What is modified bitumen roofing? It’s a roofing membrane made of asphalt blended with a polymer (either a rubber-like SBS or a plastic-like APP) and reinforced with a polyester or fiberglass mat, sold in rolls and used on flat and low-slope roofs. It’s usually installed in two or more layers, a base sheet and a top cap sheet, and attached with a torch, hot asphalt, cold adhesive or a factory self-adhesive backing.
Often called “mod bit,” it’s a close relative of traditional built-up roofing (BUR), designed to be more flexible and simpler to install. Here’s how it works, how SBS and APP differ, the main installation methods, and where it makes sense.
In this article
- How Modified Bitumen Roofing Works
- SBS vs. APP Modified Bitumen
- Installation Methods
- Multi-Ply and Hybrid Systems
- Where Modified Bitumen Roofing Is Used
- Pros and Cons of Modified Bitumen
- How Long Does Modified Bitumen Roofing Last?
- Maintenance and Repair
- Modified Bitumen vs. Roll Roofing
- Frequently Asked Questions
How Modified Bitumen Roofing Works
Plain asphalt gets brittle when cold and soft when hot. Adding a polymer “modifies” it, so the membrane stays more flexible and durable through temperature swings. That’s the core idea behind the name.
According to Polyglass, a major membrane manufacturer, modified bitumen first appeared in commercial roofing in Europe in the mid-1960s and was widely adopted in the United States by the 1970s. It was developed partly to address drawbacks of built-up roofing, such as its weight, labor and difficulty of repair.
The layers of a typical system
- Base sheet. The first layer of membrane, attached to the roof deck, insulation or cover board.
- Cap sheet. The top, weather-facing layer. Most cap sheets are surfaced with mineral granules for UV protection; others have a smooth surface for a coating, or a factory-applied reflective finish.
- Reinforcement. Each sheet has a mat inside, usually polyester, fiberglass or a combination.
The Asphalt Roofing Manufacturers Association (ARMA) describes a typical SBS torch-applied system as one base sheet plus one cap sheet. Polyglass notes that most installations need at least two layers, and some use three or more.
SBS vs. APP Modified Bitumen
The two main types are named for the polymer added to the asphalt.
| SBS (styrene-butadiene-styrene) | APP (atactic polypropylene) | |
|---|---|---|
| Type of modifier | Synthetic rubber (elastomeric) | Plastic (plastomeric) |
| Feel and behavior | Flexible and stretchy | Stiffer once installed |
| Cold-weather performance | Better low-temperature flexibility | Harder to handle in cold; can crack if unrolled when very cold |
| Application methods | Torch, hot asphalt, cold adhesive, self-adhered, some mechanically attached | Mostly torch or heat-welded; some self-adhered |
| Noted strengths | Elongation and flexibility under movement and temperature swings | UV and weather resistance; flows well when heat-welded |
SBS: the rubber-like option
SBS is a thermoplastic elastomer, which means it behaves like rubber at normal temperatures. The British Columbia roofing contractors’ manual notes that SBS improves low-temperature flexibility and elasticity. Polyglass describes SBS as able to tolerate much lower temperatures than APP and suited to colder regions. GAF highlights its greater elongation.
For roofs in cold northern climates, SBS is the more common choice for that reason.
APP: the plastic option
APP makes the membrane more rigid once installed and melts at higher temperatures, which is why it’s typically torch-applied. GAF notes APP’s resistance to UV and weather, and Polyglass says it flows easily when heat-welded, which helps on smaller roof areas.
Its weakness is cold handling. The same roofing manual reports that most manufacturers set a lower application limit for APP of around 40°F, and warns that unrolling it in very cold weather can crack the sheet.
Installation Methods
How the membrane is attached matters as much as which type it is. The choice affects safety, cost, weather limits and which products can be used.
Torch-down (heat-welded)
A roofer uses a propane torch to melt the asphalt on the underside of the roll as it’s unrolled, fusing it to the layer below. ARMA describes the process: the torch melts the compound and burns off a thin protective film, and the sheet is unrolled slowly onto the roof. Granules are then pressed into any asphalt that oozes from the seams to protect it from sunlight.
- Pros: Strong, fully fused seams; no kettle of hot asphalt; works with both SBS and APP.
- Cons: Open flame on a roof is a real fire risk, especially near wood decks, insulation and edges. It requires trained crews and careful fire-watch practices, and it isn’t a good fit for every building.
Hot-mopped
Asphalt is melted in a kettle and mopped onto the roof, and the membrane is rolled into it. This is the same technique used for traditional built-up roofs. It’s used mainly with SBS.
- Pros: Well-established method; the hot asphalt itself adds a waterproofing layer.
- Cons: Fumes and burn hazards; requires kettle equipment; less common on small residential jobs.
Cold-applied (adhesive)
Instead of heat, the membrane is set into a liquid cold-process adhesive spread with a squeegee or applicator. GAF and Polyglass both list cold adhesives for SBS membranes.
- Pros: No open flame or kettle of hot asphalt.
- Cons: Adhesives need time and suitable temperatures to cure; application rates must follow the manufacturer’s instructions closely.
Self-adhered (peel-and-stick)
The sheet comes with a factory-applied adhesive covered by a release film. The installer peels the film and presses the sheet into place.
- Pros: No torch, no kettle, cleaner install. Polyglass notes self-adhered membranes avoid the fire hazard of torching, and some manufacturers make low-temperature self-adhered SBS sheets for cold-weather work.
- Cons: Requires a clean, dry, primed surface and good contact; adhesion can suffer if applied below the product’s minimum temperature.
Mechanically attached
Some base sheets and membranes are fastened with screws and plates, particularly as the first layer over certain decks or insulation. The cap sheet is then adhered on top by one of the methods above.
Multi-Ply and Hybrid Systems
Modified bitumen is almost always a multi-ply system. GAF describes asphaltic roofing as two or more waterproofing layers, which gives built-in redundancy: if the cap sheet is punctured, the base sheet still keeps water out.
Methods can also be mixed in one roof. ARMA publishes environmental data for a hybrid system with a self-adhered SBS base sheet and a torch-applied SBS cap sheet. That setup can keep the open flame off the deck and insulation, while still giving the top layer fused seams.
Common configurations include:
- Two-ply: one base sheet and one granulated cap sheet; the most common setup.
- Three-ply or more: an extra ply sheet or BUR felts under the cap for added thickness and redundancy.
- Hybrid: different attachment methods for base and cap, or modified bitumen over built-up plies.
The specific layup, fasteners, insulation and cover boards must match the manufacturer’s tested assembly and local code, which is what qualifies the system for the manufacturer’s warranty.
Where Modified Bitumen Roofing Is Used
Modified bitumen is designed for flat and low-slope roofs. The International Residential Code and International Building Code both require a design slope of at least 1/4 unit vertical in 12 units horizontal (about a 2 percent slope) for modified bitumen roofing, so even a “flat” roof should drain.
Typical uses include:
- Commercial and multifamily buildings with low-slope roofs
- Flat roof sections on homes, including additions and rear extensions
- Porch roofs, dormer roofs and garages with low pitches
- Duplexes, townhomes and older urban homes with flat roofs
It isn’t a substitute for shingles on steep roofs, and on low-slope roofs it competes with single-ply membranes such as EPDM and TPO, as well as traditional built-up roofing. If you’re comparing options for a home or building, our flat roofing page covers the main systems, and our commercial roofing page covers larger low-slope projects.
Pros and Cons of Modified Bitumen
Pros
- Multi-layer redundancy. Two or more plies protect against a single puncture or seam failure.
- Flexibility. SBS in particular handles expansion, contraction and building movement well.
- Toughness. Reinforced, granule-surfaced sheets stand up to foot traffic and maintenance better than many thin membranes.
- Familiar and repairable. Damaged areas can be cut out and patched with compatible modified bitumen.
- Reflective options. White granule caps and coatings can reduce heat absorption.
- Several installation choices. Torch-free options exist where open flames aren’t welcome.
Cons
- Fire risk with torch-down. Torch application requires trained crews and careful fire-safety practices.
- Weather limits. APP in particular is difficult in cold weather, and adhesives and self-adhered sheets have minimum application temperatures.
- Installation skill matters. Seams, laps and flashings depend heavily on the crew.
- Dark surfaces absorb heat. Standard granules are often dark unless you choose a reflective product.
- Heavier than single-ply. Multiple asphalt plies weigh more than a single sheet of EPDM or TPO.
- Not for DIY. Torching in particular is a job for professionals only.
How Long Does Modified Bitumen Roofing Last?
Polyglass puts the average lifespan of a modified bitumen roof at approximately 20 years, and potentially longer with proper upkeep and maintenance. Actual service life depends on the number of plies, membrane quality, installation, drainage, sun exposure and maintenance.
Manufacturer system warranties can be longer than that for premium multi-ply assemblies. GAF, for example, offers coverage terms of up to 35 years on qualifying systems. A warranty length tells you what the manufacturer will cover under its conditions; it isn’t a promise of how long your particular roof will last.
What shortens the life of a mod bit roof:
- Ponding water from poor slope or clogged drains
- Granule loss that exposes the asphalt to UV
- Seam and flashing failures at walls, curbs and penetrations
- Foot traffic and dropped tools, especially around rooftop equipment
- Blisters from trapped moisture or air under the membrane
Maintenance and Repair
A low-slope roof gets more out of regular attention than a steep shingle roof does, because water doesn’t run off as quickly. A practical routine:
- Clear drains, scuppers and gutters, especially after fall leaves and before winter.
- Look for standing water a day or two after rain.
- Check seams and laps for lifting, and flashings for cracks or separation.
- Watch for bare spots where granules have worn away, as well as blisters, cracks and punctures.
- Re-coat smooth-surfaced or coated roofs as the coating manufacturer recommends.
Small defects are usually repaired by cleaning the area and applying a patch of compatible membrane, sometimes with a torch or adhesive depending on the original system. Leaks at walls and penetrations often trace back to flashing details rather than the field of the roof. If you’re seeing leaks or damage, our flat roof repair page explains what’s involved.
Modified Bitumen vs. Roll Roofing
Modified bitumen is sometimes confused with mineral-surfaced asphalt roll roofing, since both come in rolls with granule surfaces. Standard roll roofing is a simpler, single-layer asphalt product often used on sheds and outbuildings. Modified bitumen uses polymer-modified asphalt and reinforcement mats and is installed as an engineered multi-ply system, making it the more durable choice for occupied buildings with flat roofs.
If you’re curious how basic roll roofing goes on, see our guide on how to install roll roofing.
Frequently Asked Questions
Is modified bitumen the same as torch-down roofing?
Torch-down is one way to install modified bitumen, not a separate material. Mod bit can also be hot-mopped, set in cold adhesive or installed as a self-adhered sheet.
What is the difference between SBS and APP modified bitumen?
SBS uses a synthetic rubber modifier and stays more flexible in cold weather, while APP uses a plastic modifier that is stiffer and usually torch-applied. SBS is generally preferred in cold climates.
How long does a modified bitumen roof last?
One major manufacturer puts the average at about 20 years, with longer service possible with good maintenance. Actual life depends on the number of plies, installation quality, drainage and upkeep.
Can modified bitumen be used on a house?
Yes. It is common on flat and low-slope sections of homes, such as additions, porches, dormers and garages, as long as the roof has at least a 1/4-inch-per-foot design slope.
Can modified bitumen be installed in cold weather?
It depends on the product and method. SBS and cold-weather self-adhered sheets handle low temperatures better, while many manufacturers set APP’s minimum around 40°F.
Can you walk on a modified bitumen roof?
Granule-surfaced modified bitumen tolerates occasional foot traffic better than many thin membranes, but frequent traffic and dropped tools still cause wear. Walkway pads are often added around rooftop equipment.
Modified bitumen is a proven, multi-layer choice for flat and low-slope roofs. SBS offers cold-weather flexibility, APP offers heat and UV resistance, and the installation method (torch, mop, cold adhesive or self-adhered) should match the building, the season and the safety needs of the job.
If you’re unsure what’s on your flat roof or how much life it has left, our roof inspection page explains what a professional evaluation covers.
