Owner-Built Home & Homestead
(Page 7 of 15)
"Rubble" is the term usually applied to stone not machine cut or dressed. When such rough stones are built in courses with level beds, the wall is called coursed rubble. When the stones are of varying sizes and shapes, and laid without regard to the direction of the joints, the wall is known as random rubble. Sometimes the stones are roughly squared and faced and laid on level beds to form more or less continuous courses, and such a wall is known as dressed rubble. Whichever pattern is used, the finished wall should appear to be a unit rather than a conglomeration of rocks. The amateur stone mason frequently creates his "monsterpiece" by failing to observe a few common-sense design practices. First of all, rock shapes, colors and textures should be matched, allowing only enough variety for interest. In general, the larger stones should be placed in the lower courses, with small stones in the upper. Stones of the same size or shape should never be set side by side. The first layer of stones should be embedded a few inches into the wet foundations.
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The ultimate strength of a rubble wall depends as much upon the manner in which it is bonded or tied together as it does upon the quality of stone and mortar. At about every 5 feet in each course a "bond" stone long enough to extend through the wall should be placed. It is important also that successive courses break joints (vertical joints should never occur over those of the lower course). The same is true of horizontal joints (besides the importance of keeping horizontal joints as near level as possible). A good stone mason never sacrifices strength, as he might if thin stones were set on edge and then backed with loose fill. With a minimum 12-inch thick wall, stones should extend at least 4 inches into the wall. Stones just below an opening should be laid so as to spread the weight carried by the wall on either side of it.
Due to the irregular size of the mortar joints and the numerous voids that have to be filled in, a stone wall may contain as much as a third of its volume in mortar. Cement mortar for laying rubble stone should combine, in volume proportions, 1 part cement, 3 parts sand and 1/2 part fire clay. Hydrated lime is often used in place of fire clay, with more tendency to discolor the stones. Locally obtainable pit or bank sand is generally considered best for mortar, though river bed sand that is free from loam and vegetable matter also can be used successfully. For rough work, a combination of coarse and fine sand makes the strongest mortar. It is a good practice to moisten all stones prior to laying them in the wall, and for several days afterward.
The main element in any type of hand-laid stone masonry is simply time. Under average conditions a mason and his helper can place about 2 tons or 60 cubic feet of wall in one day. Including cost of labor and materials, the contract price for stone work starts at $2 a cubic foot. Consequently, our patent office contains hundreds of applications which purport to reduce this costly time element.
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