reinforcing polyester fabric can be embedded into the elastomeric coating material.
The coating is also available in the common colors of the old oil paints. Single layer re-coatings can be applied about every 5-10 years to extend the life, and traditional appearance, of old tin roofs.
A roof by any other name...
Tin is only one type of metal roofing system. Sheet metal, lead, copper, zinc, tin plate, terne plate, and galvanized iron have all been used as roofing materials.The original metal roofing material produced in this country was sheet iron, first used at the end of the 18th century. Rolling mills produced large sheets that required fewer seams than later plated materials. Iron must be painted to prevent rust.
Lead sheet, more often used for flashings and lining wood gutters, was only briefly used for roof coverings. While lead doesn't rust, it never gained widespread use for two reasons. Expansion and contraction caused small tears in the lead, and when warm, the soft metal would creep down steeply pitched roofs.
Tin used in roofing is actually tin plated iron. It was one of the most common metal roofing materials used throughout the 19th century. Terne, an alloy of lead and tin, was also used as a plating material. Tin plate is bright and shiny when first installed, while terne has a duller appearance. Any iron-based roofing material needs to be painted to guard against corrosion. The protective plating eventually wears away or is damaged, leaving the iron beneath vulnerable to rust.
Painting metal roofs
If your roof has been maintained regularly, it still needs to be protected from corrosion.
Traditional linseed oil paint was the only available product to protect the metal until the latter part of the 20th century. The linseed oil was tinted by grinding in earth pigments, with iron oxide (red) being the most common.
You can use acrylic primers and paints on metal roofs, but they won't last as long as the elastomeric coatings. Typically a metal roof needs to be painted every two to four years, while the new coatings can last as long as ten years. As with any paint job, good surface preparation is the key to success.
You'll need to remove any surface rust with a wire brush. Next, wash the roof to remove surface dirt and debris. Rinse it well and allow it to dry thoroughly before painting.
With your roof repaired and in good shape, put it on a regular maintenance schedule.
William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.