MEASURZMENT OF The method of deter mining the quantity of brick masonry is governed by voluminous trade rules or by local customs, which are even more arbitrary than those for stone masonry (§ 585, which see).
The quantity is sometimes computed in perches, but there is no uniformity of understanding as to the contents of a perch. It ranges from 161 to 25 cubic feet.
Brick-work is occasionally measured by the square rod of ex terior surface. No wall is reckoned as being less than a brick and a half in thickness (13 or 131 inches), and if thicker the measurement is still expressed in square rods of this standard thickness. Unfor tunately the dimensions adopted for a square rod are variable, the following values being more or less customary: 1611 feet square or 2721 square feet, 18 feet square or 324 square feet, and 161 square feet.
The contents of a brick wall are frequently found by multiplying the number of cubic feet in the wall by the number of brick which it is assumed make a cubic foot; but as the dimensions of brick vary greatly (see § 83), this method is objectionable. A cubic foot is often assumed to contain 20 brick, and a cubic yard 600. The last two quantities are frequently used interchangeably, although the assumed volume of the cubic yard is thirty times that of the cubic foot. The former value is about correct for the average brick.
The volume of brick masonry is frequently stated in thousand bricks, the contents being obtained by measuring the area of the face of the wall and allowing a certain number of bricks to each square foot, the number varying with the thickness of the wall. A 4-inch wall (thickness = width of one brick) is frequently assumed to contain 7 bricks per sq. ft.; a 9-inch wall (thickness = width of two bricks), 14 bricks per sq. ft.; a 13-inch wall (thickness = width of three bricks), 21 bricks per sq. ft., etc.; the number of brick per square foot of the face of the wall being seven times the thickness of the wall in terms of the width of a brick. The size of bricks differs
materially in different localities, but not infrequently the above relations are employed even though they are considerably in error for a particular size of brick.
Not infrequently the contents of the wall and also the number of bricks laid are stated in thousands of bricks wall measure, in which case the volume is computed as in the preceding paragraph; and sometimes the number of bricks laid is stated in thousands of brick kiln count, i.e., the number of brick actually purchased which, on account of breakage, is 1 to 5 per cent more than the number actually laid, according to the quality of the bricks and the number and the size of openings.
Since well-established custom has all the force of law, unless due notice to the contrary is given, the only relief from such arbitrary, uncertain, and indefinite customs is to specify that the masonry will be paid for by the cubic yard,—gross or net measurement, according to the structure or the preference of the engineer or architect.
In making estimates for the number of bricks required, an allow ance must be made for breakage, and for waste in cutting brick to fit angles, etc. With good brick, in massive work this allowance need not exceed 1 or 2 per cent; but in buildings 3 to 5 per cent is none too much.