Modified Bitumen Roofing: Why It Works for Chicago Buildings
Chicago's climate is notoriously tough on buildings. Summer temperatures exceeding 90°F, winter lows dropping below zero, and constant freeze-thaw cycles throughout spring and fall create challenging conditions for any roofing system. Modified bitumen has earned its popularity in the Midwest precisely because it handles these conditions exceptionally well.
What is Modified Bitumen?
Modified bitumen (often called "mod bit") is an asphalt-based roofing system enhanced with plastic or rubber polymers. These modifiers improve upon traditional built-up roofing by adding flexibility, durability, and temperature resistance. The material comes in rolls that are applied in overlapping layers.
Two Types of Modification
APP (Atactic Polypropylene)
APP-modified bitumen is known as "plastic asphalt." It offers excellent UV resistance and handles high temperatures well. APP membranes are typically torch-applied, with the installer using an open flame to melt the underside as the roll is unrolled.
SBS (Styrene-Butadiene-Styrene)
SBS-modified bitumen is called "rubber asphalt." It provides superior flexibility, especially in cold weather. SBS membranes can be torch-applied, cold-applied with adhesive, or self-adhered. For Chicago's climate, SBS is often the preferred choice.
Why Mod Bit Works in Chicago
- Flexibility in extreme cold prevents cracking during harsh winters
- Multiple layers provide redundant waterproofing
- Self-healing properties allow minor damage to seal itself in summer heat
- Excellent resistance to ponding water common on flat roofs
- Compatible with various insulation types for energy efficiency
- Proven track record in Midwest climate for 40+ years
Local Advantage
Modified bitumen's ability to remain flexible at temperatures below -20°F makes it ideal for Chicago winters. Unlike some single-ply membranes that can become brittle, mod bit maintains its integrity through temperature extremes.
Installation Methods
Modified bitumen can be installed using several methods:
- Torch-applied: Traditional method using open flame to adhere sheets
- Cold-applied: Uses adhesive instead of heat (ideal for occupied buildings)
- Self-adhered: Peel-and-stick application for faster installation
- Hot mopped: Traditional asphalt method (less common today)
Expected Performance
A properly installed two-ply modified bitumen system in the Chicago area typically lasts 20-30 years. With quality installation and regular maintenance, many systems exceed this range. The multiple layers provide peace of mind that a single puncture won't lead to immediate leaks.
