Roofing Takeoff Guide: Squares, Slope Factor, Flashing, and Insulation

A roofing takeoff is the full count of every square of roofing, linear foot of flashing, board of insulation, and penetration on a roof. This guide covers the math in simple language with real pitch factors, waste percentages, and the shortcuts good estimators use every day on steep and low-slope roofs.

1. What a Roofing Takeoff Is

A roofing takeoff is the full bill of materials for a roofing project. The biggest line items are the roof surface itself (membrane, shingles, or metal), the insulation underneath, the flashings at every edge and penetration, and the fasteners and adhesives that hold it all together.

Roofing is priced by the square. One roofing square = 100 square feet of roof surface. When you hear "this is a 38-square roof" that means 3,800 SF of roofing.

The basic outputs

• Roof area in squares (actual surface, not plan area)
• Insulation (squares or boards, thickness, R-value)
• Underlayment (squares or rolls)
• Flashing and edge metal (linear feet by type)
• Penetration count (drains, vents, curbs, skylights)
• Fasteners, adhesives, sealants (quantities per spec)

2. Squares and Slope Factor

The square

One square equals 100 sq ft. A 2,500 SF roof is 25 squares. Shingles, underlayment, membrane, and other roofing materials are sold and priced in squares or multiples of squares (a shingle bundle covers 1/3 of a square).

Why slope factor matters

The roof plan shows the footprint from above. But a sloped roof has more surface than its footprint because of the rise. Slope factor is the multiplier that converts plan area to actual roof area.

Slope factor chart

• 0-1/12 (flat): 1.000
• 2/12: 1.014
• 3/12: 1.031
• 4/12: 1.054
• 5/12: 1.083
• 6/12: 1.118
• 7/12: 1.158
• 8/12: 1.202
• 9/12: 1.250
• 10/12: 1.302
• 12/12 (45 degrees): 1.414

Formula: slope factor = square root of (1 + (rise/12)^2). Memorize the common ones (4/12, 6/12, 8/12) and the rest are easy to derive.

3. Reading the Roof Plan

Where roofing info lives

• A-series roof plan: shows layout, slopes, drains, scuppers, and penetrations
• A-series roof details: parapet caps, drain details, curb details, expansion joints
• Spec section 07 50 00 (low slope) or 07 31 00 (steep slope) for materials, warranty, and install requirements
• MEP drawings: mechanical curbs, vent pipes, exhaust fans, lightning protection
• Structural drawings: roof framing, parapet heights, edges

Key things to spot

• Slope arrows and callouts (1/4:12, 6:12, etc.)
• Drain locations and sizes
• Crickets (mini slopes to divert water around curbs)
• Expansion joints and control joints
• Overflow scuppers
• Walkway pads at equipment
• Snow guards on sloped roofs

4. Low-Slope Takeoff

Low-slope or commercial flat roofs use membrane systems: TPO, EPDM, PVC, or modified bitumen.

Membrane area

1. Measure net roof area in SF from the roof plan.
2. Subtract large skylights, atriums, and open courtyards.
3. Use slope factor of 1.00-1.02 (flat roofs have almost no slope factor).
4. Divide by 100 to get squares.
5. Add 5-10% waste for seams, cuts, details, and overlap.

Typical low-slope assembly takeoff

• Cover board: 1/2 inch HD polyiso or gypsum, equal to membrane area
• Insulation: polyiso to required R-value, usually 2 layers staggered
• Vapor barrier (sometimes): equal to insulation area
• Mechanical fasteners or adhesive: per manufacturer rate; fasteners typically 10-16 per 4x8 board
• Primer, seam tape, edge metal, termination bar

Tapered insulation

Low-slope roofs often use tapered polyiso to create a minimum slope for drainage (usually 1/4 inch per foot). Take off tapered layout from the manufacturer's diagram, or budget 30-50% more volume than flat insulation on roofs that need it.

5. Steep-Slope Takeoff

Steep-slope roofs are residential or commercial roofs above about 2/12 pitch. Common materials: asphalt shingles, metal panels, clay or concrete tile, slate.

Asphalt shingle takeoff

1. Measure plan area from the roof plan (or architectural drawings).
2. Multiply by slope factor for actual sloped area.
3. Divide by 100 to get squares.
4. Add 10-15% waste for cut-up roofs with hips and valleys; 10-12% for simple gables.
5. Add 1 extra bundle per hip and ridge run (for cap shingles).

Metal panel takeoff

Standing-seam metal comes in custom-cut panels. Order panel length equal to the sloped run of each roof plane plus 2-4 inches for eave overhang. Multiply by number of panels (roof width / panel coverage width). Add 3-5% waste.

Tile takeoff

Clay and concrete tile is sold per square or per piece. Count per-square (90-120 pieces per sq depending on model) and add 12-18% waste. Order ridge and hip tiles separately.

6. Flashing and Edge Metal

Flashing is the sheet metal that waterproofs every edge and intersection. Most flashing is priced per linear foot installed.

Common flashings

• Drip edge: at eaves and rakes (LF equals perimeter)
• Valley flashing: LF measured down each valley
• Step flashing: at sidewalls where roof meets vertical wall
• Counter-flashing: at chimneys, parapets, and walls (LF of wall intersection)
• Base flashing: at roof-to-wall intersections
• Coping: metal cap on top of parapet walls (per LF)
• Gravel stop / fascia: on low-slope roof edges
• Pipe boots and penetration flashings: count per each

Gutters and downspouts

• K-style gutter 5 or 6 inch: LF of eave
• Downspouts: count plus linear feet from roof to grade
• Splash blocks / drain connections: per each

7. Roof Insulation Takeoff

Polyiso (polyisocyanurate)

Standard commercial flat roof insulation. Sold in 4x8 boards (32 SF per board). Typical thicknesses:

• 1 inch: R-5.7 per inch
• 2 inch: R-11.4 (most common single layer)
• 3 inch: R-17.1
• 4 inch: R-22.8

Most codes require R-25 to R-35 for commercial roofs depending on climate zone. Layer two sheets of 2-inch staggered for R-22, or one 2-inch plus one 1.5-inch for tighter R targets.

Cover boards

Above the insulation, below the membrane: 1/2 inch HD polyiso, 1/2 inch gypsum (DensDeck), or 1/4 inch HD wood fiber. Order equal area to roof membrane.

Fasteners and adhesives

• Mechanical fasteners: 5-8 per 4x8 board field, 8-12 at perimeter, 12-16 at corners (wind uplift zones)
• Adhesive (low-rise foam): one 1.5 gallon canister covers about 8 squares
• Hot asphalt: about 25 lbs per square for built-up roofs

8. AI Roofing Takeoff

Roof takeoff has historically meant measuring every roof plane by hand on a PDF, applying slope factor, then estimating flashing lengths and penetrations. AI changes the workflow.

What PILRS does for roofing

• Reads the roof plan and identifies each roof plane automatically.
• Pulls slope from callouts and applies correct slope factor.
• Measures perimeter for drip edge, valleys, and parapet lengths.
• Counts penetrations, drains, and curbs from the plan and MEP coordination.
• Applies your waste factor and outputs squares, LF, and counts in a price-ready format.

Where estimators still add judgment

AI gets the geometry. Estimators add the experience: how much waste a complex cut-up really needs, which brand warranty the owner wants, whether the crew can handle the slope in July. Good tools free up your time for the thinking work.