Estimation and specification can be a difficult topic in the field of architecture and construction. But understanding the whats and whys of the process makes it very easy.
The most complicated type of estimation is the detailed estimate. Detailed estimate consists of complete and thorough calculation of the costs of every single resource and service of a project and should be delivered before the construction begins.
Using the centerline method, all the total lengths are added, after which the overlaps that occur in T-junctions are excluded.
In this method, the volumes of the structures are calculated first. After the values are obtained, it is multiplied to the rates of each material used, giving us the total amount required.
(This example only has details for brick wall construction)
Step 1: Calculate The Length Of The Walls
The first step is to get the total length of the structure. Since there are two types of walls in the plan, calculations must be done separately for each.
- Add the lengths of all sides from the center to center.
In the example problem, the plan of the wall structure is shown. For the 0.3 m thick wall:
- The total running length of all the walls, by adding the individual center-to-center lengths is 65.9 m.
For the 0.2 m thick wall:
- The total running length of all the walls, by adding the individual center-to-center lengths is 27.05 m.
Step 2: Classify And Tabulate The Different Structures
A basic construction consists of a superstructure and substructure. Substructure consists of foundation. Superstructure consists of everything above the plinth level, that is, the walls, DPC, plaster and whitewash.
- Earthwork and excavation: This refers to the excavation work done.
- Foundation: Foundation usually has the same measurements as earthwork and excavation.
- Plinth: Plinth is above the foundation.
- Walls: Different footings of walls should be taken separately.
- Damp proofing course (DPC): This course does not have thickness.
- Plaster and whitewashing: Plaster and whitewash, like DPC, has no thickness and is applied on the walls.
Step 3: Calculate Each Length By Subtracting T-Junctions
For each of the above categories, calculate individual lengths by using the given formula:
L= l-n*b/2 (where L is required length, l is the added length, n is the number of T junctions and b is breadth)
For example, taking earthwork for 0.3 m thick wall,
- l=65.9 m
- b=0.9 m
Therefore, L= 65.9-(6*0.9/2)= 63.3 m
Use the same method and formula to calculate the individual lengths for each of the elements in the table.
Step 4: Measure Each Structure And Tabulate The Values
In this step, the total volume of each structure is calculated.
- Note down the breadth and height of each from the section.
- The number of layers should also be noted. If there are two layers of DPC, then it should be counted twice.
- Multiply length, breadth, height and number of layers to obtain total volume of each.
Step 5: Subtract the volume of doors, windows and other openings
- Calculate the volume taken up by doors and windows from the walls.
- Subtract this value from the total volume calculated from the walls.
- For plasters, subtract the area (length*height)
Step 6: Calculate The Final Volumes
- Note down the volumes for each of the elements.
- Repeat the above steps for 0.2 m walls as well. In the above example the total final volume is
Step 7: Calculate The Amount For Each Structure
The last step is to calculate the rates.
- Refer to IS 7272 for the rates of each material.
- Multiply the volume with the rates to get the total amount.
- Add the amounts to get the final cost.
Step 8: Add The Miscellaneous Charges
A percentage of the cost goes to transportation and contractor's charges.
- Transportation charges: Around 5-10% of the total structure cost is added.
- Contractor's charges: Here again, 5-10% goes to the contractors.
- If any other charges are applicable, add those too.
- Add both these charges to the value obtained from the previous step to obtain the final cost.
Although these are the basic steps used to calculate detailed estimation, it has further steps, where the materials used to build the structures are taken separately and rates are calculated for them. Labour and service costs are also done separately.