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 and should not be used. Even latex paints, if applied in too
many layers, can become impermeable.
4.5 STRUCTURAL MAINTENANCE. The first problem in addressing a
structural problem is recognizing there is a problem. After many
years of painting, patching, additions, and alterations, the
structural problem often can go undetected.
Structural problems show up in the visual condition of
materials. Examples are cracks in foundations, warped or bowed
(See Figure 4-14.)
timber framing, and deteriorated materials.
Other indicators of structural damage can be sour odors or visual
fungus, which may indicate dry or wet rot. Dust piles, metal
shavings, or wood chip piles may indicate structural movement or
insect damage. In general, historic buildings do not need to
meet the same structural design requirements as contemporary
buildings. Accept moderate deflections; measure and monitor
crack dimensions and movement; and prudently assess the existing
condition.
The predominant damage to foundations is caused by movement
of piles or pile caps, rotting of wood piles, settlement, or a
change in ground water level. Therefore, there is an associated
change in the bearing capacity of the structure. The engineering
principles needed to fix a problem are best left to structural
engineers. A rule of thumb in dealing with foundation problems
is not to reinforce isolated areas without considering the
resultant forces on the rest of the structure.
For evaluating masonry damage, consider that mixtures of
mortar in historic structures were predominantly lime mixtures,
which are softer and less durable than those used in modern
construction. Portland cement mortar under movement conditions
will frequently split off brick and stone faces because it is
(As shown in Figure 4-14.)
stronger than the lime mortar.
Consider using lime mortars for repainting masonry.
Timber construction by its nature will droop or sag over
time. This by itself does not mean that the structure is not
sound. The problems with timber construction are found most
frequently when the wood is cut away to accommodate an
alteration. A second area worth investigating is the possibility
of bug infestation. If bugs are present and the timbers are
hollow or decayed, fumigation and application of bug-toxic
preservatives should be pursued. Fungus can deteriorate the
structure if the environment is not heated or cooled to prevent
the growth.
Wrought iron was the grandfather of steel by 100 years. The
early items used were fasteners, hardware, and various
compression members. Wrought iron was not frequently used as
tension members because of its brittle nature. By the early
1900s, steel became a material of choice because of its ability
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