A low-slope roofing takeoff looks easy because the roof is nearly flat: measure the area, convert to squares, price the membrane. In reality the membrane is often the smallest part of the job. Insulation layers, tapered systems, cover boards, parapet flashings, drains, curbs, penetrations, edge metal, accessories and warranty requirements decide the price, and each is measured in a different unit. This guide shows how to build a complete low-slope roofing takeoff for commercial roofs, from reading the roof plan to a worked example, with the items that most often get missed.
If you would rather have the quantities prepared from your drawings, our roofing estimating services deliver membrane, insulation, flashing and accessories by roof area. For steep-slope and general roofing math, see our roofing estimating guide and our post on roofing takeoff for contractors. This post goes deeper on the low-slope, commercial side.
What counts as low-slope roofing
Low-slope roofs are commonly defined as roofs with a slope of about 3:12 or less, and many are close to flat, relying on a membrane and a drainage design to keep water moving. The main membrane families you will price are:
- Thermoplastic single-ply, such as TPO and PVC, with heat-welded seams.
- Thermoset single-ply, such as EPDM, with adhered or taped seams.
- Modified bitumen, applied in one or more plies by torch, adhesive or self-adhesion.
- Built-up roofing (BUR), made of alternating layers of felts and bitumen.
- Liquid-applied and spray foam systems with protective coatings.
Each has different material units, accessories, labor rates and safety requirements. The National Roofing Contractors Association at nrca.net publishes technical guidance on membrane types, details and good practice that specifications often reference.
Step 1: Gather the documents
Roofing information is spread across sheets, so collect all of it before you measure:
- The roof plan, showing drains, slopes, crickets, equipment, hatches, skylights and penetrations.
- Roof details and wall sections for parapets, edges, expansion joints, curbs, drains and transitions.
- Specifications, usually Division 07, with sections such as 07 22 00 for roof and deck insulation, 07 52 00 for modified bituminous membrane roofing, 07 53 00 for elastomeric membrane roofing, 07 54 00 for thermoplastic membrane roofing, and 07 62 00 for sheet metal flashing and trim. These sections set the membrane type, thickness, attachment, insulation, cover board, warranty and testing.
- Structural drawings, for the roof deck type, which affects fastener types and attachment.
- Mechanical and electrical drawings, for rooftop units, penetrations, curbs, conduit supports and walkway needs.
Our guide to reading construction plans for estimating shows how to cross-check sheets so penetrations shown only on mechanical sheets do not get missed.
Step 2: Measure the roof area
Measure each roof level separately, because tie-ins, access and heights differ. For nearly flat roofs, plan area is close to true area, and many estimators add a small allowance for slope, tapered areas and crickets. Then add the vertical surfaces that the membrane covers. Roofing is commonly priced by the square, and one square is 100 square feet.
- Field area: roof plan area inside the parapet or curb lines.
- Wall and parapet flashing: perimeter length x flashing height. Include the face of the parapet, the top if the membrane runs over it, and the cant strip area.
- Curb and penetration flashings: counted by size, with perimeter times height for curbs.
- Roof-to-wall transitions at upper roof walls and mechanical screens.
Keep field area, flashing area and detail counts on separate lines, since the membrane, the labor rate and the accessories differ for each.
Step 3: Price the insulation system
Insulation is usually the biggest material cost on a low-slope roof. The roof section is built up from the deck: a vapor retarder or air barrier where specified, one or more layers of insulation, a cover board where specified, and the membrane. Insulation thickness and number of layers come from the specification and the energy code for the project location, so read them instead of assuming. Multiple layers are installed with staggered joints, and each layer is measured separately in square feet with its own waste.
Insulation types include polyisocyanurate, expanded and extruded polystyrene, and mineral wool. Pricing changes with type, thickness, facer and fire rating. Cover boards, such as gypsum, high-density polyiso, fiberglass or perlite, are measured the same way and may be required for fire resistance, membrane compatibility or wind uplift.
Attachment is also quantity-driven. Mechanical fasteners and plates, adhesive or hot asphalt are specified by the roofing manufacturer to meet the wind uplift design for the building. Fastener density is higher at roof perimeters and corners than in the field of the roof, so measure those zones separately and take counts from the manufacturer’s attachment pattern for the specific project.
Step 4: Calculate tapered insulation
Many commercial roofs use tapered insulation to build slope toward drains or scuppers, because the structural deck is flat. Building codes generally call for a minimum slope for membrane roofs, commonly about 1/4 inch per foot, but always check the code and the specification. Tapered systems are designed by the manufacturer from the roof plan, and the final panel counts come from their layout drawings. During bidding, estimate the volume from the average thickness.
Average thickness = (minimum thickness + maximum thickness) / 2. The maximum thickness equals the minimum plus the slope times the distance from the low point to the high point. Tapered insulation is priced by board feet, where one board foot is one square foot at one inch thick.
Example: a roof section runs 30 feet from a scupper line to the high point, with a 1/4 inch per foot slope and a 1 inch minimum thickness at the scupper. The rise is 30 x 0.25 = 7.5 inches, so the maximum thickness is 8.5 inches and the average is 4.75 inches. For a 100 foot long section, the area is 3,000 square feet, and the volume is 3,000 x 4.75 = 14,250 board feet. With 5 percent waste for cuts, that is about 14,963 board feet. Add crickets behind curbs and units, which are priced separately because they use special panels.
Step 5: Flashings, edges and accessories
Flashing and accessories take a lot of labor for their material cost. Measure them carefully:
- Base flashing and counterflashing at parapets and walls, in linear feet by height and type.
- Coping, gravel stops and edge metal, in linear feet, with corners, splice plates and end caps counted separately.
- Roof drains and overflow drains: count each, with clamping rings, sumps and strainers.
- Scuppers and downspouts: count each.
- Pipe penetrations: count each by size, since each needs a boot, pocket or flashing detail.
- Equipment curbs and supports: count each, plus perimeter length for flashing.
- Expansion joints, area dividers and pressure-relief vents: linear feet or count.
- Roof hatches, skylights and smoke vents: count each and note whether flashing is by the roofer.
- Walkway pads, in square feet or linear feet along equipment routes.
- Seam tape, primers, adhesives, sealants and termination bars, taken from manufacturer coverage data.
- Lightning protection and roof-mounted fall protection anchors, if included.
The roofing, sheet metal and mechanical trades often overlap at curbs and edge metal, so write down who furnishes and installs each item. Our guide to building envelope estimating explains how to find and close these scope gaps, and our thermal and moisture protection estimation service covers insulation and waterproofing scope more broadly.
Step 6: Tear-off, repairs and re-roofing
Re-roofing adds items that new construction does not have. Measure existing roof layers, check for wet insulation, and set a unit price for replacement of deteriorated deck and insulation, since the amount is unknown at bid time. Add tear-off labor, debris disposal, chutes or hoists, interior protection, temporary roofing and protection of rooftop equipment. If the existing roof may contain hazardous materials, confirm who tests for them and who handles removal, because that is a scope and safety question as much as a cost one. State the unit price and the assumed quantity of repairs in your bid.
Step 7: Labor, safety, equipment and warranty
Labor on low-slope roofs depends on the membrane, attachment method, roof size and complexity, number of penetrations, access and weather. Production rates in squares per day from your own completed jobs are more reliable than published figures. Add:
- Hoisting and staging: cranes, hoists or material lifts, and staging of material on the roof.
- Fall protection, guardrails and warning lines, as the job requires.
- Hot work permits and fire watch for torch-applied systems.
- Weather protection and delays, especially for adhesives and cold-weather installation.
- Manufacturer inspections and warranty fees. Many commercial roofs require a manufacturer warranty, often with a specified term, so price the inspection, documentation and warranty cost.
- Testing, such as flood tests or electronic leak detection, where the specification calls for it.
For help with rates, burdens and productivity, read our guide to construction labor cost estimating. Material prices for insulation and membranes can move quickly, so see our posts on tariffs and material price changes and 2026 material cost trends.
A worked example
These numbers are illustrative. A commercial building has an 80 x 120 foot roof, a 24-inch parapet around the full perimeter, and a system of two layers of polyiso, a cover board and a single-ply membrane. Assume the membrane flashing runs 30 inches high (2.5 feet) up the parapet.
- Field area: 80 x 120 = 9,600 sq ft.
- Perimeter: 2 x (80 + 120) = 400 linear feet.
- Parapet flashing: 400 x 2.5 = 1,000 sq ft.
- Membrane area: 9,600 + 1,000 = 10,600 sq ft.
- With 8 percent for laps, overlaps and waste: 10,600 x 1.08 = 11,448 sq ft, or about 114.5 squares.
- Insulation: two layers x 9,600 sq ft = 19,200 sq ft. With 5 percent waste: 20,160 sq ft.
- Cover board: 9,600 x 1.05 = 10,080 sq ft.
- Coping or edge metal: 400 x 1.05 = 420 linear feet.
Tapered insulation, crickets, drains, scuppers, pipe boots, curb flashings, fasteners, adhesives, walkway pads, tear-off, labor, equipment, warranty and testing are then added on their own lines, using the counts from the roof plan and details.
Items that are commonly missed
- Tapered insulation and crickets that are drawn on the roof plan but not in the specification.
- Cover boards, vapor retarders and air barriers called for in the spec.
- Higher fastener counts at perimeters and corners.
- Coping, edge metal and counterflashing that belong to a sheet metal subcontractor.
- Pipe boots, pitch pockets, curb flashings and drains, which are counted by each.
- Walkway pads and protection at equipment access routes.
- Manufacturer warranty fees, inspections and testing.
- Hoisting, fall protection and weather protection.
Common low-slope roofing takeoff mistakes
- Pricing field membrane only and leaving parapet and curb flashing out of the area.
- Using one waste factor for field and detail areas.
- Ignoring wind uplift zones and using a single fastener density.
- Pricing tapered insulation as flat insulation, or leaving crickets out.
- Counting drains, penetrations and curbs from the roof plan only and not the mechanical drawings.
- Assuming edge metal and counterflashing are in someone else’s scope.
- Missing tear-off, disposal, wet insulation allowances and temporary protection on re-roofs.
The broader list is in our guide to common construction estimating mistakes, and the basics of organizing quantities are in our material takeoff guide.
Frequently asked questions
How many square feet are in a roofing square?
One square is 100 square feet. A roof of 10,600 square feet is 106 squares before waste.
What waste factor should I use for single-ply membrane?
Many estimators use about 5 to 10 percent, depending on roll width, the roof shape and the number of penetrations. Compare your ordered and installed quantities on past jobs to refine it.
How is tapered insulation priced?
Usually by board feet, based on area and average thickness, plus a separate price for crickets and special panels. The manufacturer’s layout drawing gives the final panel quantities.
What slope do flat roofs need?
Codes generally require positive drainage, with a minimum slope that is commonly about 1/4 inch per foot for membrane roofs. Check the building code and the specification.
Should I include the manufacturer warranty in the estimate?
Yes, when the specification requires one. Price the inspection, documentation and warranty costs, and state the term and type of warranty in your bid.
How do I estimate a re-roof when conditions are unknown?
Price the visible work, then give a unit price for wet insulation and deck repairs with a stated assumed quantity. This allows adjustment after tear-off.
Get a low-slope roofing takeoff prepared from your drawings
We extract roof area, insulation layers, tapered systems, flashings, edge metal and accessories by roof level, so you can apply current supplier pricing and your own crew rates. See our roofing estimation service, our quantity and material takeoffs, or the full list of estimating services by trade. For the unit conversions used in this guide, read our post on takeoff units of measure and conversions.