Re-Roofing Over Modified Bitumen Roofing (MB): What You Need to Know

Modified bitumen (MB) roofing has been a go-to for decades in Colorado and Georgia for its flexibility, durability, and performance on low-slope and flat roofs. But when it’s time for a new roof, can you re-roof directly over your old MB system?

Yes—but only if it’s done correctly.

Here’s what property owners in Denver, Atlanta, and other major cities need to understand about re-roofing over modified bitumen systems.

Can You Re-Roof Over Modified Bitumen?

You can—under specific conditions:

• The existing MB roof must be dry, clean, and stable.

• There should be no trapped moisture or underlying structural damage.

• The new roof system must be compatible (chemically and physically) with the MB surface.

ARMA (Asphalt Roofing Manufacturers Association) and the NRCA both emphasize: never install a new system over a wet or deteriorated roof.

Best Roofing Options to Install Over Modified Bitumen

Here are typical systems installed over MB:

1. New MB Membrane (Cap Sheet Overlay)

• Often used in 2-ply systems.

• Can be hot-mopped, cold-applied, or torch-applied.

• Requires primer if the existing surface is mineral-surfaced.

2. Single-Ply Systems (TPO, PVC, EPDM)

• Must use a separation sheet or insulation board if installing over MB.

• Fully adhered or mechanically attached systems are common.

• Check for chemical compatibility.

3. Metal Retrofit Systems

• Often built on top of insulation over the MB.

• Requires structural evaluation.

• Helps improve reflectivity and lifespan.

Re-Roofing Considerations for MB Roofs

Moisture Detection Is Critical

Use infrared or nuclear moisture scans. Any wet insulation or MB must be removed before reroofing.

Deck and Attachment Checks

Ensure the substrate (concrete, wood, or steel) can still meet code:

• Deck deflection should not exceed L/240 under full load.

• Fastener pullout tests should be conducted, especially with lightweight concrete.

Add a Cover Board

Cover boards like DensDeck or high-density polyiso are recommended for:

• Smoothing the substrate.

• Improving fire resistance.

• Protecting the membrane from damage during installation.

Code Compliance

The IBC and IRC allow re-roofing over one existing system only if:

• No water is present.

• The roof assembly still meets uplift resistance requirements.

When a Tear-Off Is Required

Re-roofing is not an option if:

• There are more than two roofing systems already in place.

• There is evidence of ponding water or deck rot.

• Insulation or membrane is saturated or deteriorated.

In these cases, full removal is the only compliant path forward.

FAQs

Can you install TPO over modified bitumen?

Yes, but only with a separation layer like insulation or a slip sheet to prevent chemical incompatibility and ensure adhesion.

How long does a re-roof over MB last?

A well-installed overlay can last 15–20 years, depending on the new system, local climate, and roof slope.

Do you need permits for MB re-roofing in Colorado or Georgia?

Yes. Both states require roofing permits. Local municipalities may also require moisture inspections before overlay approval.

Need an Expert Inspection?

If you’re in Denver, Atlanta, or surrounding areas and considering a re-roof over modified bitumen, Roof Nuts is here to help.

We’ll:

• Inspect your current MB system

• Recommend the right overlay options

• Handle all permits and code compliance

Colorado: RoofNutsCO.com | 303-276-2403
Georgia: RoofNutsGA.com | 404-476-6851

Get it done right. The first time. With Roof Nuts.