Modified Bitumen Parapet Wall Flashings: Why They Matter

When it comes to protecting a flat or low-slope roof, there’s one area that’s especially prone to failure if not installed right: the parapet wall.

At The Roof Champion of Columbus, I’ve seen dozens of flat roof issues that trace back to improperly detailed wall flashings, especially on modified bitumen systems. If you’re a homeowner with a flat or low-slope roof, this is something you need to understand.

Let’s break it down simply.

What Is a Parapet Wall?

A parapet wall is the short vertical wall that extends above the roof level—common on flat roofs. It looks simple, but it creates a vulnerable junction where horizontal roofing meets vertical wall.

Water loves to sneak in at this transition.

And in modified bitumen systems, sloppy parapet flashing is one of the top reasons for leaks, trapped moisture, and premature roof failure.

What Flashing Should Look Like on a Modified Bitumen Roof

Here’s what you should expect from a properly done parapet wall flashing detail on a modified bitumen roof:

• Base Ply runs up the wall at least 8 inches.

• Cap Sheet is torched, adhered, or cold-applied over the base ply, extending up the wall.

• Flashing termination is either:

  • A metal reglet flashing cut into the wall with sealant, or

  • A surface-mounted metal counterflashing fastened to the wall.

• Cant strip at the roof-to-wall corner to ease the transition and prevent sharp bending of the membrane.

• All components sealed watertight—no open seams, no shortcuts.

Common Mistakes I See in Columbus Homes

Unfortunately, I see these details botched all the time.

Here’s where contractors go wrong:

• Membrane not turned up the wall high enough.

• No cant strip at the corner—causes membrane stress and cracking.

• Surface-mounted flashing installed without proper sealant or fasteners.

• Sloppy torching techniques that lead to burned or blistered membrane.

• Relying on caulk alone to stop water instead of using proper flashing materials.

Why This Matters for Your Roof

If water gets behind your membrane at the parapet, here’s what can happen:

• Water gets trapped behind the wall.

• Insulation becomes saturated.

• Blisters or delamination appear on your roof.

• You see bubbling or leaks—especially after freeze-thaw cycles in our Ohio winters.

The worst part? Insurance often won’t cover it if it’s a result of poor workmanship.

That’s why I take these flashings seriously. We do it the right way, or we don’t do it at all.

What I Recommend as Your Roofing Advocate

If you have a modified bitumen roof—or any flat roof system—ask your roofer these questions:

• Does the flashing run at least 8" up the wall?

• Is there a cant strip at the base of the parapet?

• What type of termination is being used?

• Are they using a separate base and cap sheet system?

And if you’re not sure, call me. I’ll inspect it myself and tell you the truth.

Serving Homeowners Who Want It Done Right

I’ve helped homeowners in Powell, Dublin, Upper Arlington, Hilliard, Westerville, and all around Columbus get real solutions—not just bandaid repairs.

If your flat roof isn’t draining right or your parapet flashing looks like it’s been caulked and prayed over, don’t wait.

Let’s fix it right the first time.

Call me directly at (614) 639-3422 or visit www.roofchampcolumbus.com.

FAQ

How do I know if my parapet flashing is failing?
Look for bubbling, blistering, or signs of moisture near walls. If you see cracking around the base or caulk pulling loose, it’s a red flag.

Can parapet flashing be repaired or does it need full replacement?
Depends on the condition. Minor defects can be repaired, but widespread issues often require removal and re-flashing to prevent future leaks.

How long should modified bitumen flashing last?
If installed correctly, 20–30 years. But shortcuts cut that in half—or worse.

Do all flat roofs have parapet walls?
Not always. Some have edge metal terminations instead. But when a wall is present, flashing becomes even more critical.