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EPDM roofing in Herndon, VA: the code slope and what tapered insulation does
An EPDM membrane is a rubber sheet, and the code says little about the sheet itself beyond a standard number. What it sets is a floor for slope. This page reads the thermoset sections of the 2021 Virginia Residential Code and Construction Code, explains where tapered insulation fits, lists what a roofer measures on a flat deck, and points to the Town of Herndon paperwork.
- Code minimum slope: 1/4 inch in 12
- EPDM standard: ASTM D4637
- Cricket needed beside openings over 30 inches
The numbers in the codes
Three figures from the 2021 Virginia texts frame a low-slope EPDM roof. They come from the residential book (chapter 9) and the construction book (chapter 15), read as third-party copies. The Town building office says which one applies to a given building.
Slope floor
1/4 inch of fall in 12 inches of run, written as 2 percent. Over a 20 foot run that is 5 inches of fall (arithmetic, not a code figure).
Material standard
ASTM D4637 is the standard the construction code table names for EPDM. The residential code accepts D4637 or D5019 for thermoset sheet.
Opening width
A chimney or penetration wider than 30 inches, measured across the slope, needs a cricket or saddle on its ridge side.
What the slope rule says
- Residential R905.12
- Thermoset single-ply. R905.12.1 sets the slope at not less than 1/4 in 12 for drainage, R905.12.2 names ASTM D4637 or D5019, and R905.12.3 says to install per the chapter and the manufacturer.
- Construction 1507.12
- Single-ply, with the same 1/4 in 12 figure in 1507.12.1 and EPDM listed against ASTM D4637 in Table 1507.12.2. Ballasted roofs under 2:12 also follow 1507.12.3.
- Drains, R903.4
- Roof drains at each low point unless the roof is sloped to shed over its edges.
The phrase to notice is "for drainage". The slope figure is there so water leaves the roof, not to rate the rubber. A membrane can be sound and a roof can still hold water if the deck and insulation give it nowhere to go.
What tapered insulation adds
Above-deck insulation is a flat set of boards by default. Tapered boards are cut thicker at one end, so a stack of them builds a slope on top of a deck that has none. The code text read here is silent on the word. What it does say, in R906.1 and 1508.1, is that above-deck insulation must be covered by an approved roof covering and must pass NFPA 276 or UL 1256 as an assembly.
The EPDM Roofing Association speaks to tapering directly. For roofs at 1/8 inch per foot or less, it advises placing drains in low areas, using crickets and saddles, and says tapered insulation "may be incorporated throughout the roof or in localized areas."
The same FAQ asks designers to build in positive drainage to avoid excessive ponding. It names two reasons: standing water makes repairs harder, and it adds to moisture damage if a puncture occurs. How the membrane maker treats standing water is the maker's to state, so ask for it in writing.
What a roofer measures
None of the sources above prescribes a measuring routine. The list below follows from what they name: slope, low points, drains and openings. Each step can be recorded with a photograph and a number.
- Deck fallLevel or straightedge readings along the line toward each drain, compared with the 1/4 inch per foot figure.
- Low pointsThe spots where a reading dips, marked on a roof sketch, with their distance from the nearest drain.
- Drains and edgesCount of drains, whether the roof also sheds over an edge, and whether the bowls are open.
- Openings wider than 30 inchesChimneys and similar penetrations, and whether a cricket or saddle exists on the ridge side.
- Existing insulationThickness at each drain and at the highest point, since the difference shows how much taper is already there.
Replacing a roof that already drains
New work
- Design slope of 1/4 in 12 or more under R905.12.1.
- Drains at each low point, or a roof that sheds over its edges.
- Covering installed per the manufacturer.
Reroofing, R908.1
- The slope minimum need not be met where the roof provides positive roof drainage.
- Secondary overflow drains or scuppers need not be added to such a roof.
- Chapter 9 requirements otherwise still apply to materials and methods.
Positive drainage is not defined in the passage read. Whether a particular low roof qualifies is a call for the Town building office and the designer. For construction-code buildings, section 1512.1 sends reroofing to chapter 15 and to parts of the Virginia Existing Building Code.
Herndon paperwork
| Document | What it says |
|---|---|
| 13VAC5-63-80, state exemption | Replacement of an unlimited amount of roof covering in R-3, R-4 and R-5 buildings is exempt where design wind speed is not over 100 mph; 100 square feet or less is exempt in all groups. |
| Same section, historic district | A building official may require a permit for replacement roofing inside a locality-designated historic district. |
| Town Re-Roof sheet | Zoning approval first if the property is in the historic district, then submission by email. It states a review time of about 4 weeks. |
The sheet was read October 18, 2026. The Town site did not load by script on October 20, so confirm the current form with the Town. The Herndon roofing page sets out the full path, including the historic district split.
Questions to take to the Town and your designer
- Which book, residential or construction, governs this building?
- Does this roof provide positive roof drainage under R908.1?
- Will tapered insulation be drawn on a plan, with low points and drains marked?
- Is a cricket or saddle shown beside every opening wider than 30 inches?
- Who inspects the roof twice a year, and after storms?
Book a phone consultation to talk through a Herndon EPDM roof, and have the address and a few roof photographs ready. For the wider picture, see the EPDM roofing guide, the roofing materials overview, flat roof work, roof replacement, services, Arlington and service areas.
Codes and guides opened
All pages below were opened October 20, 2026, except where noted in the Herndon section.
Questions people ask
What minimum slope do the Virginia codes set for an EPDM roof?
Both the 2021 Virginia Residential Code and Construction Code texts read here set a design slope of not less than 1/4 unit vertical in 12 horizontal, which is 2 percent, for thermoset single-ply and EPDM. Ask the Town of Herndon building office which book governs your building.
Is EPDM thermoset or thermoplastic?
The residential code lists EPDM under thermoset single-ply roofing, with ASTM D4637 or D5019 named as the material standards. The construction code table names EPDM and ASTM D4637 specifically.
Does the code mention tapered insulation?
The code sections read here do not use the phrase. They cover above-deck insulation in general, requiring it to be covered by an approved roof covering and to pass NFPA 276 or UL 1256 as an assembly. Tapering is a design choice that the EPDM Roofing Association mentions for drainage.
What does tapered insulation do on a flat roof?
It is insulation board cut at a slope so that water has a path to a drain. The EPDM Roofing Association says it may be used across a roof or only in localized areas where more drainage is needed.
Can a flat roof with almost no slope still get a new EPDM covering?
The residential code says reroofing need not meet the 1/4 in 12 minimum where the roof provides positive roof drainage. The text does not define that phrase in the passage read, so ask the Town building office and your designer how they apply it.
What is a cricket on a flat or low roof?
It is a small raised form that sheds water around something that would block it, such as a chimney. The construction code calls for one on the ridge side of a chimney or penetration wider than 30 inches.
How often should an EPDM roof be inspected?
SPRI, the single-ply roofing trade association, says low-slope roofs should be inspected at least twice a year and after any major storm, with drains kept open and flashings checked.
Does a Herndon EPDM re-roof need a permit?
The state code exempts some replacement of roof covering, but the Town has its own Re-Roof sheet and a separate historic district review. The Herndon roofing page lays out both. 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.