Stone Masonry
Next to plain dirt, stone (or rock) is the least exploited of all materials for building construction. And like earth–which has been used for centuries in building walls, floors and roofs–rock is most readily available at little or no money cost. It can be gathered (usually free for the hauling) from any streambed, from abandoned mines and quarries, or from open fields and embankment cuts. There is hardly a region in the country that doesn’t contain a substantial resource of building stone.
Maps and aerial photographs of one’s region are generally available, and can be employed to advantage in locating building stone. Agricultural soil maps are revealing and thorough. Geologic maps indicate existing pit and quarry sites as well as the type and structure of the rock. U.S. Coast and Geodetic Survey maps cover nearly every section of the country. They are especially helpful in locating abandoned ore mines. Tailings from mines are among the best sources of building stone. From aerial photos one can locate such rock-laden features as excavations, outcroppings, cliffs abandoned railroad and road cuts and natural streambeds.
With such widespread availability, one asks, why is building stone so rarely exploited by homebuilders? Because building with stone is similar to building with earth: There is a large “time” and “labor” factor involved in gathering and placing the material into a wall. But the average Owner-Builder’s time and labor resource customarily outweighs his capital resource, so this cannot always be considered a serious handicap.
Perhaps a more pertinent answer to this query lies in the fact that stone masonry technology–more than any of the other building trade skills–has been traditionally clothed in secrecy. Carl Schmidt, in his little book on Cobblestone Architecture, illustrates this point:
Several very old men, who as little boys saw cobblestone masons at work, readily recall the jealousies among the masons. Whenever a visitor appeared while they were working, they would stop work, hide their tools and do something else until the visitor went on his way. The fact that these men succeeded very well in keeping their own methods a secret, explains the different mannerisms found in the method of laying up the walls.
Through the centuries stone masons also have succeeded in maintaining a respectable, highly paid and somewhat apostolic status in the building industry. Their “trade secrets” are maintained to this day, and include such important items as an intimate knowledge of rock, the correct mortar proportions and use of auxiliary materials, the proper selection of tools and organization of work procedure and–finally–an esthetic awareness of the rock in place: The total effect and composition of the finished wall.
Intensive research on stone masonry reveals that no pertinent literature exists on the subject that is applicable to the unskilled Owner-Builder. Stone masons maintain their closed shop. In this chapter an attempt is made to close the enigmatic gap.
With fear of over-simplifying the stone masonry skill it should be stated that the foremost prerequisite of any mason worth his mortar is an intimate–nearly intuitive–knowledge of rock. Pick up a rock. Where the inexperienced observes color, weight and form, the experienced stone mason notices bedding, seams, rift and grain. He first visualizes the rock in place, laid on its natural bedding. Bedding is recognized by a granular change in color or texture. It is mostly prevalent in sedimentary rock, where changing conditions of deposition of sediment under water occur.
Bedding joints are horizontal, but seams are generally vertical, to the rock surface. Seams are regular in limestone and irregular in granite. They occur in rock as a result of compression and tension stresses in earth forms. The direction of greatest ease of splitting in a rock is called the rift . It may be parallel to the seam. A second, more minor, direction of splitting is called the grain. Only the most experienced mason can detect grain direction.
Several simplified systems of rock identification have been devised to assist the mason in his choice of building stone. Rock classification can be physical, differentiating between unstratified and stratified rock, or it can be of a chemical nature, dividing rock into its siliceous (sandy), agrillaceous (clayey) or calcareous (limey) composition. The classical classification of rock, however, is based upon geological origin–igneous, sedimentary and metamorphic. A composite classification system of the more common building stones, along with their significant construction properties is presented below.
This chapter is one of the few with no bibliography at the end. The dearth of books may be a continuance of the “closed shop” stone masonry conspiracy mentioned earlier: In any event there are no contemporary manuals on laying up building stone. The Audel reference text on masonry is typical of what is currently available: The stone masonry techniques and tools discussed date back to antiquity. The correct hammer and chisel are identified, as is the manner of squaring huge marble building blocks.
A number of unlikely research sources were used to compile this chapter; but primarily the actual stone-laying experience of the author over the past fifteen years forms the nitty-gritty of what is to follow.
The rock classification system illustrated above can prove of only general value to the Owner-Builder mason. Let’s have a closer look at choosing your rock and building with this natural resource. Accessibility of the rock must be one of the prime criterion. An expensive quarrying or hauling operation can be a deterrent sufficient to dissuade one from using this material in his building. In some instances a particularly hard rock is called for–as in floors and steps. Rock with cleavage (a splitting quality) is generally a more valuable characteristic than a block-like monolithic quality.
Of course we desire to build a durable wall, and one that will withstand rain, wind, frost, heat and fire. A building stone “life” ranges from 10 to 200 years. Frost damage is common to softer and porous rock. Again, if rock is not laid on its natural bed-face, frost action will tend to laminate the layers. Another important rule: The strength of the mortar should equal the strength of the rock. An excessively rich mortar is more pervious than a weaker mortar because shrinkage-cracking occurs in rich mortar. Mortar joints are the most vulnerable part of the wall to moisture penetration.
Granites are the least affected by weathering: Limestone and sandstone the most. They are commonly destroyed by surface erosion (from sea salts, for instance) and atmospheric pollution. Rain will leach the cementitious materials found in some sandstone to the surface, where they become brittle, weak and finally flake off.
A number of stone preservatives are available, designed to protect rock from the aforementioned frost and moisture penetration hazards. A waterproofing agent prevents the penetration of moisture but the moisture that does gain access into the wall is not permitted to escape. This is bad. The wall should “breathe”, whatever material is used. Moreover, the outer waterproofing layer is a thin skin which differs in physical properties from the underlying material. This difference causes certain stresses to be set up which inevitably force the outside skin to flake off.
One may reason that strength should be the foremost requisite of rock for building purposes. Rock that is sound and suitable in other respects, however, is almost invariably strong enough for use in a wall. Recent tests at the U.S. Bureau of Standards on samples of Montana quartzite indicated a compressive strength of 63,000 pounds per square inch (a rather typical rock strength). A structure of such material would have to be over 10 miles high before failure would occur from crushing the lower courses!
Another good example of structural strength is illustrated in the 555-foot high Washington Monument. Pressure at the base course is 700 pounds per square inch; but marble will sustain a crushing load of 25,000 pounds.
The appearance of your dwelling should not be underestimated when choosing a building stone. Every rock has its unique color and rock of different color can be mixed in a wall. Every rock also has its unique lustre, be it vitreous, pearly, resinous, dull, metallic or whatever. Rock containing much iron should be avoided, since stains caused by oxidation of iron under atmospheric influence will discolor the mortar.
Some rock can be “worked” better than others. Angular, square-edged, quarried rock “lays up” better than roundish cobblestone boulders. The last are sometimes called “rolling stones”, because they are loosened and weathered from the parent ledge by natural processes.
Workability depends as much upon the correct mortar mix as it does upon the type of rock laid. A proper mortar is weather resistant and has adequate bond strength and compressive strength. The proportion of sand, cement, fire-clay–and especially–water must be controlled to within a narrow margin. The optimum proportion is 12 shovels of clean, washed concrete sand, 4 shovels common cement and 2 shovels fireclay.
To give you a better appreciation of this optimum proportioning, make a trial mix in the kitchen, using oven-dried sand in a measuring cup: 12 oz. sand; 4 oz. cement; 2 oz. fireclay; 5 oz. water.
The actual process of laying stone consists, first, of spreading a uniform layer of mortar, then forcing stone into its bed (a bed can also refer to the top or bottom of a stone). The mortar should be stiff enough to support the stone without letting it touch the stone underneath.
A bedding trowel is used by stone masons for spreading large mortar beds. Unlike a brick mason’s sharp-pointed trowel, the bedding trowel has a rounded end. Two sizes are commonly used. The 2 1/2 cubic foot detachable steel drum concrete-mortar mixer sold by Sears is entirely sufficient for either small or extensive masonry work.
After a course–or layer–of stone is laid, the wall behind the facing stone must be carried up, to give support to the face. This is termed backing and usually consists of a cheaper class of masonry, or poured concrete, bonded directly to the face. Bondstones act as ties, bridging face to backing. Metal strips–masonry ties–are also commonly used to tie the face to the backing.
The simplest, fastest, and in all respects neatest type of stone masonry pattern for the Owner-Builder to work is called “cyclopean” masonry. Various sizes and shapes of stone are used in cyclopean masonry, with no respect to regular courses. Joints–spaces between stones–look best cut deep. A 1/2 wide tucking trowel is used for this purpose. Master stone masons can be rightfully proud of their time-consumed “varicose vein” joints, but the effort required is not compensated for in the final result.
There are several design features of cyclopean masonry that are essential to wall that “reads” well. First of all it is essential to break the joints. Then too, rock sizes should be well proportioned and graded from the small “spans” (rocks filled into spaces too small for regular sizes) to a larger size stone that is proportional to the size of the wall. Triangular-shaped stone, or long sliver-like specimens should be placed so as to give a directional vitality to the wall. A triangular stone with the apex pointing down gives a more dynamic impression than if it points up. A common error most amateur stone masons make is to congregate the larger size rock near the base course and finish the upper portion with progressively smaller and smaller sizes. This is no design or style–looks like the builder ran out of good material.
Corners are always set first in wall work, and edges are laid first in flatwork (such as slate floors). Below is an illustrated sequence of stone laid in a typical wall panel.
Once the basic mechanics of stone laying are mastered, design subtleties in rock can be incorporated which add immense interest to the building as well as enhanced value to the masonry. The 8-foot square stone mosaic in our living room was constructed of white granite, brown sandstone, black slate and blue river-rock. Except for such art-panels, the problem of combining stone is generally a ticklish matter. Colors should be harmonious. Ordinarily only stone of similar hardness should be used.
In a building, a harmonious interplay of stone, wood and glass is always sought. Stone should contrast with these other building elements as well as with the native surroundings. On a sloping site, for instance, a massive stone foundation wall binds the building to the sloping terrain; it links the natural landscape to the formal discipline of the building.
Success in building with rock is not easy. But no material blends as well with the natural environment or reveals the personal artistry of the builder.
Stone walls should be treated with respect to the shape of the building. The recently completed Woolman School social hall (Nevada City, California) is a good example. It has circular, cloverleaf-like wall panels and the roundish building stone reaffirms the curvilinear motif. The outside concrete walls of this building were constructed with a sliding horizontal slip form, then faced with stone with a layer of fiberglass insulation between. Barbwire ties embedded in the concrete wall, form concrete and stone facing into a homogeneous mass. The stone-faced circular slip form construction methods developed on this project is without doubt the best system for an inexperienced Owner-Builder to tackle.
The stone wall-panel illustration
(Click here to see illustrations) is an example of
better-than-average masonry. The rock forms are natural–and thus
restful–and rock sizes are pleasingly proportioned to the total size of
the panel. Triangular, square and various other shapes are thoughtfully
distributed to create a dynamic, readable composition. Deeply recessed
joints assist
the eye in its movement and re-grouping experience.
The
most obvious re-grouping consists of rocks 3, 11, 12, 10, 27, 18, 26
and 29. A readable directional quality is attained without lining-up
joints. Notice how rock 34 breaks the joint line between 19 and 33, 14
and 21, 6 and 16 and 8. Vitality is also achieved by strategically
placing triangular forms such as rocks 24, 11 and 30. The downward
pointing apex adds a dynamic “unbalanced” aspect to the composition.
A
final feature that qualifies this panel for professional status is the
thoughtful placement of base, corner and top rock courses. Top corner
rock 25, for instance, is more massive than bottom corner rock 1. Base
rocks 1 and 2 are powered over by corner rock 9. Top rock 30 compliments
its lower neighbor rock 32, thereby creating a re-grouping which
consists of rocks 30, 32, 20 and part of 18.
A
few detractive criticisms of this panel may also be in order: Rock 43
is the only spall, or fragment, used even though places exist for at
least a half-dozen more, such as between rocks 8 and 16 and between
rocks 13 and 12. Notice how beautifully spall 43 integrates neighboring
rocks 33, 38, 24 and 21. Corner rock 37 should never have been used:
The top slope makes it difficult to set the next corner rock 39. The top
corner rock 40 adds further to this conflict: Its effect is to wedge
rock 39 out at the top while at the same time it appears to be slipping
from its bed. The left-hand side of this panel has much more stability
and grace than the right-hand side.
The
sequence of rock laying is indicated numerically: Notice that one
begins at the left-hand corner and works to the right. Corners are
always set first and interior spaces then filled in. Generally larger
rocks are set first, with smaller ones filled in around them. It is
simpler to fit smaller rocks around large ones than it is to find a
place for a large one.
Large rock 26,
for instance, is bedded on rocks 17 and 18 and small rock 27 is set
after the cavity has been fully defined. Top rock 30 is temporarily
propped into position so that the top is level with the top of the wall.
A filler rock (32) is then found to fit the cavity. Small rocks,
especially spalls (see 43), are always set after the larger rocks are in
place.
