Why the slate did not go back on
When a slate roof starts shedding tiles, the instinct is to replace it in kind, and sometimes that is right. Here it was not, for three reasons that stack on top of each other.
The first is weight. Slate is stone, and a slate roof weighs several times what an asphalt roof weighs over the same area. The framing under a house like this was built for that load when the house went up — but it has been carrying it through every winter since, and old framing settles, dries out, and occasionally gets modified by someone adding a bathroom. Once the slate is off, putting the same load back on is a question for someone qualified to answer it, and answering it properly costs money and time before a single new tile is laid. Going lighter removes the question entirely.
The second is the material. New slate is quarried stone, and matching an existing roof means matching colour, size and thickness from a quarry that is still producing that stone. Salvaged slate solves the matching problem and creates a different one: you are buying tile that has already spent most of a century on somebody's roof, with no way to know how much of its life is left. Either path is expensive, and neither is quick.
The third is the people, and it is the one that gets underestimated. Slate is not laid like anything else. The nails are set so the tile hangs rather than being clamped tight, the exposure and headlap are unforgiving, and the finished roof is a field of brittle pieces that crack if you walk on them wrong. It is a distinct trade, it does not transfer over from asphalt, and the number of people who do it well is small and shrinking. That is not only a problem for the installation. It is a problem for every repair over the following thirty years, when a couple of cracked tiles means finding a specialist and waiting for the calendar to open.
So what does a slate-look asphalt shingle give up? Length of service, first — a real slate roof can outlast several asphalt roofs, and it is fair to say so plainly. And it gives up something at close range: stone has actual thickness, and the shadow line along a slate course is a real edge, where a laminated shingle builds its shadow out of a second layer and a printed pattern. Standing in the yard looking up, you can tell.
What it keeps is most of what the roof was doing for the house. It keeps the grey and the broken, variegated pattern at the distance from which a roof is actually seen — the street and the sidewalk. It keeps the structure carrying a fraction of the load it was carrying before. It keeps the roof repairable by any competent roofer in town, next month, at ordinary cost. And it keeps the house dry now, rather than after a structural review and a search for matching stone.
If you are looking at a slate roof that is dropping tiles, the question is not slate versus asphalt in the abstract. It is what your framing will take, what matching stone will cost and how long it will take to get someone to lay it — and whether that difference buys you something you can see from where you actually stand.
