About

Waterproofing a flat roof requires precise material estimation to ensure complete coverage without excessive waste. This tool calculates the requirements for Torch-on or Self-adhesive bitumen roofing felt systems. Crucially, it accounts for the "effective coverage" of each roll, as significant material is lost to mandatory overlaps.

Standard practice dictates a 100mm (4-inch) side lap and a 150mm (6-inch) end lap to ensure a watertight seal. Failing to account for this often leads to material shortages mid-job. The calculator also estimates the necessary primer (for surface adhesion) and bitumen mastic (for detailing edges and vents).

Formulas

Effective Area per Roll:

A_eff = (W_roll - Lap_side) × (L_roll - Lap_end)

Total Rolls Required:

Rolls = ceil(RoofArea × LayersA_eff) × WasteFactor

Reference Data

Material	Standard Consumption / Size	Usage
Underlay Roll	16m × 1m	Base layer
Cap Sheet Roll	8m × 1m	Top layer (mineral finish)
Bitumen Primer	0.3 to 0.5 L/m²	Prepares substrate
Mastic	3 lin. meters per cartridge	Sealing flashings

Frequently Asked Questions

A 2-layer system (Underlay + Cap Sheet) provides significantly better durability and waterproofing than a single layer. The underlay absorbs movement, while the cap sheet resists UV/weathering.

Industry standard is 100mm (10cm) for side laps and 150mm (15cm) for end laps (where the roll finishes). This ensures water cannot track back under the seam.

For concrete or timber decks, allow 0.3 to 0.5 liters per square meter. Porous surfaces may require more.