If your commercial building has an older modified bitumen roof, you’ve probably seen the signs of aging: cracks, blisters, loose seams, worn areas, ponding water, or the occasional bucket making an unwanted appearance inside the building.
Modified bitumen, often called MOD bit or modified asphalt roofing, has been used on flat and low-slope commercial buildings for decades. These multi-layer roofing systems can provide durable protection, but like any commercial roof, they eventually wear and develop problems.
Michigan’s temperature swings, snow, ice, UV exposure, foot traffic, and repeated freeze-thaw cycles can add even more stress to an aging roof.
Springfield Roofing has spent more than 40 years working on commercial roofs throughout Northern and Central Michigan. Here’s what building owners should know about modified bitumen roofing, common problems to watch for, and how to determine whether it’s time to repair or replace the roof.
What Is Modified Bitumen Roofing?
Modified bitumen is an asphalt-based commercial roofing membrane designed primarily for flat and low-slope roofs.
Traditional asphalt is combined with polymers to improve the material’s performance and flexibility. The membrane is manufactured into rolls that are installed in layers to create a continuous roofing system.
There are two primary types of modified bitumen roofing:
SBS Modified Bitumen
SBS stands for styrene-butadiene-styrene.
SBS modifies asphalt to create a more flexible, elastic membrane. This flexibility can be particularly beneficial in colder climates because the roofing material is designed to accommodate expansion and contraction as temperatures change.
APP Modified Bitumen
APP stands for atactic polypropylene.
APP-modified asphalt tends to produce a more rigid membrane than SBS and is commonly associated with heat-welded or “torch-down” roofing applications.
Both systems can provide effective commercial roofing when properly designed, installed, and maintained. The specific system on your building will influence how it ages and how repairs should be approached.
How Is Modified Bitumen Roofing Installed?
Modified bitumen isn’t one specific installation method.
Depending on the roofing system and manufacturer, modified bitumen membranes may be:
- Heat-welded or torch-applied
- Hot-asphalt applied
- Cold-adhered
- Self-adhered
- Mechanically fastened
Many systems use multiple layers, providing redundancy if one portion of the roofing assembly becomes damaged.
The top layer, often called the cap sheet, may have a granulated surface that helps protect the membrane from UV exposure and physical wear.
Common Modified Bitumen Roofing Problems?
A modified bitumen roof doesn’t suddenly become bad because it reaches a certain age.
Instead, problems tend to develop as the roof experiences years of weather, UV exposure, temperature changes, building movement, moisture, maintenance traffic, and previous repairs.
Here are some of the conditions we commonly look for when evaluating an aging MOD bit roof.
Cracking and Splitting
Cracks are one of the most visible signs that a modified bitumen roof is aging.
Over time, UV exposure, weathering, and thermal cycling can affect roofing materials. As the building and roofing system expand and contract through Michigan’s temperature swings, deteriorated areas may become more susceptible to cracking or splitting.
Small cracks may initially appear to be a surface problem, but they shouldn’t be ignored.
Once an opening allows water into the roofing assembly, moisture can affect the insulation and potentially the roof deck beneath it.
Roof Blisters and Bubbles
Raised or bubbled areas on a modified bitumen roof are commonly referred to as blisters.
Blistering can occur when moisture, air, or gases become trapped between layers of the roofing system. As the roof heats up, pressure within the trapped area can cause the membrane to rise.
Not every blister means the roof needs to be replaced.
However, blisters should be evaluated, particularly when they are large, widespread, damaged, or located in areas that experience regular foot traffic.
A broken blister can create an opening that allows water into the roofing assembly.
Open or Failing Seams
Modified bitumen roofing is installed in sheets or rolls, which means seams are an important part of the system.
As the roof ages, seams and laps should be inspected for:
- Separation
- Cracking
- Loose edges
- Failed adhesives
- Deteriorated sealants
- Previous repairs that are failing
Even a relatively small opening can provide a path for water to enter the roofing system.
Ponding Water
Flat roofs aren’t actually supposed to be perfectly flat.
Commercial roofs should be designed to direct water toward drains, scuppers, gutters, or other drainage points.
When water remains on a modified bitumen roof for extended periods, we want to know why.
Ponding water can be associated with:
- Poor original roof slope
- Compressed or damaged insulation
- Clogged drains
- Structural movement
- Improperly designed drainage
- Low areas created during previous roofing work
Standing water also increases the consequences of an existing defect. If a crack, failed seam, or puncture develops beneath an area that regularly holds water, you’ve now given that opening a nearly continuous water source.
That’s not a great combination.
Granule Loss and Surface Wear
Many modified bitumen cap sheets have a granulated surface that protects the underlying membrane.
Some granule loss can occur as a roof ages, but concentrated or widespread loss deserves attention.
Areas with significant granule loss may expose more of the underlying membrane to UV radiation and weather.
We also look closely at areas around:
- Roof access points
- HVAC equipment
- Service paths
- Drains
- Areas with frequent maintenance traffic
Those areas tend to see more physical wear than untouched portions of the roof.
Flashing Problems
Your roof isn’t just the big open area in the middle.
Walls, curbs, HVAC units, pipes, vents, drains, and other penetrations all have to be incorporated into the waterproofing system.
Those transitions are often where roof problems show up first.
Building movement and thermal expansion can place stress on flashing materials and transition points. Over time, you may see cracking, separation, deteriorated sealants, or failed repairs around these areas.
If your building repeatedly leaks around the same HVAC unit, wall, or penetration, the flashing system deserves a closer look.
Punctures and Physical Damage
Commercial roofs aren’t always treated gently.
HVAC contractors, electricians, plumbers, maintenance crews, snow and ice, dropped tools, equipment, and general foot traffic can all damage the roof.
Modified bitumen’s multi-ply construction can provide additional protection, but it isn’t indestructible.
Punctures, cuts, damaged cap sheets, and worn traffic areas should be repaired before moisture has an opportunity to spread into the roofing assembly.
Recurring Roof Leaks
One isolated leak does not necessarily mean you need a new roof.
A pattern of leaks is more concerning.
If you’re repairing one area this month, another area next month, and something else every spring, the question eventually becomes:
Are we still maintaining this roof, or are we just chasing failures?
That’s an important distinction.
Recurring leaks across different areas of a roof can indicate that deterioration is becoming widespread rather than isolated.
Can a Modified Bitumen Roof Be Repaired?
Absolutely, depending on its condition.
Modified bitumen roofing can often be repaired when deterioration is localized and the surrounding roofing material remains in serviceable condition.
Repairs may make sense for:
- Isolated punctures
- Small cracks or splits
- Localized seam failures
- Individual flashing problems
- Limited areas of membrane damage
- Specific leak locations
Compatible modified bitumen materials and roofing products can often be used to repair splits, holes, flashing details, and other localized failures.
But there’s an important distinction between repairable damage and system-wide deterioration.
If the roof is cracking throughout, repeatedly leaking, holding moisture beneath the membrane, or experiencing widespread flashing and seam failures, continuing to patch individual areas may no longer be the best use of your roofing budget.
Should You Repair or Replace a Modified Bitumen Roof?
There isn’t a universal age at which a modified bitumen roof automatically needs replacement.
Condition is more important than the number on the calendar.
Generally, repair may make sense when the roofing system remains in good overall condition and the problem is confined to a limited area.
Replacement may make more sense when deterioration is widespread or repairs are becoming a regular operating expense.
| Roof Condition | Repair May Make Sense | Replacement May Make Sense |
|---|---|---|
| Small isolated leak | ✓ | |
| Localized seam failure | ✓ | |
| Individual puncture | ✓ | |
| Localized flashing problem | ✓ | |
| Roof generally remains serviceable | ✓ | |
| Widespread cracking or splitting | ✓ | |
| Numerous failing seams | ✓ | |
| Multiple recurring leaks | ✓ | |
| Widespread wet insulation | ✓ | |
| Extensive flashing deterioration | ✓ | |
| Repairs are becoming frequent | ✓ |
A commercial roof inspection can help determine where your roof falls on that spectrum.
When Should a Modified Bitumen Roof Be Replaced?
Replacement becomes worth considering when the roof is experiencing problems throughout the system rather than in isolated areas.
Warning signs can include:
- Widespread cracking or splitting
- Numerous open or deteriorated seams
- Extensive blistering
- Multiple recurring leaks
- Large areas of wet insulation
- Deteriorated flashing throughout the roof
- Significant surface deterioration
- Frequent repair calls
- Damage to underlying roofing components
- Repairs that no longer provide lasting results
Age can certainly be part of the conversation, but we don’t recommend replacing a commercial roof simply because it’s old.
We want to know what condition it’s actually in.
What’s Under Your Modified Bitumen Roof?
This is one of the most important questions when considering a commercial roof replacement.
The surface only tells part of the story.
Water can travel within a roofing assembly, which means the location of an interior leak doesn’t always tell you exactly where water entered the roof.
During a roof evaluation, Springfield Roofing may examine:
- Existing membrane condition
- Number of existing roof layers
- Insulation condition
- Evidence of trapped moisture
- Roof deck condition
- Drainage
- Flashings and penetrations
- Previous repairs
- Perimeter details
Depending on the roof and the problems we’re investigating, additional evaluation methods may be recommended to better understand conditions beneath the membrane.
That information helps determine whether the roof can continue to be repaired, whether a recover may be possible, or whether removal and replacement are the better options.
Can You Install a New Roof Over Modified Bitumen?
Sometimes.
A commercial roof does not always have to be completely torn off before a new roofing system can be installed.
Depending on the existing assembly and applicable requirements, an aging modified bitumen roof may be a candidate for a roof recover.
But we’re not covering up a bad roof just because it’s easier.
Before recommending a recover, we need to evaluate factors including:
- Existing moisture
- Condition of the insulation
- Roof deck condition
- Existing roof layers
- Drainage
- Attachment requirements
- Building code requirements
- Manufacturer specifications
Wet or damaged roofing materials may need to be removed and replaced before a new roofing system is installed.
In other situations, conditions may require a more extensive tear-off.
The roof needs to tell us what we’re dealing with first.
Replacing Modified Bitumen With PVC Roofing
When an aging modified bitumen roof reaches the end of its useful service life, building owners have several replacement options.
At Springfield Roofing, our primary flat and low-slope roofing system is Duro-Last PVC roofing.
PVC is a single-ply roofing membrane, so it is fundamentally different from an asphalt-based modified bitumen system.
Duro-Last roofing systems use heat-welded seams to create a continuous waterproof membrane. The system also uses custom-fabricated deck sheets and accessories, allowing a significant portion of the membrane fabrication to occur under factory-controlled conditions.
That can reduce the amount of field seaming required during installation.
Whether PVC is the right replacement for your existing MOD bit roof depends on the building, existing roof assembly, insulation, drainage, roof deck, rooftop equipment, and long-term plans for the property.
We evaluate those conditions before recommending a roofing system.
Modified Bitumen Roofs and Michigan Weather
Michigan gives commercial roofs plenty to deal with.
A roof may experience summer heat, heavy rain, snow accumulation, ice, below-freezing temperatures, and repeated freeze-thaw cycles, sometimes within the same season.
Those temperature changes cause roofing assemblies and buildings to expand and contract.
SBS modified bitumen is specifically formulated for flexibility, which is one reason these systems can perform in colder climates. But even a well-designed roof isn’t immune to aging, damage, drainage problems, or years of exposure.
Regular commercial roof inspections become particularly important after winter.
Catching a loose seam, damaged flashing, clogged drain, or small membrane defect before spring rain arrives is considerably easier than finding it after water starts dripping onto your conference table.
Modified Bitumen Roof Repair and Replacement in Michigan
Springfield Roofing has worked on commercial roofs throughout Michigan since 1985.
With locations in Kingsley and Big Rapids, we inspect, maintain, repair, recover, and replace flat and low-slope commercial roofing systems throughout Northern and Central Michigan.
If you have an aging modified bitumen roof, we’re not going to tell you it needs replacement just because it’s old.
We’ll look at the roof.
If it can reasonably be repaired, we’ll discuss repair. If preventative maintenance can extend its service life, we’ll discuss maintenance. And if the roofing system is reaching the point where you’re spending good money chasing bad leaks, we’ll talk about replacement.
The goal is to determine what makes sense for your building and your roof, not simply sell you the most expensive option.