A Bill of Quantities should make it easier for a contractor to price a particular project since all the materials and work to be carried out is listed. Each contractor should be working from the same information therefore a fairer system is employed when pricing competitively. Less mistakes should be made compared to an estimator taking quantities off drawings since is this procedure the estimator is effectively making up his own bill of quantities.
STANDARD METHOD OF MEASUREMENT
The Standard Method of Measurement, 7th Edition, (SMM7) and the Code of Measurement Practice (MC), both agreed by the Royal Institution of Chartered Surveyors and the Building Employers Confederation, set out rules for the measurement and description of building work. The SMM is a document which provides not only a uniform basis for measuring building work but also embodies the essentials of good practice.
If all bills of quantities are prepared in accordance with these rules then all parties concerned are aware of what is included and what is to be assumed. Without the use of such a set of rules the quality of bills of quantities can vary widely.
THE PREPARATION OF A BILL OF QUANTITIES
The orthodox method of preparation of a Bill of Quantities can conveniently be broken down into two main processes;
1. The measurement of the dimensions and the compilation of the descriptions from the drawings and specification. This process is commonly known as taking-off.
2. The preparation of the bill. This involves the calculation of volumes, areas etc., (squaring the dimensions). Traditionally this was followed by abstracting to collect similar items together on another sheet called the Abstract Sheet. From this abstract the bill was written. This process is commonly known as working-up.
Dimensions and quantities are scaled or read from drawings and entered in a recognised form on a specially ruled paper called 'dimension paper'.
A typical form of dimension paper is shown below.
The timesing column is used when there are several similar items having the same measurements, and to indicate that the measurement is to be multiplied it will be 'timesed' as shown in the example below.
Measurements taken from drawings are set down in the dimension column.
The calculated volumes, areas, etc., of the measurements in the dimension column are set out in the squaring column.
The description of the work to which the dimensions apply is written in the description column.
There are two sets of columns in the width of a single A4 sheet as shown above. No written work should be carried across the central vertical division. There is usually a narrow binding margin (not shown) on the left of the sheets.
In building contracts the taking-off of dimensions is usually divided into sections under three main subdivisions:
(a) Carcass (b) Finishings (c) External Works
Plumbing and engineering services are taken as part of the Finishings section and Drainage as part of the External Works section of the Take-off.
A typical example of part of a take-off is shown below:
The descriptions should be clear and concise and include variables such as:
Physical dimensions, capacity, output, duty, pressure rating, material used, form of construction (e.g. sectional boiler), accessories, manufacturer's catalogue reference, British standard reference, other standard references, local agent, etc.
Some items are measured as 'extra over', that is they are not to be priced at the full value of all their labour and materials, as these have to a certain extent already been measured. For example fittings such as bends and junctions to drain pipes are measured as 'extra over'. This means that the pipe is measured along its full length including the fittings length and the estimator when pricing the item assesses the extra cost for the fittings as 'extra over' the pipe length.
This comprises the squaring of dimensions, as shown previously and transferring the resultant lengths, areas and volumes to an Abstract Sheet.
The purpose of abstracting is to split up the building into its constituent parts for measurement. Similar items are collected together and classified primarily into SMM sections.
Abstract Sheets are ruled with a series of vertical lines which are spaced about 25mm apart, and usually double foolscap width. A typical Abstract Sheet is shown below but is reduced to A4 size.
For small building services projects it may be possible to avoid using the Abstract Sheet since we don't use m3 of material . The linear lengths of pipework and materials and square areas of materials may be measured from drawings and put onto the dimension paper and transferred from this to a Bill of Quantities.
The writing of the bill is theoretically copying out the descriptions and quantities from the abstract or dimension paper. The bill of quantities has been traditionally divided up into work sections similar to those contained in the SMM as follows overleaf:
A Preliminaries/General conditions
E In situ concrete/Large precast concrete
G Structural/Carcassing metal/timber
K Linings/Sheathing/Dry partitioning
M Surface finishes
P Building fabric sundries
Q Paving/Planting/Fencing/Site furniture
R Disposal systems
S Piped supply systems
T Mechanical heating/cooling/refrigeration
U Ventilation/Air conditioning systems
V Electrical supply/power/lighting systems
W Communications/Security/Control systems
X Transport systems
Y Mechanical and electrical services.
These sections are further sub-divided into, for example:
R11 Foul drainage above ground
R12 Drainage below ground
S10 Cold water
S11 Hot water
S12 Hot and cold water (small scale)
S13 Pressurised water
T10 Gas/Oil fired boilers
T11 Coal fired boilers
T30 Medium temperature hot water heating
T31 Low temperature hot water heating
T32 Low temperature hot water heating (small scale)
T33 Steam heating
T40 Warm air heating
T41 Warm air heating (small scale)
T42 Local heating units
U10 General supply/extract
U11 Toilet extract
U12 Kitchen extract