This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol2", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
(Contributed by W. H. Brown, F.S.I.)
The preparation of a Bill of Quantities consists of three operations, known as "Taking off," "Abstracting," and "Billing," and in this and the following chapters it is proposed to describe generally these three processes, together with the modes of measurement in each trade. First of all, a thorough knowledge of every detail of construction is absolutely necessary, and of the trade method of carrying it out, and this knowledge requires to be kept constantly up to date in order to avoid obsolete modes of measurement. Take, for instance, the case of drilling, say, twelve equidistant holes in an iron or steel plate. Formerly this would have taken twelve times as long as drilling one hole, but at the present time the twelve drills would be set to the required pitch, and the twelve holes drilled in one operation; from which it will be seen that if these holes were simply numbered without any note as to their position, the Contractor could not price them correctly. The great aim of the Quantity Surveyor should be to so measure and describe each item that the Builder will be able readily to understand it and accurately assess its value. If this principle be constantly kept in mind it will help the Surveyor over many a difficulty, especially where an unusual mode of construction is shown or specified.
It is proposed to deal with the three operations of "Taking off," "Abstracting," and " Billing" in the order in which they are performed, - the first operation being described as Taking off if the dimensions are taken from the drawings, and Measuring if they are taken from the actual building.
"Taking off," then, consists in taking the dimensions of the various items from the drawings and entering them with their proper descriptions on "dimension paper," each page of which is ruled for two columns of dimensions as below; and when the dimensions are taken from the actual building they are usually entered in a dimension book similarly ruled.
1. B. inM.
The first column on the left is called the Timesing column. Where the same dimension occurs two or more times, the number of times is entered in this column. The next column is for the dimension, and the third column, called the Squaring column, is for the products of the dimensions. For example, "twice 242 feet by 20 feet one-brick wall in mortar" would be written as shown above. Dimensions should always be written in the same order to facilitate reference, namely, length x width x height.
In taking off, system is of the utmost importance. There are three main systems, differing in the order of taking off. The first may be described as "taking off in trades," that is, each trade is taken off separately. This is a most laborious method, and is pretty well obsolete among practical surveyors, as items are likely to be overlooked; moreover, it makes future reference and consequently the adjustment of variations very tedious operations. On the other hand, it facilitates the work of the Abstractor, and indeed in some of the trades an abstract can often be dispensed with altogether. This, however, is not an important consideration, as the time of the "Taker-off" is much more valuable than that of the "Abstractor."
The second system consists in taking off each portion of the work complete in all trades. For instance, window openings are taken off complete, starting with the sashes and frames, glass and furniture, continuing with the deductions of brickwork, facings, plaster, and paper, and working up to the lintels, arches, sills, and finishings, such as architraves and window boards. This system saves time for the "Taker-off," greatly lessens liability to error, and facilitates the adjustment of variations at the completion of the job. On the other hand, it takes much longer to abstract.
Speaking generally, the most convenient system is probably the third - a modification of the last, the main difference being the separation of the carcase and finishings. This facilitates the subdivision of the work, and is particularly advantageous in a job where there is much variation in the character of the finishings, as these can be taken room by room. It also lessens the labour of abstracting, without rendering the adjustment of variations unduly difficult.
In large works, where several Takers-off have to be employed, the character of the building must decide the easiest method of subdividing the work, and no hard-and-fast rule can be laid down.
The Taker-off should use plenty of headings on his dimension sheets, so that any item can be instantly located within a few sheets. In a large job, or one with much variation in the height of walls, it will be found advantageous to take off a wall at a time, under a proper heading, such as North Wall of Main Building. This will probably mean rather more dimensions, but these will be written down in less time than it takes to make big collections and think out the various additions for extra heights, besides which there is less liability to error, and much time is saved when it comes to adjusting variations at completion, making System No. 3 quite equal in this respect to System No. 2.
After giving briefly the order of the two systems Nos. 2 and 3, it is proposed to adopt the latter as being the more convenient for our purpose, and to explain the modes of measurement as far as possible in the order of that system, rather than deal with them under the various trades, as is done in most of the text-books. The student will thus be enabled to follow the subject through in the order in which the work is actually done, and each item will appear in its proper relation to that portion of the building to which it belongs.
The following list is not intended to comprise every possible item which a building might contain, but to give such an idea of the system that the student may readily be able to allot any item not mentioned to its proper place in the list:-