How to Calculate Bitumen Waterproofing for Roofs & Foundations (2026 Guide)
Bitumen waterproofing is a building's first line of defense against water — it protects foundations from soil moisture and salts, and roofs from rain leaks. Calculating it accurately prevents a material shortage during execution or a costly surplus. In this guide you will learn step by step: how to calculate the waterproofing area (for footings and roofs), the bitumen quantity per square meter, the four types and the rate of each, and the number of coats by code. All numbers here match exactly the free Bitumen (waterproofing) Calculator on “Site Engineer”. ⚠️ Note: this guide covers bitumen waterproofing specifically, not thermal insulation.
🧮 The Waterproofing Formula
- Footing area (full perimeter) = 2 × (length + width) × height + base (optional).
- Number of coats by code: SBC 2 · ECP 2 · BS 3 · AASHTO 2.
- Type rate: cold 1.5 L/m² · torch 4.5 kg/m² · sheet 3.5 kg/m² · cementitious 1.2 kg/m² — per coat.
- Waste fixed at 10% (×1.10). For torch-on: each 10kg roll covers 2.2 m².
⚠️ This calculation is for bitumen waterproofing only (not thermal insulation). The number of coats follows the chosen code, and the quantity is divided by the can/drum size for the number of containers.
📋 Steps to Calculate Waterproofing
- Determine the waterproofing area: For footings (full perimeter): 2 × (length + width) × height, plus the base area if it will be sealed. For roofs: length × width.
- Choose the bitumen type: Cold (liquid), torch-on (heat-bonded rolls), self-adhesive sheet, or cementitious — by location and budget.
- Set the number of coats: Follows the code: SBC, ECP, and AASHTO use two coats, and BS uses three.
- Calculate the quantity: Quantity = area × number of coats × type rate, then add 10% waste.
- Calculate the containers: Divide the quantity by the can/drum size (e.g. 20 L) and round up. For torch-on: area ÷ 2.2 = number of 10kg rolls.
- Prepare the surface and apply: Clean the surface and apply a primer, then lay the coats with overlap at joints, and turn the waterproofing up the sides.
✅ Worked Example
Waterproofing a footing 4m × 3m × 0.5m high (full perimeter + base), cold bitumen, SBC code (two coats), 20-liter can:
Enter the same values in the Bitumen Calculator and you'll get the exact same result — these values come from the calculator's own formulas.
📊 Bitumen Types & Rates (for the 19m² example, 2 SBC coats)
| Type | Rate (per m²/coat) | Unit | Qty for 19m² (2 coats) |
|---|---|---|---|
| Cold (liquid) | 1.5 | L | 62.7 L |
| Torch-on | 4.5 | kg | 188.1 kg (9 rolls) |
| Self-adhesive sheet | 3.5 | kg | 146.3 kg |
| Cementitious | 1.2 | kg | 50.2 kg |
🛡️ The Saudi Angle: Why Waterproofing Matters
In the Saudi environment, waterproofing is a necessity, not a luxury: foundations are exposed to soil moisture, salts, and chlorides that attack concrete and steel, and roofs are exposed to seasonal — sometimes heavy — rain leaks. Foundation waterproofing is applied before backfilling, directly on the sides and top, and roof waterproofing before finishing.
Tips: clean the surface well and apply a primer for adhesion; apply two perpendicular coats; ensure overlap at sheet/roll joints to prevent water ingress; and turn the waterproofing up the sides to stop water seeping in from the edges.
🚫 Common Waterproofing Mistakes
- One coat instead of two: the code requires two coats (three in BS) for reliable sealing — one coat leaves pinholes.
- Ignoring overlap: not overlapping sheet/roll joints creates leak paths — keep a sufficient overlap.
- Wrong type for the location: using cold bitumen on an exposed UV-hit roof, or skipping cementitious in tanks.
- Forgetting footing waterproofing before backfill: after backfilling it is hard to fix — seal the sides and top right before filling.
- No surface preparation: applying on a dirty or damp surface or without primer weakens adhesion and causes peeling.