Even before a building is completed, Nature begins to destroy it systematically. Gravity, wind, and seismic movements constantly test the stability of the structure. The ultraviolet wavelengths of sunlight fade organic building materials and break down their molecules. Rainwater dissolves carbon dioxide and sulfur dioxide from the atmosphere to form weak carbonic and sulfuric acids that eat away at stone and encourage the oxidation of metals. Where two adjacent metals of dissimilar electrolytic potentials are wetted by rainwater, a galvanic reaction occurs, generating electric currents that cause rapid decomposition of the anodic metal. Water encourages the growth of molds, mildew, and fungi that attack many building materials, particularly wood and wood products. Water also encourages several types of wood-destroying insects, as well as weeds, vines, and trees whose roots burrow into tiny building cracks and wedge them inexorably larger. Falling water spatters soil onto the lower reaches of exterior walls, nourishing insects and fungi. As water freezes in soil, it heaves and cracks foundations and pavings. Freezing water spalls chips from the surfaces of concrete and masonry. Wind transports dust, spores, and seeds and deposits them on buildings. Mice and rats set up residence inside and gnaw passages through a building. Domestic animals rub, chew, and scratch building surfaces and deposit decay-breeding excrement in obscure corners. The human inhabitants of a building do their share to tear it down, too, tracking in moisture and dirt, spilling, spattering, staining, charring, banging, scuffing, scratching, breaking, discarding debris, wearing out doors and drawers, keeping destructive pets, rearing destructive children, and creating smoke, soot, and cooking fumes that stain building surfaces. Nature bears no special grudge against buildings; for the most part these are the same natural forces that level mountains, divert rivers from their beds, change lakes into meadows and meadows into forests, and turn old materials into new ones throughout the natural world. Change is the constant factor in Nature. Birth, growth, maturity, decline, death, decay, and rebirth are the stages in all natural cycles. So too with buildings, but we humans like to keep the cycle under human control, to maintain each building in use until its death suits our purposes.
The forces of deterioration in buildings can be grouped into three categories: Some forces pose such strong or immediate threats to the usefulness of the building that they must be neutralized at all cost. Other forces are inevitable, but we can cope with them satisfactorily on a day-to-day basis. And paradoxically, certain forces of deterioration can contribute to the beauty and utility of the building—if we will let them.
Among the first category of forces, the most dangerous are those that threaten the stability of a building's foundations. We avoid frost heaving by founding the building below the deepest level at which the soil freezes in winter. We avoid excessive settlement by designing the foundations so that they do not exceed the bearing capacity of the supporting soil. To prevent erosion of the soil from around and under foundations, roof-water drainage systems must be kept in good working order, and any major pipe leaks inside or near the building must be repaired promptly. In dry, windy locations, plantings or other protective devices must be employed to protect the soil against wind erosion. Areas immediately adjacent to basement walls must be kept clear of trees to avoid root damage. Driven piles made of untreated wood should be periodically inspected to ensure that wells or pumps at nearby construction projects have not lowered the water table below the tops of the piles, for unless the piles are totally submerged, decay will set in.
If a building's foundations have settled but the building is not irreparably damaged, underpinning is usually an effective remedy, in which new foundations of larger capacity are constructed under or alongside the existing ones, and the building is jacked onto the new ones.
Structural weaknesses also fall into the "dangerous" category. Ordinarily a building's structure is strong enough to begin with, but if faulty design or increased loadings cause structural inadequacies when the building is being used, new beams, columns, or bracing can be added.
Structures require maintenance. Steel structural members must be protected from dampness and rust by either the surrounding layers of a building, or by the maintenance of paint or some other protective coating on all exposed surfaces. Bolted joints in timber frames need to be retightened to compensate for wood shrinkage after the building has been heated for an initial period after construction; access holes or access panels must be provided in any covering materials to allow this. Wood fungi and wood-destroying insects must be kept out of wooden structures. Some wood fungi, such as the blue-gray stain found on recently sawed, unseasoned lumber or the white flecks seen in some woods, are merely unsightly and cause no structural damage. Others, with colloquial names like dry rot and wet rot, are exceptionally destructive. Wood-boring and wood-eating insects, of which there are many species in various parts of the world, can endanger the stability of a structure.
Most wood-destroying organisms consume wood as food, need moisture, and need air. The chief means of controlling the organisms are to poison the wood, to keep the wood completely dry to deprive the organisms of moisture, or to keep the wood completely submerged in water to deprive them of air. Various chemical substances called preservatives are used commercially to poison wood against both insects and fungi. They are not very effective if merely applied to the surface and should instead be dispersed throughout the cells of the wood by pressure treatment in a factory. Certain species of wood are naturally resistant to insects and decay by virtue of chemical substances that grow naturally in them. Redwood, cypress, and cedar are the most common American woods with this property.
Keeping wood either dry or wet may seem like contradictory aims, but wood in either condition is safe from attack. Wood that is moist but not submerged or wood that is alternately wetted and dried offers both moisture and air to destructive organisms and is therefore strongly attacked. Wood that is in or very near to the soil is especially vulnerable. Wood laid directly on a brick or stone foundation should be protected from capillary moisture with a damp-proof layer of impervious plastic or asphalt. Wood beams resting in pockets in foundation walls should have similar protection and should be given plenty of space to "breathe" on all sides. It is wise to use preservative-treated wood wherever a wood frame comes in contact with a foundation wall.
Where one piece of wood is connected to another in an exterior location, rainwater is held between them by capillary action, and decay is rapid. The covered bridge illustrates one logical response to this problem: The hundreds of joints in the wood trusses supporting the bridge are kept dry by a waterproof roof and walls (21.1). If the bridge were not covered, it would become structurally unsound within several years because of the decaying of its connections. Where weather protection of wood connections is not possible,
Foundation preservative treatment, or at least a heavy application of paint, stain, or asphalt, can slow deterioration, but exposed wood connections should be avoided except in benches, railings, fences, and other secondary constructions. Even in these, periodic inspection and maintenance are required to replace rotting components before they become dangerous.
Termites are especially liable to attack a building that has untreated wood parts extending into the ground or one with wood scraps or tree stumps buried near the building. Termites are capable of attacking dry wood, however, by carrying water from the ground to the wood. The most common (and most damaging) type of termite found in the United States is the subterranean termite, which nests in the soil but takes its food from the wood of a building above. In order to reach the building, the insects build hollow shelter tubes of mud, wood dust, and excreta over the intervening foundation construction and then tunnel into the wood itself. Where termite infestations are common, the soil in the immediate vicinity of the building should be poisoned, and sheet metal termite shields should be installed between the foundation and the wood structure (21.2). The shields do not prevent the entry of termites, but they do cause the insects to have to build their shelter tubes over the shield, where they can be seen, rather than through cracks in the foundation. If tubes are seen on the shields, an exterminator can be called.
Dry wood termites, which need very little moisture, do not require contact with the soil and are found in wooden buildings in the tropics and subtropics. Infested buildings must be covered with plastic sheets and fumigated with poison gas. Other types of termites and wood-destroying insects are found in various parts of the world, each requiring its own set of preventions and cures.
Closely associated with decay and termite prevention is the prevention of water leaks in buildings. Aside from fire or earthquake, nothing can bring down a building faster than the internal decay caused by a leaky roof. Roofs, rainwater drainage systems, walls, and windows must be maintained diligently. Plumbing leaks and conditions of excessive condensation must be attended to quickly. In the ordinary wood-frame house, water damage and decay from condensation are common in wooden window sashes across the bottom of each pane of glass and in wooden subfloors beneath toilet tanks. Leakage problems and their associated water damage and decay occur frequently next to chimneys and around roof gutters. Through decay of the roof boards around leaks, such problems compound rapidly unless taken care of as quickly as they occur.
Roof maintenance includes keeping drainage systems operational, leakproof, and free of clogging debris, removing soil and growing plants that may settle in crevices, and inspecting for leaks or signs of deterioration. Shingles of any sort are gradually eroded by water, ice, and wind; are decomposed by the destructive powers of the sun; and may be split or torn by ice, wind, or tree branches. Membrane roofs erode less rapidly but are susceptible to damage by expansion and contraction of the roof deck, vapor blistering and rupture, and abrasion from excessive foot traffic.
The mortar joints in masonry are vulnerable to damage by the freezing and thawing of absorbed water. Masons can shape and compact mortar joints to make them shed water rather than trap it or absorb it (21.3). Even with such precautions, however, the mortar deteriorates progressively, usually over a period of many decades, and from time to time it is necessary to rake out the damaged mortar near the surface and repoint the masonry with new mortar. Climbing vines accelerate the deterioration of masonry because their roots penetrate the surface and their leaves keep it moist between storms. Building owners must weigh this factor against the undeniable charm of climbing vines, their contribution to summertime cooling through shade and leaf-surface evaporation, and their insulating role in winter.
Continuous inspection is required to maintain a building's fire hazards at an acceptable level. Rubbish must be cleared promptly. Chimneys, appliances, and the loads on electrical circuits must be checked periodically to ensure safe operating conditions. Exits must be kept clear. Fire doors must be kept closed but operable. Misplaced, missing, or deteriorated extinguishers must be replaced, and alarm and emergency lighting systems must be checked regularly. In certain types of buildings, especially schools, occasional fire-exit drills are advisable for the safety of the occupants.
Intolerable health hazards in plumbing systems include blocked or leaking pipes, pipes that are prone to freezing and therefore to bursting, and cracked or defective plumbing fixtures. Inadequate heating and a lack of natural light or ventilation are hazards sufficient to warrant vacating a building until they are corrected, as are accumulations of garbage or infestations of vermin. Window screens to keep out flies and biting insects are a necessity for health in most areas of the world.
Last in our discussion of critical maintenance problems in buildings are the human enemies of buildings, namely vandals and arsonists. When no ulterior motive is apparent, vandalism and arson seem to occur more frequently in buildings that are either in poor repair or abandoned. Inhabited buildings in good repair are likely to suffer damage only if they seem psychologically threatening to the vandals. Schools and low-income housing are frequent targets, especially if they appear authoritarian and depressing. Buildings in which each inhabitant feels a personal involvement would seem to
Weather- Resistant Concave
Non -Weather- Resistant Weathered
Raked Mortar Joint Profiles
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