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TPO roofing in Herndon, VA: the storm record and the drainage a flat roof needs
A TPO membrane keeps water out only while water can leave the roof. NOAA's Storm Events files hold 20 reports under a Herndon place label, including 12 wind rows, 4 flood rows and 3 hail rows. None is a roof report, but together they show what a low-slope roof here has to shed. This page sets the rows beside the slope, drain and overflow rules in the 2021 Virginia codes and the ponding guidance from the asphalt roofing trade association.
- 20 NOAA rows carry a Herndon label
- Ponding means water standing 48 hours or longer
- Code minimum slope for TPO: 1/4 inch in 12
The NOAA rows under a Herndon label
The National Centers for Environmental Information publish Storm Events as yearly files. This page uses the 2016 to 2026 Virginia files and keeps the Fairfax County rows whose begin-location label reads Herndon or Herndon Hgts. The label is NOAA's own place name, not the Town boundary.
| Event type | Rows | What the rows say |
|---|---|---|
| Thunderstorm wind | 12 | Estimated gusts of 50 to 56 knots; the 56 came on July 14, 2023 |
| Flood | 3 | Heavy rain flooded a street, closed a road and put a creek out of its banks in 2018 and 2019 |
| Flash flood | 1 | July 14, 2023: a stream ran over a footbridge |
| Hail | 3 | 1.0 inch in 2019, 1.5 inches in 2023, 1.0 inch in 2024 |
| Tornado | 1 | April 6, 2017, rated EF0, damage mainly to trees |
Source: NOAA NCEI Storm Events details files, Fairfax County rows, counted October 20, 2026. Twenty rows fall on 14 separate dates.
Every row sits between April and August
Five rows are in April, two in May, eight in July and five in August. None carries a June date or a date outside those months. Storm Events lists reported events only, so a quiet year in the file is not proof of a quiet year.
The water rows are ground-level
The flood narratives name Autumn Ridge Circle, Seneca Road near Kentland Drive and Sugarland Run. They show how hard the rain fell. They say nothing about any building.
One date holds three rows
On July 14, 2023 the file has a wind row, a hail row and a flash flood row under a Herndon label. A flat roof can face wind, hail and a rain burst in a single afternoon.
What ponding is and why it matters
ARMA defines ponding water as water that remains on a roof "for 48 hours or longer."
Asphalt Roofing Manufacturers Association, ponding water page, October 2024
ARMA names three sources: rain, melting snow or ice, and runoff from rooftop equipment. It lists what standing water can lead to: deck deflection, dirt and debris, ice damage, biological growth and water infiltration. It also joins the National Roofing Contractors Association and the International Institute of Building Enclosure Consultants in recommending a design slope of at least 1/4 inch per foot so a roof drains freely.
The page is written about asphalt systems, not thermoplastic ones. What it offers a TPO owner is the drainage vocabulary and the 48-hour test. How a membrane maker treats standing water is set by that maker, so it belongs in the questions list below.
ARMA's list of practices is also a list of things to look for on a flat roof. Each can be seen from a ladder or a photograph:
- Drains and scuppers placed at the low points of the roof, and kept clear.
- Tapered insulation where the deck itself has no slope.
- A cricket upslope of rooftop equipment and a saddle between drains.
- HVAC condensate lines run to a proper drain, not onto the membrane.
- Regular inspection, since a pond may show only after rain.
Slope, drains and overflow in the codes
Two code books bear on a thermoplastic roof in Virginia: the Virginia Residential Code and the Virginia Construction Code. Both were read as third-party copies on UpCodes, which republishes the 2021 Virginia text. Check the official text and the Town building office for your address.
- Minimum slope
- Residential R905.13.1 and construction code 1507.12.1 each set a design slope of not less than 1/4 unit vertical in 12 horizontal (2 percent) for a thermoplastic or single-ply membrane.
- Material standard
- R905.13.2 lists ASTM D4434, D6754 or D6878 for thermoplastic coverings, and R905.13.3 says to install per the chapter and the manufacturer's instructions.
- Drains at low points
- R903.4: roof drains at each low point, unless the roof is sloped to drain over its edges.
- Ballasted low-slope
- Construction code 1507.12.3 covers ballasted roofs under 2:12 and points to section 1504.5.
- Crickets and saddles
- 1503.5 asks for one on the ridge side of a chimney or penetration wider than 30 inches, in sheet metal or the roof covering material.
Overflow: where the two books say it
Residential code, R903.4.1
- Secondary drains or scuppers where parapet-type edges would trap water if the primary drains back up.
- Overflow drains of the same size as the roof drains, inlet 2 inches above the low point.
- Or overflow scuppers three times the size of the roof drains, at least 4 inches high, in the adjacent parapet walls.
- Overflow goes to an approved location and is not tied into the roof drain lines.
Construction code, 1502
- 1502.2 requires secondary drains or scuppers when the roof edge entraps water if primary drains clog "for any reason."
- 1502.3 sizes scuppers so ponding depth stays within what the roof was designed for.
- Scuppers must open at least 4 inches, and flow through the primary system is not counted.
- Sizing refers to section 1611 and the plumbing code.
The residential and construction texts are different books with different scopes. The Town building office, not this page, says which one governs a building. Ask about overflow in writing.
What a drainage plan should name
The codes and ARMA leave a lot to the designer. A scope for a Herndon flat roof is easier to review when it spells out the drainage in plain terms.
- The slope that exists. State the slope of the deck, and where tapered insulation will add more. The code figure is a minimum for the design, so the plan should say how it is reached.
- The low points. Name each drain, scupper or edge where water leaves, with its location on the roof plan.
- The overflow path. Say whether the roof has secondary drains or scuppers, their size, and the height of the inlet above the low point.
- The obstructions. List chimneys, equipment curbs and wide penetrations, and say whether a cricket or saddle is drawn at each.
- The condensate. Say where air-conditioning condensate lines end.
- The checks after rain. Ask who looks at the roof after a heavy storm, and what counts as ponding in the membrane maker's terms.
Permit steps, the Town's Re-Roof sheet and the historic district rules sit on the Herndon roofing page. This page stays with water and drainage.
Questions for a Herndon flat roof
Take the answers back to the Town building office or your engineer. Book a phone consultation to talk through a Herndon TPO roof, and have the address, a few roof photographs and any drain locations ready.
- Where does water stand on the roof two days after a storm, and are those spots near a drain?
- Is there an overflow scupper or secondary drain, and how high is its inlet above the low point?
- Do the drain bowls and scuppers need cleaning or repair before any membrane work?
- What does the membrane maker say about standing water, in writing?
- Does the scope show crickets, saddles and tapered insulation on a drawing?
For the wider picture of single-ply systems, see the TPO roofing guide, the roofing materials overview, flat roof work, commercial roofing, roof repair and services. The Herndon roofing page covers the local permit path, and service areas lists the other towns.
Records opened and dates
NOAA files were downloaded October 18, 2026 and counted October 20, 2026. The ARMA page and the code copies were opened October 20, 2026.
- NOAA National Centers for Environmental Information, Storm Events details files (Virginia, 2016 to 2026)
- Asphalt Roofing Manufacturers Association, the effects of ponding water
- 2021 Virginia Residential Code, chapter 9 (third-party copy on UpCodes)
- 2021 Virginia Construction Code, chapter 15 (third-party copy on UpCodes)
Questions people ask
Does NOAA record roof leaks in Herndon?
No. Storm Events rows describe wind, hail and flooding as reported to the National Weather Service. A flood row names a road or stream, not a roof. The rows show the kind of weather a flat roof here has to handle, nothing more.
What counts as ponding water on a TPO roof?
The Asphalt Roofing Manufacturers Association defines ponding water as water that remains on a roof for 48 hours or longer. Its page is about asphalt systems, so ask the membrane maker in writing how it treats standing water on a TPO roof.
What slope do the Virginia codes require for a thermoplastic roof?
The 2021 Virginia Residential Code and Construction Code copies read here both set a design slope of not less than 1/4 unit vertical in 12 horizontal for thermoplastic or single-ply membranes. Ask the Town building office which code applies to your building.
What is an overflow scupper?
It is a secondary opening in a parapet wall that lets water leave if the main drains back up. The residential code text sets its inlet 2 inches above the low point of the roof and a minimum opening height of 4 inches.
Does a sloped-to-edge roof need drains?
The residential text says roof drains go at each low point unless the roof is sloped to drain over its edges. Whether a given roof meets that is a question for the designer and the Town building office.
Who decides where a cricket or saddle goes?
The commercial code text calls for a cricket or saddle on the ridge side of a chimney or penetration wider than 30 inches. The roof designer or engineer decides where else one helps. Ask for it to be drawn in the scope.
Do hail reports in Herndon mean a TPO roof is damaged?
No. Three NOAA hail rows carry a Herndon label, at 1.0 to 1.5 inches. A report does not say what any roof sustained. Only an inspection of the membrane can, so ask for photographs of any suspect area.
Does a Herndon re-roof need a permit?
The state code exempts some replacement work, but the Town has its own Re-Roof sheet and historic-district review. The Herndon roofing page lays out both paths. Confirm with the Town before work starts.
Talk through your roof with Roofers of Arlington
Share the address, what you are seeing on the roof and the decision you need to make. We will tell you what to expect before anything is scheduled.