TPO, PVC or EPDM? A Plain-English Guide for a Flat Commercial Roof

Flat roofs aren't really flat. A commercial roof that looks level from the parking lot still has a slight slope, built so water finds a drain, a scupper or an edge. Roofers call these low slope systems, and the membrane on top is what keeps your building dry. Three names come up in almost every proposal: TPO, PVC and EPDM. Here's what they are, in plain English, if you own or manage a building in the Yakima Valley.

What a membrane roof is

Instead of overlapping shingles, a low slope roof uses large sheets of waterproof material laid over insulation and a deck. Water doesn't run off a low slope as fast, so the roof has to hold up to standing moisture and stay sealed at every seam. The sheets, the way they're joined, and the way they're tied into walls, drains and rooftop equipment make the difference.

New Castle's about page lists TPO, PVC and EPDM as the low slope products it installs. It also says its leaders have a background in low slope systems averaging 10 years of service in the roofing industry each. That's the lens for the rest of this guide.

EPDM: the rubber sheet

EPDM is a synthetic rubber membrane. It's most often seen in black, though lighter versions exist. It comes in large sheets, so a big roof can have fewer seams. Seams are generally joined with adhesive or seam tape rather than heat.

Owners like its straightforward nature, and small repairs are usually simple patches. What to watch are the seams and the flashing details, because bonded joints see the most stress where the roof meets a wall, a pipe or a curb.

TPO: the welded light-colored sheet

TPO is usually white or light gray, which reflects sunlight. Its seams are joined with hot air welding, which fuses two sheets into one rather than gluing them. A good weld becomes part of the membrane.

Weld quality depends on the crew's skill and the conditions on the day. So ask who does the welding and how the seams get checked. A contractor who can answer that clearly is telling you something about how the job will go.

PVC: the chemical-resistant sheet

PVC is also a heat-welded membrane. It's known for standing up to chemicals and grease, which is why you often see it on restaurants and on buildings where exhaust fans send residue onto the roof. If your roof carries equipment that discharges anything onto the surface, say so in your first conversation. That can change the right choice.

TPO and PVC look similar from a distance and share welded seams, but they're different materials with different strengths. Your roofer should be able to explain why one fits your building better than the other, without leaning on a brand name.

The parts you don't see from the ground

The membrane gets the attention, but leaks usually start at the details: drains, pipes, curbs around rooftop units, wall edges and the metal at the perimeter. Whoever installs your roof has to flash each of those properly.

New Castle fabricates sheet metal in-house, so edge metal and flashing can be made to fit your building rather than ordered off a shelf. Their commercial page puts it this way: “We do in-house metal fabrication, saving you time and money by cutting out the middle man.”

The same page lists schools, airports, offices, retail, warehouses and manufacturing facilities among the buildings its commercial team has worked on, and says the company has helped dozens of Yakima Valley businesses get a durable roof. Wapato-based, but working across the valley.

How to compare proposals

Two bids can look alike and describe very different roofs. Put these questions to each contractor and compare the answers side by side.

• Which membrane is it, and why was it picked for this building?
• How are the seams made, and how are they checked?
• What happens at drains, pipes, equipment curbs and wall edges?
• Is the existing insulation staying or going, and what is that decision based on?
• Who makes the edge metal, and to what standard?
• What does the warranty cover, and is it in writing?

A proposal that names the membrane and walks through those details tells you far more than one that only lists a total. It also shows the contractor has actually looked at your roof.

Walk the roof with the contractor if you can. Point out any spot where water has ponded or a ceiling has stained inside. A good roofer will take notes, climb over to the trouble areas and tell you what they see, good or bad.

Don't choose by color or brand name alone. Think about how the building is used, how much rooftop equipment you have, how often service people walk the surface, and how well the roof drains today. Those facts point toward a membrane better than any brochure does.

To talk through your building, call New Castle at (509) 941-8952, Monday through Saturday, 7am to 5pm, or use the contact page. Free estimates are part of how they work.