PVC roofing in Alexandria, VA: the weather record and the welded seam

The storm rows, cut by month and date

The National Oceanic and Atmospheric Administration (NOAA) publishes its Storm Events database as yearly files. For the City of Alexandria, the files released between March 23 and September 18, 2026 hold 75 rows, and the first one is dated 2017. The sorting that matters for a seam is not by event type but by when the rows fall and how many separate days produced them.

Source: NOAA Storm Events bulk files, rows labeled ALEXANDRIA (C) in Virginia, counted by month on October 24, 2026. January, February, April and November have no rows. Distinct dates are counted within each month.

Read the table as a calendar, not a forecast. Thunderstorm wind clusters in July, flash flooding in August, and 2021 is the heaviest year with 20 rows. The August 14, 2023 date alone supplies seven flash flood rows and one wind row. These are reports that someone logged, and the counts cannot say what happened to any one roof.

What the wind rows do and do not show

A welded lap joins sheets to each other. It is not what keeps a roof on the building, which is the job of the attachment and the tested assembly. The strongest rows in the file are a useful reminder of that gap between weather and roofs.

• May 26, 2021. A thunderstorm wind row estimated at 60 knots. The narrative describes trees blown down on West Braddock Road and a tree on power lines on Mount Vernon Avenue.
• July 29, 2023. The strongest row, estimated at 70 knots, with large trees, branches and wires down across the city.
• August 14, 2023. Seven flash flood rows and one wind row on one date, with road closures and stranded vehicles in the narratives.

None of the 75 narratives mentions a roof or shingles, and wind magnitudes in the file are estimates. Flash flood rows describe water on streets, not on roofs.

For uplift, the guide specification asks for a complete roof assembly rated against wind loads and validated by Factory Mutual testing, and notes that an FM 1-90 rating correlates to a maximum calculated uplift of 45 pounds per square foot with a safety factor of 2. The Virginia commercial code lists PVC sheet under ASTM D4434, as read on a copy of the code, and sends edge metal on low-slope single-ply roofs to ANSI/SPRI ES-1 tests. Confirm the edition with the City.

Summer sun and the crew on the roof

PVC seams are made in the open, so the season of the work matters to the people doing it. The National Weather Service explains that its heat index was devised for shade and light wind, then adds one line about sunshine.

The same page describes the Wet Bulb Globe Temperature, which folds in solar radiation and wind and is aimed at outdoor workers. Neither measure describes the temperature of a roof membrane, and the page does not mention roofing. What it supports is a plain question for a scope: how does the crew plan its hours on a summer day, given that the weld work is done by hand tools and machines on an exposed surface?

How a hot-air weld is made

PVC is a thermoplastic: heat softens the sheet and cooling sets it. That is why a welded lap can become one piece with the sheet. SPRI, in a 2019 article by Dave Nordentoft, says the whole job comes down to three controls.

1. Temperature. Hot air is blown between the overlapped sheets so the upper and lower surfaces soften together.
2. Speed. How fast the welder moves along the lap sets how long the heat acts on the surfaces.
3. Pressure. A roller presses the heated surfaces together. SPRI says this is how they bond at the molecular level.
4. Test welds first. The article says contractors with automatic welders run test welds at the beginning of each day and save the temperature and speed settings that worked.

The article is careful about hand welding, which it says is still approved. Ambient air temperature, line voltage, how fast the technician moves and how evenly the roller is pressed all change the result, and it says these factors can, though infrequently, leave partial bonds that are subject to failure over time. Automatic machines reduce some of that, since the machine weight supplies steady pressure. Its figure for a good seam is a 1.5 inch weld that becomes stronger than the membrane beside it when properly performed.

PVC-specific rules come from the federal guide specification, Unified Facilities Guide Specification 07 54 19, August 2023 edition. It bans torch equipment for seams and repairs, and requires heat guns used within the membrane manufacturer's written instructions. It also bans heat guns or open flame to speed the drying of adhesives, and keeps bonding adhesive off the sheet surfaces in seam or lap areas.

How a weld is checked

This is a federal document written for military and government projects. It does not bind a private building in Alexandria, and it is quoted here as a published example of how a checking duty can be written down. Ask the manufacturer's instructions and your roofer which checks apply to your sheet.

Lap widths in a federal specification

The same specification sets how far sheets overlap before the weld, and the figure depends on how the sheet is held down.

Adhered sheet, side overlap

At least 3 inches. The specification keeps bonding adhesive off the sheet in lap areas.

Mechanically fastened sheet, side overlap

5.5 to 7 inches, a figure the specification notes as typical so that 3 inches of seam width remains beyond the fastener plate.

End laps

At least 4 inches for both methods.

One more detail comes from the same section: the direction of laps must let water flow over them and not against them. The text also calls for felt-backed membrane placed directly on a concrete deck or other hard surface. The wording is from the specification, and the figures belong to that document.

Weld clauses for a written scope

• Which seam method does the quote use for field seams and for flashing, and does it follow the manufacturer's published instructions for the named sheet?
• What weld width does the quote state, and is that the width the instructions require?
• Who makes test welds at the start of each day, and are the settings recorded?
• Is every weld probed along its whole length, by whom, and on the day it is made?
• How are defects handled: rewelded, or cut out and covered with a patch welded on all sides, and is the patch re-probed?
• If a peel test is cut, who does it and where is the sample patched?
• What hot work precautions apply, and is a fire watch part of the plan?
• Does the building need a permit, an association vote or historic review before the work starts?

Book a phone consultation to talk through an Alexandria PVC roof: have the address, the building type and what you can see of the roof. For the City rules see the Alexandria roofing guide, and for the material in general the PVC roofing page, flat roofing, commercial roofing, materials and services. Back to the Alexandria hub or call (571) 397-6072.

References, opened October 24, 2026

• NOAA National Centers for Environmental Information, Storm Events bulk CSV files, rows for Alexandria (C), Virginia, released March 23 to September 18, 2026.
• National Weather Service, Heat Index.
• Unified Facilities Guide Specification 07 54 19, Polyvinyl-Chloride Roofing, August 2023.
• SPRI, Introduction to Hot-Air Welding, September 20, 2019.
• Virginia Construction Code 2021, chapter 15, UpCodes copy, because the code publisher blocks automated access.