Historic wooden roofs using straight edge-grain heartwood shingles
have been known to last over sixty years. Fifteen to thirty years,
however, is a more realistic lifespan for most premium modern
wooden shingle roofs.
 These weathered historic 19th-century handsplit and dressed shingles were found in place under a later altered roof. See also, below. Photo: John Ingle. |
Contributing factors to deterioration include
the thinness of the shingle, the durability of the wood species
used, the exposure to the sun, the slope of the roof, the presence
of lichens or moss growing on the shingle, poor ventilation levels
under the shingle or in the roof, the presence of overhanging
tree limbs, pollutants in the air, the original installation method,
and the history of the roof maintenance. Erosion of the softer
wood within the growth rings is caused by rainwater, wind, grit,
fungus and the breakdown of cells by ultraviolet rays in sunlight.
If the shingles cannot adequately dry between rains, if moss and
lichens are allowed to grow, or if debris is not removed from
the roof, moisture will be held in the wood and accelerate deterioration.
Moisture trapped under the shingle, condensation, or poorly ventilated
attics will also accelerate deterioration.
In addition to the eventual deterioration of wooden shingles,
impact from falling branches and workmen walking on the roof can
cause localized damage. If, however, over 20% of the shingles
on any one surface appear eroded, cracked, cupped or split, or
if there is evidence of pervasive moisture damage in the attic,
replacement should be considered. If only a few shingles are missing
or damaged, selective replacement may be possible. For limited
replacement, the old shingle is removed and a new shingle can
be inserted and held in place with a thin metal tab, or "babbie."
This reduces disturbance to the sound shingles above. In instances
where a few shingles have been cracked or the joint of overlapping
shingles is aligned and thus forms a passage for water penetration,
a metal flashing piece slipped under the shingle can stop moisture
temporarily. If moisture is getting into the attic, repairs must
be made quickly to prevent deterioration of the roof structural
framing members.
When damage is extensive, replacement of the shingles will be
necessary, but the historic sheathing or shingle lath under the
shingles may be in satisfactory condition. Often, the historic
sheathing or shingle laths, by their size, placement, location
of early nail holes, and water stain marks, can give important
information regarding the early shingles used. Before specifying
a replacement roof, it is important to establish the original
shingle material, configuration, detailing and installation. If the historic shingles are still in place, it is
best to remove several to determine the size, shape, exposure
length, and special features from the unweathered portions. If
there are already replacement shingles on the roof, it may be
necessary to verify through photographic or other research whether
the shingles currently on the roof were an accurate replacement
of the historic shingles.
"The Repair and Replacement of Historic Shingle Roofs" an Historic Preservation Brief
