Waterproofing membrane calculator
Waterproofing membranes are laid with overlaps at the side and end of every sheet, so you always need more membrane than the area itself. The calculator adds the overlap and divides by the area of a roll.
Your measurements
Result
- Area incl. overlap
- 55.00 m²
- Rolls needed
- 6 pcs
One layer of membrane. Flat roofs and terraces usually get two layers of bitumen membrane — double the area.
How the calculation works
- Area with overlap = area to waterproof + overlap percentage (10% by default).
- Roll area = width × length (for example 1 × 10 m = 10 m²).
- Rolls = area with overlap ÷ roll area, rounded up.
Worked example
For a 50 m² terrace with 10% overlap you need 55 m² of membrane. With 1 × 10 m rolls that is 55 ÷ 10 = 5.5, so 6 rolls.
What the calculator assumes
- 10% covers side laps of 8–10 cm and end laps of 10–15 cm.
- The calculation is for a single layer.
When to adjust the result
- Terraces and flat roofs usually get two layers of bitumen membrane — double the area.
- The membrane turns up parapets and walls by at least 15–20 cm: add perimeter × upstand height.
- Bitumen primer (about 0.3 L/m²) and corner and outlet details are not included.
Waterproofing details that matter
Most leaks start at junctions, not in the middle of a surface. Reinforce internal corners, wall–floor junctions and pipe penetrations with the system's sealing tape and collars before applying the main coats. Liquid membranes need the full thickness stated by the manufacturer, which usually means two coats in different directions.
Apply only on a clean, sound, appropriately dry substrate with the primer the system requires, and respect drying times between coats. In wet rooms, take the membrane up the walls well above splash height and behind the whole shower area, then lay tiles with adhesive compatible with the membrane.
Frequently asked questions
How much overlap for bitumen membrane?
Typically 8–10 cm along the sides and 10–15 cm at the ends — follow the membrane manufacturer's instructions.
One or two layers on a flat roof?
Two-layer bitumen systems are the norm on flat roofs and terraces; single-ply synthetic membranes are laid in one layer.
Liquid membrane or sheet?
Liquid membranes follow complex shapes and are common under tiles; sheets give a uniform thickness and suit large flat areas. Follow one system's rules from primer to tape.
How thick should a liquid waterproofing be?
As stated by the manufacturer for the use — typically a minimum dry thickness of around 0.5–1 mm in wet rooms, applied in at least two coats.