How a TPO Roof Is Installed, Step by Step

A TPO installation runs in six steps. Strip the old covering, inspect and repair the deck, lay insulation, fasten the membrane, heat-weld every seam, then flash every edge and penetration. The welding is the step that decides whether it leaks. Whether your building needs a full strip or can take a recover depends on how wet the existing insulation is, and that is checked, not assumed.

Rancho Roofing works on low-slope sections across San Diego County. This page is written so a homeowner standing in the driveway can tell whether the job is being done properly, which is more useful than a description of the product.

One fact frames all six steps. R&E Roofing’s April 2026 guide reports that seam failures are the most common cause of leaks on a low-slope surface. Everything below is either making seams or protecting them.

What goes down before the membrane?

The old covering comes off, the deck gets checked, and insulation goes down.

Tear-off is first, unless the building is getting a recover. A recover lays new material over the old, which saves the tear-off cost and is legitimate when the existing insulation is dry and the deck is solid. Over wet insulation it traps the problem and shortens the life of everything above it. The insulation has to be opened and checked, not assumed from the ground.

Then the deck. Soft or delaminated sheathing gets cut out and replaced. This is the step that changes a price mid-job, which is why a written allowance for deck replacement belongs in the quote.

Insulation follows. It carries most of the energy performance and a large share of the cost, and it is also what gives the surface its slope on many buildings. Tapered insulation is how a dead-flat deck gets the fall it needs to drain.

How is the membrane fastened down?

Either mechanically, with plates and screws hidden under the next sheet, or fully adhered with glue across the whole surface.

Mechanically attached is faster and cheaper. Fasteners with round plates go through the membrane along one edge of each sheet. The next sheet laps over them, so the fasteners end up under a welded seam rather than exposed.

Fully adhered bonds the whole sheet to the insulation below. It costs more, it holds better against wind uplift, and it moves less under the sheet, which matters on an exposed roof. Premium Roofing’s June 2026 comparison notes that a fully adhered installation gives the best wind uplift performance regardless of the membrane type.

Neither method is a shortcut. The wrong one for the building is the shortcut.

How are the seams joined?

With hot air, not glue, and that is the whole reason to choose TPO.

A hot-air welder runs along the overlap and melts the two sheets into a single piece of material. Done properly there is no adhesive involved and no separate product to fail later. Premium Roofing’s June 2026 figures put a welded TPO seam at 100 or more pounds per inch of strength, against roughly 60 for EPDM’s glued and taped seams.

A careful installer probes the finished seams with a hooked tool to find any spot that did not take. That check takes minutes and it is the difference between a roof that is watertight and a roof that looks watertight.

If nobody probed the seams, nobody knows whether the roof is finished.

What should the edges and penetrations look like?

Every pipe, drain, curb, and wall line gets its own flashing detail, welded in and mechanically held at the top.

The open field of a membrane roof almost never fails. Water gets in where the sheet has to stop and turn: around a vent pipe, at a drain, up a parapet wall, around a skylight curb.

Look for a proper boot or a field-fabricated corner welded at every penetration, not a smear of sealant. Look for termination bar and counter-flashing where the membrane runs up a wall, not membrane tucked behind stucco and hoped for. Look for edge metal that grips the membrane rather than sitting on top of it.

RoofRanked’s March 2026 guide gives a maintenance figure worth knowing here. Catching a seam separation early is roughly a $200 repair. Ignoring it for two years turns it into about $2,000 of interior damage plus the repair. The details you can see from the ground are the ones that decide that.

What has to be true about the weather?

The surface has to be dry, because welding and adhesive both need a dry sheet.

This is why flat work stalls in wet weather and why pressing on through drizzle is a warning rather than a favour. A seam welded over a damp sheet can look perfect and hold for a season.

It is also why the marine layer matters here. A surface that is damp until mid-morning shortens the working day, and rushing to make up that time is where mistakes enter. For independent guidance rather than a contractor’s opinion, the National Roofing Contractors Association publishes installation standards. If you want a low-slope section looked at properly, we install membrane systems.

How can I tell if the insulation under my old roof is wet?

Not from the ground. It takes a test cut, a moisture meter, or an infrared scan taken after sunset, when wet areas hold heat longer than dry ones. Soft spots underfoot and a surface that stays dark after rain are hints, not proof.

What is a mil, and why does it keep coming up?

It is a thousandth of an inch, and it measures membrane thickness. RoofRanked’s March 2026 guidance is that 60 mil from a major manufacturer is the solid choice, and 45 mil is a thinner margin for error. The price difference on a typical surface is a few hundred dollars.

Can a TPO roof be walked on afterwards?

For maintenance, yes, with care. For regular foot traffic to reach equipment, walkway pads get welded down along the route. Without them, repeated traffic wears the membrane in exactly the places people step, which are also the places nobody inspects closely.

Does the whole roof have to be done at once?

A section can be done on its own where it is bounded by a wall or an expansion joint. Splitting a single continuous surface across two jobs means creating a seam between old and new material, and that seam becomes the weakest part of the finished roof.