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Wind Uplift Zones on a Commercial Roofing Takeoff: Field, Perimeter, and Corner Fastener and Adhesive Density

By The Sharp Pencil team

Do not take off fasteners, plates, and adhesive with one average density for the whole roof. Find the wind design method the project uses (ASCE 7 through the building code, FM Global for an FM insured building, or both), lay out the field, perimeter, and corner zones that method defines, take off the area of each zone separately, and apply the fastening or adhesive rate that the specified assembly calls for in that zone.

Why send your takeoffs to Sharp Pencil

The expensive part of a takeoff backlog is the jobs you no-bid because nobody has the hours. Each point below answers a specific way a takeoff costs you time or trust.

• Usually delivered within 48 hours. A takeoff that sits in a week-long queue is no help when the bid is due sooner.
• Every quantity names its sheet. Each quantity carries the sheet it came from and a confidence grade, so you can check any number.
• No silent blanks. Any checklist line the drawings could not settle carries a written reason, not a blank or a guess.
• Questions instead of guesses. Anything the set leaves open, such as a missing building height or an unstated wind design method, comes back as a drafted RFI you can copy and send to your architect. Paste the answers into your job page and the takeoff is updated; one re-run is included when you answer the questions.
• Priced with your numbers. Send us your costs once in a simple price book, and each takeoff also comes back with a client-ready proposal and a re-price sheet, and the takeoff workbook adds a Pricing tab.
• UNPRICED instead of a guessed number. Where a cost is missing, the line says UNPRICED and shows the quantity, so nothing is hidden inside the total.
• A roofing professional reviews every paid job before it goes out. The review comes with every paid takeoff, not as an upgrade.
• Answers in one thread, not an email chain. Ask about your job in a thread on your job page. If a question needs a roofing estimator on our team, the answer comes back in the same thread.
• Your plans stay yours. Nothing is shared or sold. See what happens to your plans.
• Monthly plans, with your first takeoff free. Our monthly plans are the main way to buy: Shop 12 at $2,750 a month, Shop 25 at $5,500 a month, and Shop 60 at $11,000 a month. $395 is the price per single takeoff. Either way, try your first one free on a job you are actually bidding before you pay anything.

What are wind uplift zones on a low-slope roof?

Wind pulls hardest on the edges and corners of a roof, so the design methods split the roof into zones and require more holding power where suction is highest. On a mechanically attached or adhered single-ply roof, more holding power usually means more fasteners and plates, tighter rows, narrower sheets, or a heavier adhesive rate in the perimeter and corners. That is why one average density undercounts the edges.

Two methods show up on commercial bid sets:

• ASCE 7, through the building code. GAF's explainer on IBC and FM notes that the IBC requires roof wind loads to be determined under ASCE 7 (Chapters 26 to 30) and that mechanically attached or adhered roof coverings must resist the components and cladding pressures from that standard (GAF, IBC and FM).
• FM Global, for FM insured buildings. The same GAF article explains that FM insured buildings must meet both the IBC and FM requirements, using FM Approved assemblies found through RoofNav, and that in an FM Approval listing such as "1-60" or "1-75", the "1" means Class 1 for fire resistance from below the deck and the second number is the wind uplift capacity in pounds per square foot. FM's roof wind design guidance is in Property Loss Prevention Data Sheets 1-28 (Wind Design) and 1-29 (Roof Deck Securement and Above-Deck Roof Components).

GAF also warns that a vague "Meet FM" line in a spec on a building that is not FM insured can pull the whole FM process into the job unintentionally. For a takeoff, that is an RFI, not a guess.

How ASCE 7 defines the roof zones

The zone layout changed between editions, so the edition matters before you draw a single zone.

Source: GAF, A Guide to Using ASCE 7-16. In this table, h is the building height.

Two takeoff consequences follow from the guide and from Professional Roofing's review of the 2016 edition (Thomas L. Smith, "How do I load thee?"):

1. Under ASCE 7-16 the zones depend only on building height. A missing or unclear roof height on the drawings is a takeoff problem, not a detail.
2. The perimeter and corner bands got wider and the pressures went up in most zones. The GAF course on ASCE 7 notes that the largest increases are in Zone 1 and that on some taller, smaller footprint buildings only Zones 2 and 3 exist (GAF on BNP Continuing Education). A density carried over from an older job can be wrong in both directions.

For reference, SPRI's wind design seminar lists the ASCE 7-16 external pressure coefficients (GCp) for these zones, for small effective areas, as 0.9 (Zone 1'), 1.7 (Zone 1), 2.3 (Zone 2), and 3.2 (Zone 3), against 1.0, 1.8, and 2.8 for the three zones in ASCE 7-10 (SPRI, ASCE 7 seminar). You do not calculate pressures in a takeoff. The designer of record does. But the coefficients show why the corners cost more.

Check which ASCE 7 edition the local code adopts. Newer codes may reference a later edition, so confirm the edition and the zone layout on the drawings or in the spec before using the table above.

How FM tightens fastening in the perimeter and corners

When a job uses FM's prescriptive enhancements for Zones 2 and 3, fastening density rises at the edges. GAF's ASCE 7-16 guide quotes FM Global Property Loss Prevention Data Sheet 1-29 (as revised in February 2020) for conventionally mechanically attached single-ply systems: in Zone 2 (perimeter), the distance between fastener rows is no more than 67% of the row spacing needed for the wind uplift resistance rating for Zone 1, or one row of intermediate fasteners is added; in Zone 3 (corners), the distance between fastener rows is no more than 50% of that Zone 1 row spacing, or one row of intermediate fasteners is added (GAF, A Guide to Using ASCE 7-16, main text and Appendixes B and C).

GAF also notes that those Zone 2 and Zone 3 enhancements are to be based on Zone 1, not on Zone 1'. FM data sheets are revised over time, and the current edition and the FM Approved assembly listing for the job govern. If the spec names an FM rating, such as 1-90, the assembly listing in RoofNav sets the fastening, and the takeoff should follow the listed assembly for each zone rather than a generic pattern.

How to take off fasteners and adhesive by zone

1. Identify the wind design basis. Look for a wind design note on the roof plan or structural sheets, the design wind pressures by zone, an FM rating in the spec, and the ASCE 7 edition. If the set shows zone pressures, use them. If it shows nothing, write the RFI.
2. Confirm the building height and plan dimensions for each roof area. Every zone width in Table 1 depends on them, and FM row spacing enhancements are based on the Zone 1 pattern. Treat each roof level at a different height as its own roof for zoning.
3. Lay out the zones and take off each zone area separately. Field (or Zones 1' and 1), perimeter, and corners each get their own SF line. Add the zone areas back up and check that they equal the roof area, so no area is included twice.
4. Get the attachment rate for each zone from the specified assembly. For mechanically attached systems that is fastener and plate spacing, row spacing, and sheet width by zone. For adhered systems it is the adhesive coverage rate, and for foam adhesive ribbons the ribbon spacing, by zone. Insulation and cover board attachment usually changes by zone too, so take it off the same way.
5. Count by zone, then total. Multiply each zone area by its rate, keep the subtotals on separate lines, and only then sum the materials for the order. Perimeter sheets and half sheets also change seam and lap quantities, so carry them with the perimeter zone.
6. List what is still open. Missing height, unstated edition, an unlisted FM rating, or a spec that says "Meet FM" on a building with no stated insurer all go on the RFI list.

Professional Roofing notes that treating awkward leftover areas as the higher zone can simplify field work at little cost. If you choose that simplification, write it into your bid clarifications so the quantity and the spec match.

Worked example: how much the perimeter band moves

This is arithmetic only, on an assumed building: one roof area, 200 ft by 300 ft in plan, 30 ft tall. It shows the perimeter band changing between ASCE 7 editions. It does not show pressures or fastening rates, which come from the project.

The band area is the total roof area (60,000 SF) minus the inner rectangle: 176 ft by 276 ft (48,576 SF) for a 12 ft band, and 164 ft by 264 ft (43,296 SF) for an 18 ft band. Under ASCE 7-16 the 18 ft band is Zone 2 with the Zone 3 corners inside it, and Zone 1 is another 0.6h band inside that, so the higher-density area grows again. On this assumed roof, the edge band alone is about 19% of the roof under ASCE 7-10 and about 28% under ASCE 7-16. Spreading one average density across that area is how edge fasteners and adhesive go missing.

Common mistakes

• Using one fastener or adhesive rate for the whole roof.
• Using ASCE 7-10 zone widths on a job designed to ASCE 7-16, or the reverse.
• Zoning a multi-level roof as one roof with one height.
• Forgetting that insulation and cover board attachment also changes by zone.
• Leaving out the extra seams, laps, and half sheets that perimeter rolls create.
• Treating "Meet FM" as a full FM wind design without asking whether the building is FM insured.

FAQ

Why do fasteners increase at the roof perimeter and corners?

Wind suction on a roof is highest at the edges and corners, so ASCE 7 and FM assign higher design pressures there, and the specified assembly needs more attachment in those zones to resist them.

What are ASCE 7-16 roof zones 1', 1, 2, and 3?

For low-slope roofs on buildings 60 feet or less in height, Zone 3 is an L-shaped corner zone 0.6h long and 0.2h wide, Zones 2 and 1 are each bands 0.6h wide, and Zone 1' is the interior area that remains, which may not exist on every roof.

How does FM change fastener row spacing in Zones 2 and 3?

GAF's ASCE 7-16 guide summarizes FM Data Sheet 1-29 as calling for Zone 2 fastener rows at no more than 67% of the Zone 1 row spacing (or one intermediate row), and Zone 3 fastener rows at no more than 50% of the Zone 1 row spacing (or one intermediate row). Check the current FM data sheet and the FM Approved assembly for the job.

Do adhered roofs change by wind zone too?

Often, yes. The specified assembly can call for different adhesive coverage rates or foam ribbon spacing in the perimeter and corners, so take off adhesive by zone the same way you take off fasteners.

What should go on the RFI list for wind uplift zones?

The building height for each roof level, the ASCE 7 edition used, the design wind pressures by zone, whether the building is FM insured and which FM rating applies, and the fastening or adhesive schedule by zone for the specified assembly.

Can Sharp Pencil take off fasteners and adhesive by wind zone?

Yes, when the set gives the information. Zone areas and attachment quantities come back as separate lines with sheet cites, and missing items such as building height, design pressures, or the FM rating come back as drafted RFIs on the Questions tab. Usually delivered within 48 hours, and the first takeoff is free on a live bid.