ESTIMATING
What is an Estimate?
Before starting any work for it’s execution
the owner or client or builder or contractor should have a thorough knowledge
about the cost of the work that can be completed within the limits of his found
or probable cost that may be required to complete the work. It becomes
therefore necessary to prepare the probable cost or estimate for the intended
work from its plan and specification. Otherwise, it may so happen that the work
has to be stopped before its completion due to shortage of funds or materials. In
this reasons the above an estimate for any public construction work is asked to
be prepared and submitted previously so that sanction of necessary funds may be
obtained from the authority concerned.
Thus an estimate for any construction work may be defined as the process of calculating the quantities and cost of the various items required in the work. To prepare an estimate, drawings consisting the plan, the elevation and the sections through important points, along with a detailed specification giving specific description of all workmanship, properties and proportion of materials, are required.
Different Types of Estimates.
An estimate prepared from the plans and
specifications and consulting the current market prices of materials is never
the actual cost of work. Because the cost of materials and labour may vary
during the period of its actual execution or due to variations and
modifications of actual dimensions shown in the drawing or due to some
unforeseen contingencies. The difference between the estimated and actual cost
will depend upon the skill and accuracy of estimator. There are different types
of estimates and they are
1. 1. A Detailed Estimate.
2. 2. A Preliminary or approximate or rough estimate.
3. 3. A Quantity estimate or quantity Survey.
4. 4. Revised estimate.
5. 5. A Supplementary Estimate.
6. 6. A Complete Estimate.
7. 7. Annual Maintenance Estimate.
A Detailed Estimate: This includes the quantities and cost of everything required for satisfactory completion of work and this is the best of most reliable and mostly used that can be made. A detailed estimate is accompanied with (a) Report (b) Specifications. (c) Detailed drawing showing plans different sections, layout plan. Key or Index plan etc.. (d) Design data and calculations. (e) Basis of rate adopted in the estimate. Such a detailed estimate is prepared for technical sanction, administrative approval and also to execute a contract with the contractor. The method of preparation a detailed estimate has been in another article.
A Preliminary or approximate or rough
estimate: This is an approximate estimate made to
findout an approximate cost in a short time and
thus enable the responsible authority concern to consider the financial
aspect of the scheme for according sanction to the same. Such an estimate is
framed after knowing the rate of similar works and by the use of any one of the
following methods of estimate:-
a. (a) Unit rate estimate.
b. (b) Plinth area estimate.
c. (c) Cube rate estimate.
a. Unit rate
estimate: In this method all costs of a unit
quantity such as per k.m. for a highway per metre, of span for a bridge, per
classroom for school building, per bed for hospital, per litre for water tank
etc. are considered first and the estimate is prepared by multiplying the cost
per corresponding unit by the number of units in the structure.
b. Plinth area
estimate: In this method plinth area should be
calculated by taking the external dimensions of the building at the plinth.
Court area and other open areas should not be included in the plinth area. At
the beginning, when plan of a building has not yet been prepared or available
determine the total floor area of all the rooms corridor, verandh, kitchen,
W.C. and bath according to the requirement of the client, and of the total area
thus found, may be added for walls and waste to get the approximate total
plinth area. The plinth area thus found shall be multiplied by the plinth area
rate for similar type design and specification of building at the locality.
c. Cube rate
estimate: The method of estimating building cost
by the cubic metre (or cubic foot) of volume is more accurate in general, than
the method of plinth area method. Because cost of building depends not only on
plinth area but also on their respective height. The best of estimating cost by
the cubic rate is to find the volume of the building (length x breadth x
height) and then multiply the volume by the local cubic rate for similar type
of building. Length and breadth should be measured external to external
excluding plinth offset, corbelling, string course etc. The height should be
measured from the top of the flat roof (or half way of the sloped roof) to half
the depth of the foundation below the plinth. Parapet is not to be included.
A Quantity estimate or quantity Survey:
This is the complete estimate of the
quantities of materials that may be required to complete the work concerned.
Revised estimate: When a sanctioned estimate is likely to be exceeded by more than 5 percent either from the rates being found insufficient due to change to price level or from any cause whatever, except important structural alterations an estimate is prepared which is called a revised estimate. In case where important structural alterations are contemplated through not necessarily involving an increased outlay revised estimate should also be submitted for technical sanction. The method of preparation a revised estimate is same as the detailed estimate. A cooperative statement showing in an abstract from the probable variation or deviation of each item of work, its quantity, rate as compared with the original estimate stating the reasons of variations or deviation should be attached with it.
A Supplementary Estimate: While a work is in progress some additional works may be
thought necessary for development of a project which was not foreseen when the
original estimate was prepared and
the expenditure for such supplementary work cannot be meet up from savings
elsewhere within the Grant, an estimate is then prepared to cover up all such
works which is known as supplementary estimate. The method of preparation of a
supplementary estimate is same as that a detailed estimate and it should be
accompanied by a full report of the circumstances which render it necessary.
The abstract must the supplementary amount.
A Complete Estimate: This is an estimated cost of all items which are related to the work in addition to the main contract or to the detailed estimate.
One may think that an estimate of a structure includes only the cost of land and the cost of the main contracts or labour, materials and supervision. But there are many other cost items to be included. A picture of a complete estimate is diagrammatically shown as below.
Annual Maintenance Estimate: After completion of a work it becomes necessary to maintain
the same for its proper function and for the same estimate is prepared for the
items which require renewal, replacement, repairs etc. in the form of a
detailed estimate.
NEXT POST ON “HOW TO PREPARE DETAILED ESTIMATE”
