Modified bitumen roofing is one of the most widely used systems for commercial flat roofs and low-slope buildings. It was developed in the 1960s as an improvement to traditional built-up roofing (BUR) systems.

The system combines asphalt with polymer modifiers to improve flexibility, durability, and weather resistance. These polymers allow the roofing membrane to expand and contract without cracking, making it well suited for buildings exposed to temperature changes.

There are two main types of modified bitumen membranes used in commercial roofing:

SBS (Styrene-Butadiene-Styrene)
SBS membranes are rubber-like and flexible. They are commonly installed using torch-applied methods, cold adhesive, or self-adhered systems. These membranes are ideal for roofs that experience movement or thermal expansion.

APP (Atactic Polypropylene)
APP membranes are plastic-like and are typically installed using heat welding or torch-down application. They offer excellent resistance to UV exposure and weathering.

Both systems are typically installed in multiple layers, providing superior waterproofing protection compared to single-ply roofing systems.

How a Modified Bitumen Roof Is Installed (Step by Step Guide)

When it comes to commercial flat roofs, Modified Bitumen Roofing is one of the most reliable systems available. Known for its strength, durability, and multi-layered protection, modified bitumen has been a top choice for decades.

But how is it actually installed? Let’s break it down into four simple steps.

Step 1: Prep the Surface

Before installation begins, the roof deck must be cleaned and repaired. Any debris, cracks, or uneven spots are addressed to create a smooth foundation. A clean, solid surface ensures that each layer bonds correctly and extends the life of the roof.

Step 2: Lay the Base Sheet

Next, a base sheet is rolled out across the surface. This forms the first layer of protection, acting as the foundation for the modified bitumen membrane. The base sheet may be installed with adhesives or mechanical fasteners depending on the system.

Step 3: Torch Down the Membrane

Here’s where the durability comes in. The modified bitumen membrane is rolled out and heated with a roofing torch. This bonds the sheet securely to the base layer, creating a watertight seal. (Some systems use cold adhesive instead of heat for specific applications.)

Step 4: Seal Seams & Finish

Finally, all seams are sealed for complete waterproofing. The result is a smooth, durable, multi-layered roofing system built to withstand harsh weather and heavy foot traffic.

Why Choose Modified Bitumen?

• Proven performance for commercial flat roofs

• Multi-layer protection against leaks

• High durability under foot traffic

• Flexible in hot and cold temperatures

• Cost-effective compared to full replacements

Watch the Video

We’ve put together a short video that shows this process in action — from start to finish — in less than a minute.

Watch: How a Modified Bitumen Roof Is Installed in Four Simple Steps

Ready for a Modified Bitumen Roof?

At Central Roofing, we’ve been installing and maintaining commercial roofs in Southern California for over 30 years. Whether you’re considering modified bitumen or another system, we’ll help you find the best solution for your building.

Schedule Your Free Commercial Roof Inspection Today

FAQ About Modified Bitumen Roof Systems:

1. How long does a Modified Bitumen roof last?
A properly installed Modified Bitumen roof can last 20–30 years with regular maintenance. Its multi-layered design makes it one of the most durable flat roof systems.

2. Is torch down roofing safe?
Yes, when installed by trained professionals. Roofing contractors use controlled propane torches to bond the membrane securely. In sensitive areas, cold adhesive systems can be used instead.

3. What are the benefits of Modified Bitumen roofing?
Modified Bitumen offers excellent waterproofing, durability under heavy foot traffic, flexibility in hot and cold weather, and cost-effectiveness compared to a full roof replacement.

4. Can a Modified Bitumen roof be coated?
Absolutely. Applying a silicone or acrylic roof coating can extend the life of a Modified Bitumen system, improve energy efficiency, and reduce long-term maintenance costs.

Need a Modified Bitumen Roof Installed?

If your commercial building needs a new roof or replacement system, modified bitumen roofing may be an excellent option for durability and long-term protection.

Central Roofing has over 30 years of experience installing and maintaining commercial roofing systems throughout Southern California, including modified bitumen, built-up roofing, and roof coatings.

Our team provides:

• Commercial roof inspections
• Modified bitumen roof installation
• Roof repairs and maintenance
• Leak detection and emergency service

If you’re considering a modified bitumen roofing system for your building, contact Central Roofing today to schedule a roof inspection.

Related Commercial Roofing Resources

You may also find these guides helpful when researching commercial roofing systems:

• Commercial Roof Repair Services
• Emergency Roof Leak Repair Guide
• Acrylic Roof Coatings: Pros and Cons
• How to Fix a Leaky Skylight