Calculate bitumen quantities for flat roof waterproofing, felt underlays, and bitumen membrane systems. Supports torch-on, cold-applied, hot-pour, and felt applications.
Oxidised (blown) bitumen is used for flat roof waterproofing. More viscous than paving grade, it resists flow at high temperatures and UV degradation.
SBS (Styrene-Butadiene-Styrene) modified bitumen offers better flexibility. Applied by torch-on flame or self-adhesive peel-and-stick. Typical life: 20–30 years.
Torch-on membrane: 4–5 kg/m². Cold-applied: 1.5–2.5 kg/m². Hot-pour: 3–4 kg/m². Felt underlay: 1–2 kg/m². Values per layer — multiply for multiple layers.
Enter roof area and application type to calculate
Standard application rates and uses for each roofing bitumen system.
| Application Type | Rate (kg/m²) | Layers Typical | Primary Use |
|---|---|---|---|
| Torch-On SBS Membrane (4mm) | 4–5 | 1–2 | New flat roof waterproofing |
| Cold-Applied Bitumen | 1.5–2.5 | 2–3 | Repair, flashing, detail work |
| Hot-Pour Bitumen | 3–4 | 2–3 | Built-up roofing, large areas |
| Bitumen Felt Underlay | 1–2 | 1 | Underlay for tiles/metal roofs |
| Self-Adhesive Membrane | 3–4.5 | 1–2 | Peel-and-stick waterproofing |
| Bitumen Mastic Coating | 0.5–1.5 | 2–3 | Surface protection, re-coating |
The calculator multiplies roof area by the application rate and number of layers to give total bitumen weight, then divides by roll coverage to estimate material rolls needed.
Roof area: 200 m². Application type: torch-on SBS membrane at 4.5 kg/m². Layers: 2.
Bitumen: 200 × 4.5 × 2 = 1,800 kg (1.8 t)
Rolls: ⌈200 × 2 ÷ 10⌉ = 40 rolls
Add 10–15% for laps, upstands and flashings → order approx. 44–46 rolls.
Roof area: 80 m². Application rate: 2.0 kg/m². Single layer.
Bitumen: 80 × 2.0 × 1 = 160 kg
Rolls: ⌈80 ÷ 10⌉ = 8 rolls
Cold-applied membranes are peel-and-stick — no gas torch required.
Roof area: 350 m². Three-layer built-up roof (BUR) at 3.5 kg/m² per layer.
Bitumen: 350 × 3.5 × 3 = 3,675 kg (3.68 t)
Rolls: ⌈350 × 3 ÷ 10⌉ = 105 rolls of felt interlayer.
Hot-pour oxidised bitumen (Type III or IV) is applied between each felt layer.
From residential re-roofing to large commercial flat roof installations.
Estimate membrane roll counts and total bitumen weight before ordering from the merchant. Use the roll calculator to avoid over-ordering on tight-margin residential jobs. Adjust layers and rate per your specific product data sheet to match the spec.
Quickly price roofing waterproofing packages for commercial and industrial flat roofs. Calculate total membrane weight for tender BOQ line items in kg or tonnes. The optional cost field lets you cross-check supplier quotes per kg against your budget rate.
Planning a flat roof extension, garage roof, or outbuilding? Enter your roof length and width in metres or feet to instantly see how many torch-on rolls you need and what the material will weigh for delivery purposes. Add 15% waste to your order for laps and upstands.
Choose the flat roof bitumen application type — torch-on SBS modified bitumen membrane, cold-applied self-adhesive membrane, hot-pour oxidised bitumen, or traditional felt underlay. Each system has different material quantities and application rates per square metre.
Measure the flat or low-pitch roof area in square metres or square feet. Include all sections of the roof deck. For torch-on or hot-pour systems, also note the number of layers required — a standard torch-on flat roof uses 2 layers (base + cap sheet), with a total bitumen content of approximately 3.5–4.5 kg/m².
Click Calculate to get total bitumen membrane rolls (standard 1m × 10m rolls = 10 m² coverage), total bitumen weight in kg or tonnes, and estimated cost if a price is entered. Add 10–15% for laps, upstands, flashings, and waste. Always check roll coverage with your specific product data sheet.
Torch-on bitumen roofing uses a gas torch to heat and bond layers of SBS (styrene-butadiene-styrene) or APP (atactic polypropylene) polymer-modified bitumen membranes to the roof deck. The heat melts the underside of the membrane, creating a waterproof bond. SBS-modified membranes remain flexible at low temperatures, making them suitable for all climates. APP membranes have better high-temperature performance.
A standard two-layer torch-on roof system uses approximately 3.5–5.0 kg of bitumen per square metre total (base layer + cap layer). Hot-pour oxidised bitumen applied between felt layers uses approximately 2–3 kg/m² per pour. Cold-applied self-adhesive membranes typically contain 1.5–2.5 kg/m² of bitumen depending on membrane thickness. Consult your product data sheet for the specific application rate.
SBS (styrene-butadiene-styrene) modified bitumen remains flexible at temperatures down to -25°C, making it ideal for cold climates. APP (atactic polypropylene) modified bitumen has excellent UV and high-temperature resistance, better suited for hot climates. Both have better performance than oxidised bitumen (also called "blown" or "hard" bitumen). Most modern roofing membranes use SBS modification with a polyester or fibreglass reinforcement carrier.
A standard two-layer torch-on SBS membrane system on a 200 m² flat roof requires approximately 1,800 kg (1.8 tonnes) of bitumen membrane at 4.5 kg/m² per layer × 2 layers. You will need roughly 40 rolls of standard 1 m × 10 m membrane (20 m² per roll for two layers). Add 10–15% for side laps, end laps, upstands at walls and penetrations, and flashings — order approximately 44–46 rolls. For a single-layer system (repairs or cap sheet only), halve the quantities.
Standard torch-on SBS membrane rolls are typically 1 metre wide × 10 metres long, giving 10 m² coverage per roll. To calculate rolls needed: (Roof Area × Number of Layers) ÷ 10, then round up to the nearest whole roll. Add 10–15% for overlaps (typically 100 mm side laps and 150 mm end laps), upstands to walls, and detail work around penetrations such as pipes, vents, and skylights. Never underestimate your roll count — running out mid-job is a costly problem.
Yes — always add a waste allowance of 10–15% to your calculated quantity. Waste on flat roofing comes from: side laps (typically 80–100 mm per roll), end laps (150 mm), upstands turned up walls and parapets (minimum 150 mm), detailing around all roof penetrations, trimming at roof edges, and occasional off-cuts from irregular roof shapes. Complex roofs with many penetrations (plant rooms, HVAC units, pipes) may require up to 20% waste allowance. It is always cheaper to have one extra roll than to stop work and re-order.
A properly installed two-layer SBS torch-on modified bitumen roof system typically lasts 20–30 years when installed to manufacturer's specifications with correct detailing at all upstands and penetrations. The main factors affecting lifespan are: quality of installation (correct torch technique, full bond, proper laps), adequate drainage to prevent ponding water, UV-reflective mineral surfacing or coating, and maintenance of flashings and penetration seals. APP-modified membranes with factory-applied mineral granule surfacing can last 25–30 years in hot climates due to better UV resistance. Regular inspections and prompt repair of any damage extend service life significantly.