If your torchdown roof is past 15 to 20 years old, leaking at the seams, or holding water after every storm, the torchdown vs PVC question usually has a clear answer: an upgrade to a heat-welded PVC membrane gives a Seattle flat roof better seam strength, better resistance to standing water, and a longer service life than another round of torchdown. A younger torchdown roof in good shape can often be repaired and kept in service a while longer.
Torchdown is still common on older homes and small commercial buildings around the Puget Sound. This guide covers how the two systems differ, the signs a torchdown roof is ready to retire, when PVC can go over the existing roof, and how to plan the switch.
Key Takeaways
- Torchdown is a modified bitumen membrane fused in place with an open-flame propane torch, and most torchdown roofs reach the end of their useful life somewhere around 15 to 20 years.
- PVC seams are joined with hot air into a single continuous sheet, with no open flame on the roof, which removes the weak lapped seams where torchdown roofs most often leak.
- Blistering, open laps, alligatored cap sheet, and water that sits more than 48 hours after rain are the clearest signs a torchdown roof is due for replacement.
- PVC can sometimes be installed over a sound torchdown roof with a separation board, but a wet or multi-layer roof has to be torn off first under the building codes Washington adopts.
Torchdown vs PVC: What Is the Actual Difference?
The two systems are built in completely different ways. Torchdown is a rolled modified bitumen sheet, an asphalt product reinforced with polyester or fiberglass. The installer heats the underside with a propane torch until it melts and bonds to the layer below, then overlaps each new roll onto the last one and torches the lap closed. A typical torchdown roof is two plies: a base sheet and a granule-surfaced cap sheet.
PVC is a single-ply thermoplastic membrane. Large sheets are mechanically fastened or adhered to the roof, and the seams are fused together with a hot-air welder. Done correctly, the weld makes the overlap a continuous piece of membrane that is often stronger than the sheet itself. There is no asphalt and no open flame involved.
That construction difference drives almost everything else in the torchdown vs PVC comparison:
- Seams: torchdown relies on melted asphalt at each lap, while PVC seams are welded into one sheet.
- Standing water: asphalt-based membranes break down faster under ponding water, while PVC tolerates wet conditions far better.
- Surface wear: torchdown cap sheets shed granules and crack as they age in sun and rain, while a PVC membrane keeps its surface.
- Installation: torchdown uses an open flame on the roof, and PVC uses hot-air welding equipment.
If you are weighing PVC against the other common single-ply option, our breakdown of how PVC compares with TPO in the Pacific Northwest covers that choice in detail.
Why Do Torchdown Roofs Struggle in the Puget Sound Climate?
Torchdown was the default flat roof here for decades, but a Seattle winter tests exactly its weakest parts. Rain falls on a flat roof for months at a stretch, so any lap that has started to separate gets a steady supply of water working its way underneath.
Ponding makes the problem worse. Low spots that hold water through a wet week keep the asphalt saturated, which speeds up deterioration of the membrane and the seams in that area. Over time the cap sheet loses granules, the surface cracks into an alligator pattern, and blisters form where moisture gets trapped between plies.
This is why so many older flat-roofed homes in neighborhoods like West Seattle end up with a cycle of patch, leak, and patch again. Each repair can be sound, yet the roof around it keeps aging.
What Are the Signs Your Torchdown Roof Needs Replacing?
Age alone does not settle torchdown vs PVC, though any torchdown roof past about 15 years deserves a close look. These are the signs that point toward replacement rather than another repair:
- Open or lifting laps. Seam edges you can lift or see daylight under are where most torchdown leaks start.
- Blisters and soft spots. Bubbles in the membrane, or areas that feel spongy underfoot, usually mean trapped moisture or wet insulation below.
- Alligatoring and granule loss. A cracked, bare cap sheet has lost the protective surface that shields the asphalt from UV and weather.
- Ponding that lasts more than 48 hours. Water that lingers two days after the rain stops is accelerating wear on every square foot it covers.
- Repeat leaks in different places. One leak is a repair. A new leak every winter in a new spot means the whole membrane is wearing out.
- Interior signs. Ceiling stains, peeling paint near the roofline, and musty odors often show a problem before it is visible on the roof.
If you only see one or two of these on a roof that is otherwise sound, targeted roof leak repair may buy you several more years. When three or more show up together, you are usually paying to repair a roof that is near the end of its life anyway, and that money tends to go further toward an upgrade.
Can PVC Go Over an Existing Torchdown Roof, or Does It Need a Tear-Off?
Sometimes it can. A recover installs the new PVC system over the existing roof instead of stripping it off, which keeps old roofing out of the landfill and shortens the job.
There are firm limits, though. The model building codes that Washington adopts through the Washington State Building Code Council do not allow a recover when the existing roof is water-soaked or deteriorated to the point it cannot serve as a sound base, or when the roof already has two or more layers of roof covering. In either case the old material comes off first.
There is also a compatibility issue specific to PVC. The plasticizers that keep PVC flexible can migrate into asphalt-based materials, and contact with asphalt damages the membrane. A PVC recover over torchdown therefore needs a separation layer, typically a cover board, between the old roof and the new membrane.
A core sample or moisture survey settles whether a recover is realistic. If the insulation underneath is wet, a full tear-off and flat roof replacement is the right call, since sealing wet material under a new membrane only traps the problem.
What Changes for Commercial Buildings Upgrading From Torchdown?
Many small commercial, mixed-use, and apartment buildings across the Seattle metro still carry torchdown roofs, and for their owners the choice has two extra angles beyond leak resistance.
The first is fire safety during the work itself. Torchdown installation puts an open flame on the roof, and in Seattle torch-applied roofing is a regulated fire-department operation: the Seattle Fire Department’s Temporary Roofing Operation permit (7901-T) specifically covers torch-applied roof material and comes with a pre-inspection checklist before work begins. On an occupied building, keeping open flame off the roof is a real advantage of hot-air-welded PVC.
The second is rooftop detailing. PVC flashings around HVAC units and vents weld to the field membrane just like the seams. Property managers with several buildings in places like Everett often pair the new roof with a commercial roof maintenance plan, so drains stay clear and small issues get caught on a schedule instead of during a January leak.
How Do You Plan a Torchdown to PVC Upgrade?
A flat roof replacement goes smoothly when the decisions happen before the crew arrives. Here is a practical sequence for owners moving from torchdown to PVC:
- Get a real assessment. Ask for an inspection with photos, a check of seams and flashing, and a moisture reading or core cut where the roof looks suspect.
- Settle recover vs tear-off. Use what the inspection found about layers and moisture to decide, and make sure a separation board is in the scope if PVC will go over torchdown.
- Fix the drainage while the roof is open. A new roof is the cheapest moment to add tapered insulation or improve drains and scuppers so water stops ponding in the same low spots.
- Review the warranty in writing. Compare what is covered: materials only, or materials and workmanship, and for how long.
- Plan the timing and budget. A dry stretch is ideal. Homeowners can also ask about financing options for residential flat roofing so the upgrade does not have to wait for another leak.
Why the Warranty Matters as Much as the Membrane
Many asphalt-based roof warranties exclude damage caused by ponding water, which is exactly the condition a Seattle flat roof faces all winter, so owners of older torchdown roofs sometimes discover their coverage is thinner than they assumed. Read the exclusions on any warranty before you rely on it.
For the new roof, ask whether the warranty covers workmanship or only the material. Most leaks on a new flat roof come from installation details at seams, edges, and penetrations, so a materials-only warranty leaves the most likely failure points uncovered. At Lifetime Flat Roofs, our Lifetime Total Systems Warranty is a true lifetime roof warranty that covers workmanship as well as materials, because we would rather stand behind the whole roof than one component of it.
How to Choose a Contractor for a Torchdown to PVC Upgrade in Seattle
A torchdown to PVC conversion involves details general roofers rarely handle: separation boards, welded flashings, tapered insulation, and drain upgrades. Look for a flat roofing specialist who can show you similar projects.
Ask who will actually be on your roof, whether one project manager owns the job start to finish, and whether the crew is manufacturer certified. Lifetime Flat Roofs has focused on flat roofing in the Puget Sound for more than 25 years. Every roofer on our crew is certified in flat roofing, every project has a designated project manager, and we do not subcontract the work. Our office is in Kirkland, and we upgrade torchdown roofs to PVC for homeowners and building owners across the Eastside, Seattle, and Snohomish County.
Frequently Asked Questions About Replacing Torchdown Roofs With PVC in Seattle
Can PVC be installed over an existing torchdown roof?
Often, yes, as long as the existing roof is dry, structurally sound, and only one layer. Because PVC reacts with asphalt, a separation layer such as a cover board must go between the torchdown and the new membrane. If the roof is water-soaked, deteriorated, or already has two layers, building code requires a tear-off first.
How long does a torchdown roof last in Seattle?
Most torchdown roofs last roughly 15 to 20 years. Installation quality, drainage, and maintenance make the biggest difference, and roofs that pond water tend to land at the short end of that range in the wet Puget Sound climate.
Should I repair my torchdown roof or replace it with PVC?
Repair makes sense on a younger roof with an isolated problem, such as one open lap. Replacement is usually the better value once the roof is past 15 years, leaks recur in new places each winter, or blistering and cracking are widespread.
Does torchdown roofing require a fire department permit in Seattle?
In Seattle, the Fire Department’s Temporary Roofing Operation permit (7901-T) covers torch-applied roof material and includes a pre-inspection checklist. Your contractor should handle any required permits as part of the project.
Does a PVC roof still need regular maintenance?
Yes. PVC is far more forgiving of wet weather, but drains, scuppers, and penetrations still need attention. A spring and fall inspection, with drains cleared before the rainy season, keeps a PVC roof performing and protects your warranty.
Ready to Retire Your Torchdown Roof?
If your torchdown roof is showing its age, schedule a free flat roof inspection with Lifetime Flat Roofs and find out whether a repair, a recover, or a full PVC upgrade is the right next step.