TPO roofing in Bethesda, MD: the wind record, the code lines and the roof edge

Eleven wind rows, nine dates

NOAA records wind in knots, and the type on every Bethesda row is EG, an estimated gust. The miles per hour below are our conversion at 1.15 mph per knot.

• July 18 and 19, 2016Four rows: three logged between 3:14 and 3:15 in the afternoon on the 18th, one on the 19th. Each is 50 knots, about 58 mph.
• April 21, 2017One row at 50 knots.
• May 2 and August 20, 2019One row each, both at 50 knots.
• April 30, 2021One row at 50 knots.
• May 22 and July 12, 2022One row each, both at 50 knots.
• July 29, 2023One row at 65 knots, about 75 mph, the highest under the label.

Five of the eleven years have no wind row under the label: 2015, 2018, 2020, 2024 and 2025. The county file is wider and shows the season.

469 Montgomery County thunderstorm wind rows on 131 dates, NOAA files for 2015 to 2025 downloaded October 19, 2026 and recounted October 20. January has no rows. Shares are rounded and are ours.

Of the 469 county rows, 209 sit at exactly 50 knots, 60 fall between 60 and 65 knots and 9 are above 65. The rows per year run from 5 in 2018 to 75 in 2019, and the file does not say why. NOAA defines the location field as the town from which the range is calculated, so it is a reference point, not the edge of Bethesda.

The wind lines in the county and code documents

Three documents put numbers or tests on a flat roof in Bethesda. They do not measure the same thing, so they are set side by side and not compared.

SPRI, the single-ply trade association, writes that Chapter 15 relies on FM 4474, UL 580 and UL 1897 for uplift and on ES-1 and GT-1 for edge securement, and that its free Directory of Roof Assemblies lists tested low-slope assemblies. The code also cites FM numbers (FM 4474 and FM 4470). FM Approvals runs a product guide and a roofing tool called RoofNav; its home page gives no roof wind ratings.

Where the roof edge test comes in

Section 1504.6 of the Maryland text covers metal edge systems on single-ply roofs under 2 in 12 slope. Gutters and counterflashing are excluded, and gutters that secure the membrane edge are tested under SPRI GT-1 instead. The edge systems must be designed for wind loads under Chapter 16 and tested under RE-1, RE-2 and RE-3 of ANSI/SPRI ES-1.

RE-1

Pulls the membrane at an angle to the deck, in pounds per lineal foot. It is required only where the edge itself holds the membrane, which the standard calls dependent termination.

RE-2

Loads the vertical face of an edge flashing or fascia whose exposed horizontal part is 4 inches or less. Results come in pounds per square foot.

RE-3

The test for copings, with top and face loads applied together, used where the exposed horizontal part is more than 4 inches.

Which one

It depends on how the membrane is attached. Ballasted, spot or ribbon adhered, and mechanically attached more than 12 inches from the edge are dependent. Adhered, or fastened within 12 inches, is independent.

The commentary adds that an adhered system is assumed to put no stress on the edge unless the metal is loosened or the membrane peels under the pressure difference, and that recent hurricane investigations showed both can happen. It also says the precision and bias of the test methods have not been determined.

Corners, perimeters and what is not tested

The 59 percent figure is about built-up roofs, not TPO. SPRI adds that edge pressures at perimeter zones will be lower than its corner results.

From code line to a label you can read

1. Measure the slope. The edge rule applies under 2 in 12. Above that the single-ply table still applies, but the low-slope edge section does not.
2. Get the design pressure from someone qualified. The county sheet gives a speed, not a pressure at a corner. An engineer or the building department turns it into one.
3. Name the attachment. Ballasted, adhered or mechanically attached, and the distance of the first fasteners from the edge, decide the ES-1 route.
4. Ask for the test paperwork. ES-1 says edge components or packaging carry written documentation of what was tested, available to the owner or a representative. Ask for it before the metal is ordered.
5. Check the installation instructions. The standard requires instructions that include fastener and cleat requirements, so the roofer should follow the tested detail, not a similar one.

On a replacement, section 1512.4 adds that rusted, damaged or deteriorated metal edgings, drain outlets and counterflashings shall not be reinstalled. Section 1512.2.1 allows a new protective coating over existing single-ply without tear-off, and section 1512.2.1.1 bars a recover where the roof is water soaked or already has two or more coverings.

Lines to put in a Bethesda TPO quote

• The roof slope, measured on site, and the drainage it relies on.
• The attachment method and, if mechanical, the distance from the edge to the first fastener row.
• The edge detail: fascia, flashing or coping, its face height and horizontal depth.
• The ES-1 documentation for that exact edge metal, and who fabricated it.
• The assembly test route (FM 4474, UL 580 or UL 1897) and the listing relied on.
• The state of the old edge metal, counterflashings and drain outlets, and which are reused.
• Any written reply from DPS, the building owner or an association that the scope depends on.

The county’s Home Improvements page says it does not include municipality or homeowner association rules. Our Bethesda roofing guide sets out the contractor license check and the permit lanes. Book a phone consultation to talk through a Bethesda TPO roof, and see the general TPO roofing guide, roofing materials, commercial roofing, roof replacement and services pages.