Roofing built for the wettest building you operate
A car wash is the only commercial building where the worst weather happens indoors. While we talk to owners along the 13th Avenue South retail strip, the 45th Street commercial corridor, and the newer express tunnels going up off Veterans Boulevard in West Fargo, the conversation almost always starts the same way: the roof looks fine from the parking lot, but the steel deck above the tunnel is rusting from beneath. That underside attack is the defining problem of car wash roofing, and it is why a membrane that performs for thirty years on a warehouse can fail in a fraction of that time over a wash bay.
Fargo adds a second variable on top of the interior humidity. We average a little under 24 inches of precipitation a year, but the temperature swing is what punishes a wash roof. Warm, saturated air rising off the tunnel hits a deck that may be near zero on a January night, and the moisture condenses inside the assembly instead of escaping. Across hundreds of freeze-thaw cycles each winter, that trapped moisture corrodes fasteners, soaks insulation, and delaminates seams. We build the assembly to control where that vapor goes, not to hope it stays out.
Why the tunnel deck fails from the inside
During a wash cycle, the air inside the tunnel is a fog of hot water, alkaline presoaks, acidic wheel cleaners, foaming detergents, drying agents, and carnauba or polymer wax. That fog does not stay below the trusses. It rises, finds the cold underside of the roof deck, and condenses there as a chemically active film. Over a steel deck the result is corrosion that starts at the flutes and the fastener heads; over a wood deck it is rot and fastener withdrawal. On more than one Fargo tunnel we have pulled a perfectly intact-looking membrane and found the deck beneath it pitted through.
This changes the whole design priority. On a normal roof the membrane is the defense and the vapor question is secondary. On a wash tunnel the vapor question comes first. We specify a continuous air and vapor barrier at the deck level, closed-cell insulation that will not wick and hold condensate, and stainless or coated fasteners rated for a corrosive interior. The membrane on top still matters, but it is the last layer of the system, not the only one.
Picking a membrane that survives the chemistry
TPO, PVC, and EPDM age very differently when detergent mist and wax build up on them. We generally favor a thick PVC for tunnel bays because its chemistry holds up better to the alkaline soaps and oily wax residue that coat an express-tunnel roof, and because hot-air welded PVC seams give a monolithic surface with no adhesive line for chemicals to creep into. Where an owner has an existing TPO field in good shape, we will keep it and concentrate the upgrade over the wash tunnel itself, since the equipment-room and lobby portions of the building see almost none of the chemical load.
- Express exterior tunnels carry the full chemical menu and the heaviest exhaust plume — the tunnel roof is treated as a corrosion-rated assembly, not a standard reroof.
- In-bay automatics and self-serve bays see less airborne wax but almost always have drainage flaws that pond water directly over the equipment they are supposed to protect.
- Full-service washes add a heated, occupied lobby and detail bays, so the roof spans multiple interior climates that each want a different detail.
Exhaust, drainage, and the vacuum canopy
Every tunnel runs high-volume fans to pull steam and vapor out, and those exhaust penetrations are where chemistry, heat, and constant airflow concentrate. We oversize the curbs, build the flashings tall, and detail them for continuous corrosive exhaust rather than reusing a generic HVAC curb wrap. Interior roof drains and overflow scuppers get the same scrutiny — a wash bay produces water year-round, so a drain that ices over on a North Dakota morning has to have a working overflow path or the ponding load lands right back on the bay below.
The vacuum canopy and pay-station canopy on the exit side are a separate building in roofing terms. They are usually metal or a membrane-clad low deck, exposed to tailpipe exhaust, tire-dressing overspray, and the full outdoor freeze-thaw cycle. The canopy-to-building transition and the canopy drain tie-ins are the single most common chronic leak we find on Fargo express sites, so we scope them as their own line item with their own flashing detail rather than folding them into the main roof.
Keeping the cars moving while we work
An idle tunnel is lost revenue by the hour, and most operators here run seven days a week outside of deep-cold closures. We sequence tunnel-roof work into the early-morning or after-close window, dry the assembly in before the bays reopen, and stage materials so a lift never blocks the queue lanes or the vacuum stalls. Equipment-room, lobby, and canopy work can usually proceed during open hours with traffic control that keeps the customer drive clear of the crew. We confirm a watertight dry-in every evening, because a car wash cannot afford an open deck over its own humidity overnight.
Car wash roofing questions
Why does the deck rust if the membrane is not leaking?
Because the moisture is coming from inside. Warm, chemical-laden tunnel air condenses on the cold underside of the deck and corrodes it from beneath, with no surface leak to warn you. The fix is an assembly that stops vapor at the deck — an air and vapor barrier, non-wicking insulation, and corrosion-rated fasteners — not just a new top membrane.
What membrane do you put over the tunnel bay?
A thick PVC, usually fully adhered, for the tunnel itself. PVC resists the alkaline detergents and wax better than TPO or EPDM, and its welded seams leave no adhesive line for chemicals to attack. The lobby, equipment room, and detail bays see far less chemical load and can run a standard single-ply.
Will the chemical exposure void my warranty?
Many standard single-ply warranties exclude chemical attack, which is exactly the exposure a wash tunnel creates. Before we specify, we confirm the membrane is rated for your chemical program and pursue a manufacturer warranty written for the wash environment rather than a generic one that will not honor a chemistry-driven failure.
Can you work without closing the tunnel?
Mostly. Tunnel-roof work happens in the pre-open or after-close window with a confirmed dry-in before the bays reopen. Canopy, lobby, and equipment-room work can run during business hours with the crew and lift staged clear of the wash and vacuum lanes.
Do you handle the vacuum canopy and pay-station canopy?
Yes. Those canopies, their drains, and the transition flashing where they meet the main building are scoped as their own items — they are the most frequent source of recurring leaks on the express sites we see in Fargo and West Fargo.