How to calculate roll waterproofing quantities: from roof area to order

The area of your roof and the area of material you order are not the same. Here is how to account for overlaps, vertical surfaces and cutting when preparing a clear material request.

Why dividing the roof area by the roll area is not enough

Suppose the roof measures 200 m². If the roll label says 10 m², it is tempting to order 20 rolls and call the calculation complete. But some material will be used for overlaps and some will be lost during cutting. Additional material will be needed for parapets, penetrations and other details. If the design specifies two layers, each must be calculated separately.

To match the purchase to the project, start with the roof plan and the specified roof assembly, rather than the number of rolls.

Step 1. Gather the dimensions

Mark the length and width of each area, the height and length of abutments, and the positions of roof drains, pipes, roof access points and equipment. Divide complex shapes into simple geometric areas.

Calculate vertical areas separately: multiply the length by the height to which the material must extend, as specified in the design detail. Do not automatically deduct small penetrations: additional detailing and reinforcement may be needed around them.

Step 2. Separate the order by layer and product grade

If the assembly has a base layer and a top layer, list them separately. Roll dimensions, surface finishes and intended uses may differ, so a total in square metres is not yet a material specification.

The MARJA catalogue includes the ROOFIZOL and IZOMEMBRANE ranges, among others. The specific grade is selected to suit the designed assembly and service conditions. The number of layers is also determined by the design: there is no single arrangement suitable for every roof.

Step 3. Allow for overlaps and cutting

The nominal area of a roll is not the area it covers in the finished waterproofing layer. Adjacent sheets overlap along their edges, while short and complex sections produce offcuts.

A more reliable approach is to draw a sheet layout showing overlaps in accordance with the selected material’s instructions. A provisional percentage allowance can help with early budgeting, but it does not replace a layout. Material use on a simple rectangular roof will differ from that on a roof with extensive equipment.

Example of a preliminary calculation

Consider a hypothetical 200 m² roof and one of its waterproofing layers. Suppose measured abutments and details add another 24 m², and a 12% allowance is included for overlaps and cutting in the preliminary estimate. These are illustrative inputs, not MARJA consumption rates.

Calculation: (200 + 24) × 1.12 = 250.88 m². Assuming a nominal roll area of 10 m² gives 250.88 / 10 = 25.088. Round up to 26 rolls for this layer.

Calculate the other layer separately, using its roll dimensions and details. If overlaps have already been included in the sheet layout calculation, do not add the same percentage again. Agree a separate allowance for damage and unforeseen work with the contractor.

Step 4. Add ancillary materials

Primer, mastics, sealants and abutment components are not included in the area of the roll waterproofing. Create a separate line for each item, including its unit of measurement.

Calculate liquid material quantities using the area to be treated and the consumption rate in the specific product’s technical data sheet. Check whether the rate applies to one coat or the complete coating. Round up to whole packs, and verify that the product is suitable for the substrate and the selected system.

What to send MARJA for product selection

Prepare a dimensioned plan, photographs of the roof and its details, and information about the substrate, existing layers and building use. State whether the work is new construction or a repair, and attach the project specification if available.

A Consultation with a MARJA Specialist — Free of Charge

If you are not sure which material suits your project, give us a call. MARJA specialists will select the optimal solution, taking into account the type of structure, the climatic conditions of your region, and your budget.

Phone: +998 77 057 57 53

E-mail: info@marja.uz

MARJA — a manufacturer of waterproofing materials in Uzbekistan since 2004.
Quality. Warranty. Service.