Failures of Support Systems
Once the condition of the roofing material has been determined,
the related features and support systems should be examined on
the exterior and on the interior of the roof. The gutters and
downspouts need periodic cleaning and maintenance since a variety
of debris fill them, causing water to back up and seep under roofing
units. Water will eventually cause fasteners, sheathing, and roofing
structure to deteriorate. During winter, the daily freeze-thaw
cycles can cause ice floes to develop under the roof surface.
The pressure from these ice floes will dislodge the roofing material,
especially slates, shingles, or tiles. Moreover, the buildup of
ice dams above the gutters can trap enough moisture to rot the
sheathing or the structural members.
Many large public buildings have built-in gutters set within the
perimeter of the roof. The downspouts for these gutters may run
within the walls of the building, or drainage may be through the
roof surface or through a parapet to exterior downspouts. These
systems can be effective if properly maintained; however, if the
roof slope is inadequate for good runoff, or if the traps are
allowed to clog, rainwater will form pools on the roof surface.
Interior downspouts can collect debris and thus back up, perhaps
leaking water into the surrounding walls. Exterior downspouts
may fill with water, which in cold weather may freeze and crack
the pipes. Conduits from the built-in gutter to the exterior downspout
may also leak water into the surrounding roof structure or walls.
Failure of the flashing system is usually a major cause of roof
deterioration. Flashing should be carefully inspected for failure
caused by either poor workmanship, thermal stress, or metal deterioration
(both of flashing material itself and of the fasteners). With
many roofing materials, the replacement of flashing on an existing
roof is a major operation, which may require taking up large sections
of the roof surface. Therefore, the installation of top quality
flashing material on a new or replaced roof should be a primary
consideration. Remember, some roofing and flashing materials are
not compatible.
 Because of the roof's visibility, the slate detailing around the dormers is important to the character of this structure. Photo: NPS files. |
Roof fasteners and clips should also be made of a material compatible
with all other materials used, or coated to prevent rust. For
example, the tannic acid in oak will corrode iron nails. Some
roofs such as slate and sheet metals may fail if nailed too rigidly.
If the roof structure appears sound and nothing indicates recent
movement, the area to be examined most closely is the roof substrate--the
sheathing or the battens. The danger spots would be near the roof
plates, under any exterior patches, at the intersections of the
roof planes, or at vertical surfaces such as dormers. Water penetration,
indicating a breach in the roofing surface or flashing, should
be readily apparent, usually as a damp spot or stain. Probing
with a small pen knife may reveal any rot which may indicate previously
undetected damage to the roofing membrane. Insect infestation
evident by small exit holes and frass (a sawdustlike debris) should
also be noted. Condensation on the underside of the roofing is
undesirable and indicates improper ventilation. Moisture will
have an adverse effect on any roofing material; a good roof stays
dry inside and out.
"Roofing" an Historic Preservation Brief
