A coating is bought by mass; a membrane is usually bought by rolls. The Whatson Waterproofing Calculator supports both methods, but their consumption inputs mean different things.
The estimate starts with a net treatment area. It does not find that area from a roof plan or automatically include upstands, pipe details and other changes in level.
For a liquid coating, read “per coat” carefully
Enter the area in square feet or square metres. Select coating, then enter coat count, kg/m² consumption per coat and container mass in kg.
If the product information gives consumption for the entire specified system, do not enter it as a per coat figure and multiply it again. Check the stated basis and required build up first.
The example defaults are editable assumptions, not a manufacturer’s application specification. A quantity result cannot establish that the chosen system suits the substrate.
Eight containers in the checked coating example
For 1,000 sq ft, use two coats, assumed consumption 0.8 kg/m² per coat, 20 kg containers and a 5% allowance.
The calculation gives 156.08 kg and rounds the purchase to eight containers. Those containers hold 160 kg in total.
The surface area is approximately 92.90 m². The calculation multiplies it by both coats before adding the allowance; you do not need to enter twice the roof area as well.
Membrane mode uses effective roll area
In membrane mode, roll length and width are metres, with overlap entered in metres too. The model subtracts the entered overlap once from each roll dimension, then estimates quantity by effective area.
For a 10 × 1 m roll with 0.1 m overlap, effective area is 9.9 × 0.9 = 8.91 m². The same 1,000 sq ft area with 5% allowance gives a checked estimate of 11 rolls.
That is an area based approximation, not a cutting layout or a design for corner laps. The roll geometry and approved detailing may require additional material.
What to add before procurement
Measure upstands and details that will actually be treated. Check whether primer, reinforcement, sealants and finishing layers are part of the system; they are not separate quantities in this tool.
The price field means PKR per container in coating mode and PKR per roll in membrane mode. Switching method requires checking the rate as well as the product inputs.
Do not infer a waterproof roof merely from buying the calculated quantity. Preparation, specified thickness, joints, drainage and application requirements need their own review against the selected product instructions.