About this calculator
This calculator takes off the materials for a flat roof build-up, from the deck upward:
- a vapour control layer (VCL), turned up at the edges to the top of the insulation;
- insulation boards in up to four layers, plus optional tapered insulation to form the fall;
- the waterproofing membrane — single-ply (PVC, TPO, EPDM) or a two-layer bituminous system (base layer and cap sheet) — including the upstands at parapets and walls;
- edge trims around the perimeter and mechanical fixings.
Enter the roof as a rectangle, or as an area and a perimeter for any other shape.
Formula
membrane area = roof area + perimeter × upstand height
rolls (a layer) = ceil(area × (1 + waste %) / ((roll width − side lap) × (roll length − end lap)))
VCL area = roof area + perimeter × insulation thickness at the edge
boards (a layer)= ceil(roof area × (1 + waste %) / board area)
tapered layer (one fall, simplified):
maximum thickness = minimum + fall % × drainage length
average thickness = minimum + fall % × drainage length / 2
volume = roof area × average thickness
edge trims = ceil(perimeter / (trim length − overlap))
fixings = ceil(roof area × fixings per m²); packs = ceil(fixings / pack size)
Worked example
A 12 × 10 m roof with 300 mm upstands, two 100 mm layers of 1200 × 600 mm boards and a single-ply membrane on 2.0 × 20 m rolls (120 mm side and 100 mm end laps), 5% waste:
area = 120 m², perimeter = 44 m
membrane = 120 + 44 × 0.3 = 133.2 m² → 133.2 × 1.05 / (1.88 × 19.9) = 3.74 → 4 rolls
boards = 120 × 1.05 / 0.72 = 175 per layer; insulation 120 × 0.2 = 24 m³
VCL = 120 + 44 × 0.2 = 128.8 m² → 2 rolls of 1.5 × 50 m (100 mm laps)
edge trims = 44 / 1.95 = 22.6 → 23; fixings = 120 × 4 = 480 → 2 packs of 250
With a 1.5% fall over 5 m starting at 20 mm, the tapered layer is 95 mm at its thickest and 57.5 mm on average: 120 × 0.0575 = 6.9 m³.
FAQ
How many fixings does a flat roof need? That comes from a wind-uplift calculation to EN 1991-1-4 for your building height, location and terrain, and from the membrane system's approval. Corners and edges need more fixings than the middle of the roof. The rate here is a single placeholder — replace it with the designed figure.
How accurate is the tapered insulation volume? It assumes one uniform fall from a high edge to a line of outlets. Falls to point outlets, valleys and crickets change the average thickness; the supplier's tapered layout gives the exact boards.
Why do I need two kinds of rolls for a bituminous roof? A two-layer system has a base layer and a cap sheet; both cover the whole roof and upstands, and their rolls often differ in length.
Which standard does this follow? None is cited: the take-off is areas and roll coverage. The fixing pattern is a wind design to EN 1991-1-4, which this calculator does not perform.
Assumptions and limits
- Every roll count is area-based with laps taken off the roll's width and length; a strip-by-strip layout, penetrations and outlets are not modelled.
- Upstands run all round the perimeter at one height; with an open edge (no parapet) set the height to 0 and use edge trims.
- The VCL upturn is the flat insulation plus the average tapered thickness at every edge.
- Fixings are counted on the roof area at one rate, without wind zones.
- All defaults (roll and board sizes, laps, waste, upstand height, fixing rate) are examples; use the chosen products' data.