If you've spent your life in the building trades, you may have done a job or two that you look back on with regret. I still remember one from about 30 years ago. A contractor I worked for had us tear off a 20-square Vermont slate roof because there were a few leaks. We replaced the slate with asphalt shingles.

Some of those slates were almost too heavy to lift — more than an inch thick, in various shades of green, gray, purple, and red. The roof was laid out in the old "graduated" style, with the large, heavy slates applied in wide courses near the eaves, and the reveal getting smaller with each course so that the lightest slates would be...

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