Friday, December 5, 2008
Carpentry is really winding down to the last few days’ worth of trim work, and the painters resume Monday! Yes, it seems like this is all taking awhile, but the quality of work has been good and the whole experience has been totally hassle-free with no horror stories about contractors disappearing, etc.
One curious wrinkle is how to handle our apparently unique and original 1941 porch roof, comprised of a soldered patchwork of copper-bearing steel alloy sheets. It is in fairly good shape except at the drip edges. Husband Kurt has done a bunch of obsessing, I mean research, and is consulting a metal roofer to figure out a way to repair the original roof in the least obtrusive manner. I’ve been up there to see it in person, and it IS really cool—pretty rusty-brown patina.
The first image at the top of Iris' post is an ad for U.S. Steel terne roofing sheets from a 1913 issue of "The Architectural Record," and below is a cut sheet for the same product from the 1920 U.S. Steel General Catalog.
Our own stamp is similar to those illustrated in the 1913 ad and 1920 catalog sheets. It reads:
8 POUNDS COATING
As described, the sheets are copper-bearing steel alloy, which in those days typically contained 0.15% to 0.25% of copper, and they were coated with a hot-dip of terne, which typically was a mixture of around 80% lead and 20% tin.
I am guessing our "KNOX" is the Knox Pressed and Welded Steel Company of Pittsburgh, or maybe of Wheatland, Ohio. Knox merged with the Blaw Steel Construction Company just before 1920 to become the Blaw-Knox Company. The Knox (or Blaw-Knox) operation must have been a competitor of American Sheet & Tin Plate Co., a division of U.S. Steel. USS is the holder of the patent for Cor-Ten steel ("Cor-Ten A" or A242 steel aka "corten"), which is no longer produced. A606 steel is a close modern equivalent of Cor-Ten. There's more info on that here.
As a result of studies done since 1900, by the time our house was built people already knew that copper-bearing steel sheet would stand up very well to the elements after developing its protective patina. They had already been making railcars out of copper-bearing steel since at least 1919, and U.S. Steel had done extensive testing in the 1930's to develop its own patented Cor-Ten A.
Our roofing contractor has pointed out that our metal sheets do not appear to have any terne coating on them (!?), rather, just a nice brown patina on the top surface. The bottom face is also uncoated and mostly untarnished and shiny. So, it is not simply that the terne coating weathered away over the years. Some of our sheets were installed face-up and some face-down; it is impossible these sheets were terne-coated on just one face that was weathered away. Besides, the hot-dip process doesn't really lend itself to one-sided coating.
Based on all this, it appears we have a roof of bare copper-bearing steel. Even though our stamp says it has "8 POUNDS COATING" for some reason it never did get that coating. Maybe, in late-Depression pre-WWII years, the tin and lead used for terne coating were in short supply, and they just offered plates without it. Or maybe the "KNOX" brand indicates this stuff comes from an irregular batch of uncoated plates made at Knox before their 1917-1919 merger with Blaw. If there was little new steel product left for residential projects, and if some of this old, uncoated, copper-bearing sheet was just sitting around in a warehouse, maybe it was used on our house because they knew it would work in a pinch. -Kurt