Water, Water & Water vs Built Structures
It is commonly believed that if an object or structure is simply protected from the outdoor elements that it will then be preserved forever. Unfortunately this is only partially true, as there are multiple factors that can contribute to the breakdown and decay of nearly everything built by humans.
Once a rock gets cut, carved, tooled, chiseled or otherwise worked in some manner it is then referred to as a stone. Stones in general resist decay better then most other materials derived from nature. But stones also vary greatly in there durability. This is largely dependent on the specific chemical makeup of the rock from which the stone was quarried. The harder and denser the stone, the stronger and more enduing it will tend to be.
For this reason granite it used primarily in most modern outdoor applications, when stone is still employed.
Many other factors can also affect the rate of erosion and the durability of stone. These factors include but are not limited to; climate and micro climatic conditions, the way in which the stone was cut, how it was placed into the wall and with what type of mortar it was bedded.
During the middle to the later part of the nineteenth century solid masonry buildings became more costly to build, and new technological advances offered alternative construction techniques. This enabled taller buildings to be built at a faster rate, with a lower overall material cost.
First invented in England, followed quickly in the United States after the Civil War, a new form of construction in which the masonry was used only as a facing material became increasingly common.
Nearly all stone faced buildings constructed since around 1900, are really just a façade of stone, which is a veneer on the outside, with a steel frame, cinder blocks, or some type of hidden structural support underneath. The term, curtain wall construction, is used today to describe buildings that may be faced with stone, but they may also be covered with glass or other materials to form an outer shell.
Since ancient times the most enduring form of construction was always stone and masonry. This was mainly due to its strength, its resistance to weathering, and the fact that it would not burn in a fire.
Solid masonry structures can be made in varying manners. They may include brick or rubble interiors to reduce the cost or to increase strength, but overall they are all composed from stone, brick, tile, mortar, cement and masonry.
Although many of the following principles may also apply to any structure, the primary focus is regarding historic masonry structures and monuments.
With any historic masonry structure there are three
major causes of deterioration which are water, water, and water. In reality it is a little more complex, and other factors can also play a roll, as previously mentioned, but the vast majority of the time, this old adage still holds true.
The first water is from above, and in a structure it will do the most harm if somehow leaks are allowed to exist, which will then cause water infiltration into the interior of a structure. On a monument without internal space the water from above will still wet, weather, pelt and slowly wear away the surface of any stone, although with a hard granite it may take thousands of years to do the same level of weathering which would occur in a couple of years on a soft limestone. Never the less, biology will eventually win out over geology, and in due time even granite will slowly erode and be washed into the rivers. Then at some distant future date thousands of years down the road, it will then turn back into a sedimentary rock like sandstone. In a wood frame structure water leakage from above acts even more quickly causing many form of rot, mold and decay.
The second source of possible water entry is from below, and is commonly referred to as rising damp. Picture the way the wick in an old fashion lantern draws the fuel up to at least a few inches above the oil level. This type of capillary rise is often greatest on a porous stone such as sandstone, which will absorb a great deal of water. Rising damp occurs the least on a hard stone with very low water absorption such as a hard piece of granite. In historic structures built in areas on a high water table, rising damp can be a silent killer. Water and moisture will be drawn up and into the walls, each and every time the ground surrounding and under the foundation gets wet. Once this moisture gets into the walls it will not easily be able to escape. It can and will rise high and deeply into walls, and if they are solid masonry, this moisture can be trapped in the walls indefinitely. Even with wood frame structures rising damp can have devastating affects. After rising up through a foundation it will end up inside the walls, and then if it is unable to evaporate outward due to tar paper or modern siding materials it will only be able to push inward into the interior space, or stay trapped in the walls.
In the case of a monument or gravestone, rising damp is the primary cause of destruction to many sandstone, and limestone tablet stones which act exactly like one piece wicks.
The moisture will rise up to the point where the evaporation rate equals the capillary rise, so at about 8- 24 inches above the ground level or grade the moisture will push to the outdoor air and take the stones mineral binders along for the ride. After thousands of wet and dry cycles, and in many regions freeze thaw cycles, the stone will begin to spall and then crumble from the inside to the outside. The outer shell of a gravestone can become case hardened, with a friable soft stone crumbling internally, until it eventually spalls out to the surface taking the stone with it.
In a building, the rising damp may or may not be visible from the outside, or it may manifest itself only on the interior walls. Historically a layer of slate was placed above the foundation at the base of the walls to greatly reduce the moisture rise above this point.
Many modern interventions have been attempted to help protect historic structures suffering from this condition with greatly varying success rates.
The third source of water is in the form of moisture that can build up inside a structure even without a leak or rising damp problem. This is often caused or increased by adding insulation and modern windows that seal the interior space and do not allow breathability. Combined with modern heating and cooling that was never historically possible, condensation can build up and cause major damage to the interior of historic structures.
If a structure or monument is sealed from the outside with some type of protective coating it must also be breathable.
Have you ever worn an old style plastic rain coat when its hardly raining out, you well get wetter from your own sweat, then from the rain. This is how the inside of a building feels when it can’t breath.