Roofs can and should
be routinely built to last at least a century. In fact, 150
years is a reasonable expectation of a roof’s longevity
if the roof is properly constructed. How do you build a roof
to last that long? You can begin by examining roofs that
are currently a century of more old and then either copy
or improve on the traditional materials and techniques used
in the creation of these roofs. As a professional slate and
tile roof restoration contractor, I have had the unusual
opportunity to inspect, repair, restore, disassemble, and
reassemble many hundreds of century old, or older, roofs
over the past 30 years. This is what I’ve learned about
roof longevity:
1. Use a long-lasting
roofing material such as slate or tile. Make sure the slate
is top quality S1 grade. Never use fake substitutes.
2. Use a long-lasting roof decking material such as natural
rough-sawn lumber or planed boards at least 3/4” thick.
Avoid CCA treated lumber and any laminated materials such
as plywood, particle board, or OSB.
3. Use copper or stainless steel nails to fasten the slate
or tile to the roof. A good grade of hot-dipped galvanized
nail will last 100 years or more, but not nearly as long
as copper or stainless steel nails, which will last indefinitely.
4. Use stainless steel flashing, such as terne-coated stainless.
Copper and lead flashings will last indefinitely if kept
painted, but will develop pitted holes in 60-70 years if
unpainted, especially in valleys. No one yet knows how
long terne-coated stainless will last, but it will presumably
outlast both copper and lead. Alternatively, build rounded
(slate) valleys and eliminate the valley flashing, which
is subject to the greatest wear on the roof.
5. Design the roof properly. Use adequate slope — the
more the better (the roof should be too steep to walk on).
The lower the slope, the shorter the life of the roof. Build
the roof sturdy enough to prevent sag over the years.
6. Make sure the roof is installed by properly trained personnel
who understand the nuances of traditional roofing and who
possess the proper tools and equipment. For example, slate
roofs should never be walked on during installation.
In addition, traditional long-lasting roofs have never required
the use of ice and water membrane. This material can be completely
avoided (see related article this issue). Felt paper, coincidentally,
is not necessary for a slate roof to function properly, although
30 pound roofing felt is recommended as a temporary covering
over the roof sheathing prior to slate installation. Tiles
roofs are more dependent on a heavy felt underlayment.
