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TPO roofing in McLean, VA: when the code lets a new sheet go over the old roof
The first decision on a low-slope McLean roof is rarely the membrane. It is whether the old roof can stay underneath. Virginia's existing-building text answers that with three bars, one set of definitions and one exception for drainage, and the Fairfax County permit pages add their own list. This page reads those texts in order, adds what two ASTM test standards and a single-ply trade association say about proving an old assembly is dry, and marks where the paperwork is silent.
- 3 conditions bar a roof recover in the Virginia text
- 75% or 30,000 sq ft triggers an energy check on replacement
- County permit list names Re-Roof/Re-Line
Three conditions that bar a recover
A TPO membrane is a thermoplastic single-ply sheet, and Virginia's building code names it by standard: ASTM D6878 on the code's single-ply materials table. When a McLean owner asks whether the new sheet can simply cover the old roof, the answer sits in the existing-building provisions of the Virginia Uniform Statewide Building Code. Read from the state's own legal site, that text lists three situations in which a roof recover is not permitted.
- Water soaked or inadequate base
- The existing roof or covering is water soaked, or has deteriorated until it is not adequate as a base for additional roofing.
- Slate, clay or cement tile
- The existing covering is slate, clay, cement or asbestos-cement tile. This one concerns tile roofs, not membrane roofs.
- Two or more applications
- The existing roof already has two or more applications of any type of roof covering.
Source: 13VAC5-63-433.3, section 602.3.2.1.2, Virginia Administrative Code, opened October 20, 2026.
The same section also lists four conditions where a recover is permitted. Three of them name a different roof type: a separate standing-seam system, a wood shake roof that needs a board cover, and a protective coating over spray foam. The fourth reads: the new covering is installed under the manufacturer's approved instructions. A membrane sheet over a membrane appears to fall under that fourth one, so the product instructions become part of the code answer. Ask the roofer for the instruction sheet that governs a re-cover.
Recover and replacement are defined terms
The code's definitions, read on a third-party copy of the 2021 existing-building code, separate the two jobs. A roof recover installs an additional covering over a prepared existing covering without removing it. A roof replacement removes the existing covering, repairs any damaged substrate and installs the new covering. Those words matter because other rules, such as the energy trigger below, are written for one of the two.
The text adds three allowances. A low-slope recover or replacement need not meet the one-quarter inch per foot minimum slope where the roof has positive drainage. Secondary drains or scuppers need not be added in that case, though existing secondary drains may not be removed unless replaced with compliant ones. And the structural check is waived where the added load raises the force in a member by 5.0 percent or less, or where a second covering of 3 pounds per square foot or less goes over a single layer.
The Virginia Construction Code adds a pointer. Its reroofing section sends reroofing and roof repair back to Chapter 15 and to the existing-building sections that apply. Chapter 15 itself sets a minimum design slope of one-quarter unit vertical in 12 for single-ply roofs and says fully adhered or mechanically attached single-ply systems are tested to FM 4474, UL 580 or UL 1897.
How an old roof is shown to be dry
The code says water soaked and leaves the proof to the people on the roof. Three neutral documents describe the tools, and each one states its own limit.
- Map the wet areas. ASTM C1153 covers infrared imaging done at night to locate wet insulation in systems that have insulation above the deck in contact with the waterproofing. It sets criteria for the camera and the weather.
- Check the map with cores. The same practice addresses verification of the infrared data with invasive methods. The trade association SPRI advises core cuts before re-covering over existing insulation, to learn what the system contains and whether a vapor barrier would stop drying.
- Test the membrane itself. ASTM D7877 describes electrical conductance methods that locate leaks in exposed or covered waterproof membranes. It says it does not replace visual, infrared or other inspection and is used together with them when specified.
- Decide area by area. SPRI says most roofs are not wet all over, and that wet insulation can be removed where the system cannot dry and replaced with new insulation. A written result can therefore split one roof into cut-out areas and kept areas.
"It is advisable to conduct moisture surveys to determine if wet insulation exists."
SPRI, answer to the re-cover question on its FAQ page, read October 20, 2026Two limits are worth repeating. ASTM C1153 says it does not find the cause of moisture or where it entered, and it does not judge whether a system still works as waterproofing. SPRI is a single-ply trade association, so its view is an industry one, and it tells owners to consult the manufacturer on re-cover criteria. A survey report is useful to an owner only if it names the method, the date, the conditions and the cores that checked it.
House text and building text side by side
McLean has both detached houses with a low porch or addition roof and larger buildings with a flat roof, and Virginia keeps two code texts. The residential text below is the 2021 model code as shown on a third-party Virginia page; ask the county whether Virginia amended the section.
| Point | Existing-building text (state legal site) | Residential text, section R908 (third-party copy) |
|---|---|---|
| Name of the full job | Roof replacement: remove covering, repair substrate, install new | Roof replacement removes existing layers down to the roof deck |
| Bars on a recover | Water soaked or inadequate base; tile; two or more applications | The same three |
| New coating over single-ply | Names only a coating over existing spray foam | Names a new protective coating over thermoset and thermoplastic single-ply without tear-off |
| Slope and drains | Exceptions where positive drainage exists | Same exceptions in section R908.1 |
Sources: 13VAC5-63-433.3; 2021 International Residential Code chapter 9 on UpCodes (Virginia page), opened October 20, 2026.
The coating row shows the texts are not copies of each other. Which one governs a roof depends on the building group, which is a question for the building official, not a roofer or this page.
Where the Fairfax permit list fits
- Fairfax County Land Development Services lists Re-Roof/Re-Line and Roof Repair among the commercial miscellaneous permit types, applied for online in its PLUS system.
- Its commercial no-permit list includes replacement of less than 100 square feet of roof covering. The state text says 100 square feet or less. The two wordings differ at exactly 100.
- The state text exempts unlimited roof covering replacement in groups R-3, R-4 and R-5 unless the nominal design wind speed is over 100 mph, and lets a building official require a permit in a locally designated historic district.
- The county page says reviewers may include building plan review, the Fire Marshal and zoning, and that inspections follow once a permit issues. The call line is 703-222-0801.
The energy figure needs care. Section 601.4.5 applies to roof replacement of more than 75 percent or 30,000 square feet of the roof area, whichever is less, where the roof is part of the thermal envelope and its insulation sits entirely above the deck. The text defines roof area by walls, levels, edges, dividers, joints and parapets. It does not say how a recover is treated, so a re-cover of a large roof should be put to the county in writing. The McLean roofing guide shows the permit layers for a house.
What to put in the written scope
A scope that says re-cover or tear-off should state the evidence for the choice: how many layers the cores showed, which areas tested wet, and which document the manufacturer supplied for a re-cover.
- Layer count. How many existing applications, from cuts, and where the cuts were taken.
- Moisture method. Infrared, conductance, core cuts or a mix, with the date and the weather.
- Wet areas. A plan showing which areas of insulation come out and which stay.
- Drainage. Whether the roof has positive drainage, and which secondary drains or scuppers exist.
- Edge metal and wind. The chapter 15 wind and edge tests the roofer is working to, from FM, UL or ANSI/SPRI ES-1 documents.
- Permit. Who applies, which record type, and what the county said about energy and inspections.
For the membrane itself, see the general TPO roofing page, and the flat roof, roof replacement and commercial roofing pages for scope and process. The materials overview compares systems. Book a phone consultation to talk through a McLean TPO roof; the McLean guide and the services page sit behind it.
Where each rule was read
All pages below were opened on October 20, 2026. ASTM pages were read as archived copies of the astm.org abstract pages.
- 13VAC5-63-433.3, Chapter 6 Alterations, Virginia Administrative Code and 13VAC5-63-80, permit exemptions
- Virginia Existing Building Code 2021, definitions and Virginia Construction Code 2021 chapter 15 (third-party copies of code text)
- 2021 International Residential Code chapter 9 (third-party copy)
- Fairfax County, Does my project require a permit and commercial miscellaneous permit guide
- ASTM C1153, wet insulation by infrared imaging and ASTM D7877, electronic leak detection in membranes
- SPRI single-ply roofing FAQ
Questions people ask
Can a new TPO sheet go over an existing membrane in Virginia?
The state text permits a recover where the new covering is installed under the manufacturer's approved instructions, and bars it in three situations: a water-soaked or inadequate base, a slate or tile roof, or a roof that already has two or more applications. Whether your roof meets those tests is a finding about that roof, so ask for it in writing.
How many layers can a McLean roof carry before tear-off is required?
The Virginia text bars a recover where the existing roof has two or more applications of any type of roof covering. On a plain reading, an original membrane plus one earlier recover already counts as two. Ask Fairfax County Land Development Services how it reads the count for your building, and ask the roofer how many layers a core cut shows.
What does water soaked mean in the code?
The code does not define it and names no test. The text says only that a recover is not permitted where the roof or covering is water soaked or too deteriorated to serve as a base. That is why owners ask for a moisture survey and core cuts, which the sources below describe, rather than a visual opinion.
Does an infrared scan prove the insulation is dry?
No. ASTM C1153 describes night-time infrared imaging that locates wet insulation above the deck, and it states that the practice does not determine the cause or the point of entry. It also covers checking the infrared data with invasive methods. Treat the scan as a map and the cores as the check.
Is a permit needed to put a TPO membrane on a McLean roof?
The state permit text exempts 100 square feet or less of roof covering in all groups, and unlimited replacement in groups R-3, R-4 and R-5 unless the nominal design wind speed is over 100 mph. The county's commercial permit page lists Re-Roof/Re-Line. Ask Land Development Services at 703-222-0801 how your address and building group are handled.
Does re-covering avoid the energy-code insulation upgrade?
The Virginia energy trigger is written for roof replacement: more than 75 percent or 30,000 square feet of the roof area, with insulation entirely above the deck, in the thermal envelope. The same code defines replacement as removing the old covering, and the trigger text does not mention a recover. Ask the county whether a re-cover of your size is checked.
Does a re-cover need the 2 percent minimum slope?
The existing-building text excuses a low-slope recover or replacement from the one-quarter inch per foot minimum slope where the roof provides positive drainage. It also excuses adding secondary drains or scuppers in that case, though existing secondary drains may not be removed unless replaced. A roofer should state which drains exist.
What should I have ready before a call about a McLean TPO roof?
Book a phone consultation and have the street address, the building type, the roof area if you know it, any earlier roofing work you know of, and any leak history to hand. The layer, moisture and permit questions on this page are the ones to settle before you collect a written scope.
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.