Waste Factors in Costing Building Projects

Waste is a product or material that is unwanted. waste can be defined as non-value-adding works. The term non-value-adding activity is used to differentiate between physical construction waste found on-site and other waste which occurs during the construction process Wastes have not been paid much attention in the current construction management. Previous studies have shown that significant amounts of project values were lost due to these wastes.

Adding Wastage Factor to Quantities

Estimators usually add some extra material to the quantities to prevent loss in project value. Waste factors are usually listed by a percentage of the actual quantity of material left in place. That is, if the actual quantity of concrete required is 100 m3 and the waste factor is 5%, the amount of concrete to be procured should be 105 m3. Waste factor is on top of and in addition to the calculated quantities obtained from quantity take off. the actual material quantities required for a work may be higher due to waste. Waste factor is used to increase material quantity, to ensure that enough material is procured to realistically complete the work.

How to determine the wastage factor

Finding the correct waste factor requires data analysis and cost tracking. Wastage will vary from job to job, and team to team. This factor can vary dramatically and is highly dependent on the performance and work procedures adopted by the crew as well as the design of the project. Keeping track of the costs of similar Projects and compare the ordered materials vs the installed ones can give us the wastage figure. Although there are a lot of factors involving in calculations, but having an average is a tremendous help. Setting up an Excel sheet to track and analyse your jobs can lead you to a figure of wastage. The following table is showing how to keep track of the Plasterboard:

Wastage Factor Calculation Table Free Sample

Knowing the Size of Plasterboard (2400 x 1200 mm), area of each sheet would be: 2.88 m2 By dividing the Total Area of plasterboard (usually can be find out by drawings) by the area of each board, the number of sheets needed can be calculated. But the actual no of sheets is being used on jobs are more than what needed. By dividing the No of sheet actually used by the number of sheets needed, we can calculate the wastage on each job.

Wastage Factor Calculation in Costing Projects Table Free Sample

As you see, the wastage is varying from job to job, because the complexity of the designs and the productivity of the team that installing the boards are different. But the Average is 8.07%. This can be a very good figure for our wastage factor. By adding of the Performance Level of the team to our table, we might find a more accurate figure:

Wastage Factor Calculation in Costing Projects Table Free Sample Experts

By analyzing the data on the above table, we can find the best answer for our wastage factor:

Wastage Factor Calculation in Costing Projects Table Free Sample

It’s also good to make notes of each job so you can tell if it was a simple job or all odd sizes for future reference. In order to purchase or order the correct amount of material, you need to know and an appropriate wastage allowance.

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How to use wastage factors in cost estimates

Most of the estimators use a Quantity Calc Sheet (also known as take-off sheet) to keep the records of the quantities and the calculations, as well as keep track of changes in plans or design. The result of the take-off sheet is being used in Bill of Quantity sheets. Most builders or building companies have their own take-off sheets that have been designed to meet their individual requirements. This is simply inked paper that's suitable for the presentation of dimensions, gathering of items and pricing by the estimator. By having wastage as a separate column in the Take-off sheet, we can easily add an extra allowance for wastage, an keep the records of them in our take-off sheet.

Download a Sample Takeoff Sheet

How to use wastage factors in Breinz

In Breinz Estimating Software, on the Item quantity page, if you use Calc Sheet (or take-off sheet) as the primary way of your calculations of the quantity (see the left panel on the image below) you can add wastage for any line for each individual Item.

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