Roofing takeoff

A roof covering is measured over the outer covered surface, not the building footprint. Each plane's boundary starts at the eave or drip edge, out to the outer fascia face and including the eave overhang. It runs rake to rake, including the gable overhang, and stops at the ridge.

RICS NRM2 measures the area of roof covering over the covered surface, with steep-slope tile and slate under Work Section 18 and sheet, membrane, and built-up coverings under Work Section 17. German VOB/C DIN 18338 bills the covered area (gedeckte Flaeche) to the outer edge, and North American practice follows the same boundary under NRCA guidance. Hips and valleys are shared boundaries between two planes, never deductions: each plane is traced separately to that line.

Because a roof plane is tilted, the plan polygon a takeoff traces is smaller than the real surface. The true area equals the plan area multiplied by the slope factor, which is pure Pythagorean geometry. For a rise-in-12 pitch the factor is the square root of ((rise divided by 12) squared plus 1). A 4 in 12 pitch gives 1.054, a 6 in 12 gives 1.118, an 8 in 12 gives 1.202, and a 12 in 12 gives 1.414. US and international practice traces the plan and multiplies by the factor, while UK, European, and Australia and New Zealand quantity surveyors measure the true length up the slope off a section.

The biggest error is applying one factor to a roof with mixed pitches: every facet must carry its own pitch and be converted and summed separately.

Vent pipes, chimneys, skylights, hatches, and small curbs are not deducted from the field area. Roofers cut around them with offcut waste and the flashing is taken as a separate item, so netting them out understates both material and labour. Only a large open well, such as a courtyard or atrium, is deducted.

The size at which a deduction begins is the one genuinely region-specific number. Under RICS NRM2 no deduction is made for voids up to 1.00 square metre, and a rooflight larger than that is deducted then enumerated separately. German VOB/C practice over-measures roof openings, meaning it keeps them, up to roughly 2.50 square metres, while integrated rooflights and solar units are always billed separately regardless of size. US practice has no codified figure, absorbing typical penetrations in waste and deducting only a large well by judgement. Australia and New Zealand follow the RICS lineage near 1.00 square metre.

Ridges, hips, valleys, verges, and eaves are taken as separate linear items because each is a distinct product with its own rate. RICS NRM2 Work Section 18 takes them as linear extra-over items, and DIN 18338 measures each along its centre line. Hips and valleys are sloped, so their length is the rake length, the square root of (rise squared plus run squared), longer than the plan diagonal, while ridges and eaves are horizontal and their plan length equals their true length. Starter strip and hip and ridge cap are derived from those lengths, not the field area.

Underlayment, felt or synthetic, is sized from the net roof area plus the lap and rounded to whole rolls, with minimum head and side laps set by code, for example IRC R905.1.1. Ice barrier applies only where the cold-climate trigger does. Where IRC Table R301.2(1) applies, IRC R905.1.2 requires a self-adhered eave membrane reaching to 24 inches inside the exterior wall line, so the band width up the slope is the overhang projection plus 24 inches. Editions before 2024 also required 36 inches up the slope at pitches of 8 in 12 or steeper, removed in 2024, and valleys receive the membrane separately. Canada's National Building Code requires eave protection in cold-climate jurisdictions, commonly near 900 millimetres, while the United Kingdom, continental Europe, and Australia and New Zealand have no equivalent requirement.

The slope routes the entire takeoff. NRCA draws the line at 3 in 12, about 14 degrees. At or below that the roof is low-slope, a continuous membrane or built-up system measured by field area with a slope factor near 1. Above 3 in 12 the roof is steep-slope, discrete shingles or tiles, where the slope factor matters most and starter and cap come off the linear lengths. The IBC and IRC code minimum for asphalt shingles is 2 in 12 with double underlayment, while NRCA best practice is 4 in 12 and steeper.

Built-up and modified-bitumen roofs multiply the field area by the number of plies, with the base sheet, cap sheet, insulation, and cover board each its own area quantity, so counting a single layer under-orders a four-ply assembly by roughly four times. Single-ply membranes such as TPO, EPDM, and PVC are one layer but add seam length at the overlaps, fastener density, and cover board separately. Tapered insulation that builds slope to the drains is a volume, average thickness times area. Wall and curb flashing is measured by both its linear run and its vertical height, since the upstand height sets the developed sheet-metal width. Fasteners are a per-square count: IRC R905.2.6 sets four nails per shingle as standard and six in high-wind zones, graded under ASTM D3161.

Waste applies to the ordered material, never to the measured boundary. The common shingle bands run about 10 percent for a simple gable roof up to 15 to 20 percent and beyond for a complex, cut-up roof, with tile and slate near 5 to 10 percent, metal across roughly 5 to 20 percent, and single-ply across 5 to 15 percent. These bands are estimating and manufacturer practice rather than a numbered clause: ARMA, the asphalt-roofing authority, gives only a 2 to 10 percent trim-waste range tied to configuration.

Which quantity an output represents depends on its purpose, and this split is structural by region. US procurement loads waste into the ordering quantity, taking the net area, adding a waste percentage, and rounding up to whole bundles, rolls, or squares, where three bundles cover one square. UK, European, and Australia and New Zealand practice measures net under RICS NRM2 and VOB/C DIN 18338 and carries waste inside the unit rate. The reporting unit follows the same split: roofing squares of 100 square feet in the US, square metres elsewhere, and both on Canadian projects. A quantity should never carry a waste percentage and a waste-loaded rate at once. Exayard reads the drawings and applies these rules automatically, tracing each plane to its outer edge, converting through the per-facet slope factor, and producing the covering, edge, and supporting-layer quantities for the system and region in use.