Modified Bitumen Roofing

What modified bitumen is, and why it still wins jobs

Modified bitumen is asphalt that has had a polymer blended into it to change how it behaves across a temperature range, then rolled into a reinforced sheet. It is the direct descendant of the multi-ply built-up roof — the tar-and-gravel assembly that covered most of this region's older commercial and institutional buildings — and it inherited that system's defining quality: redundancy. A modified bitumen roof is normally two plies. A base sheet is attached to the substrate and a cap sheet, usually surfaced with mineral granules, goes over it with the laps offset. Water has to defeat two independent membranes in two different places to reach the deck.

That redundancy is the whole argument. Single-ply is one sheet; a puncture through it is a leak. On a roof carrying constant service traffic, sitting under a mature tree line, or supporting equipment that gets worked on monthly, a two-ply system with a granulated wear surface simply tolerates abuse that a single-ply membrane does not. It is also the reason modified bitumen persists on church, school, rowhouse and institutional stock long after the industry declared single-ply the default.

The second argument is repairability. Modified bitumen is compatible with itself, with asphalt, and with a wide family of asphaltic mastics and flashing cements. A competent roofer can make a durable repair to a mod bit roof at almost any age with materials that are available everywhere. That is not true of every membrane, and on a building that will be repaired more often than it is replaced, it matters.

SBS versus APP: the choice that governs everything else

There are two polymer families and they are not interchangeable.

SBS — styrene-butadiene-styrene

SBS makes the asphalt elastic. An SBS sheet stretches and returns, stays flexible at low temperature, and tolerates the thermal movement and building movement that opens joints in more rigid systems. It is the more forgiving of the two in a climate that swings the way ours does, and it is what we specify on most Mid-Atlantic commercial work. SBS can be installed in hot asphalt, in cold adhesive, torch-applied, or as a self-adhered sheet, which means the same product can be specified for a building where flame is unacceptable.

APP — atactic polypropylene

APP makes the asphalt plastic rather than elastic. It softens and flows under heat, which is why APP is fundamentally a torch product: the flame melts the underside of the sheet and it bonds as it cools. APP has excellent ultraviolet resistance and stands up well to sustained high roof-surface temperature, which is why it is common in hotter climates. Its trade-off is behavior in cold: an APP sheet is stiffer at low temperature and less tolerant of movement, and it is not the sheet you want on a building that flexes.

The short version, and the one worth remembering when reading bids: SBS is elastic and can be installed several ways; APP is plastic and is essentially a torch system. A proposal that does not tell you which one it is quoting has not told you what you are buying.

Four ways it gets installed, and how to choose

Torch-applied

An open-flame torch melts the underside of the sheet as it is rolled out. It is fast, it produces an excellent bond, and it introduces an open flame onto your building. It is the correct method on many roofs and the wrong one on any building where a combustible deck, an open structural cavity, a parapet with a void behind it or the occupancy makes ignition risk unacceptable.

Hot asphalt

The traditional method: a kettle heats asphalt, which is mopped down and the sheet set into it. It produces the monolithic assembly the built-up tradition is built on. It also means a kettle at ground level, asphalt fumes, and an odor that will find its way into the building if the air intakes are not managed. On dense urban sites there is frequently nowhere to put the kettle at all.

Cold adhesive

No flame, no kettle. The sheet is set in a solvent- or polymer-based adhesive applied cold. It is slower and the material cost is higher, and it has temperature and cure windows that constrain scheduling. In exchange, it is the method that lets you re-roof an occupied school, a congregation's sanctuary or a wood-deck building without the risk profile of the other two.

Self-adhered

A factory-applied adhesive backing, released and rolled. No flame, no kettle, no adhesive on site, and the cleanest option for occupied buildings. It is temperature-sensitive at application and demands a very clean, primed substrate, and it is the method we most often propose for small urban roofs where staging equipment is the real problem.

On a great many buildings the honest recommendation is a hybrid: a self-adhered or mechanically attached base sheet, then a torch- or cold-applied cap. What decides it is the deck, the occupancy, the building's insurer, the access, and the weather window — not what the crew is used to.

How modified bitumen roofs fail

• Granule loss. The mineral surfacing is the ultraviolet shield. Once it washes into the gutters in quantity, the asphalt below is exposed and ageing accelerates. Granule accumulation at downspouts is one of the earliest and most visible warnings a roof gives.
• Blistering. Trapped moisture or air between plies expands under solar heat. Small stable blisters are often best left alone; blisters that have broken open are an entry point and need cutting out and patching.
• Alligatoring and crazing. Surface asphalt oxidizes, shrinks and cracks in a pattern that looks exactly like its name. On a smooth-surfaced roof it is the standard end-of-life signature and, caught early enough, the standard candidate for a coating restoration.
• Open laps and fishmouths. A lap that was not fully bonded lifts at its edge. On a torch roof this usually traces back to insufficient bleed-out at installation; on cold-applied work, to adhesive applied outside its window.
• Base flashing failure at parapets and curbs. The most common leak source on this system by a wide margin. Vertical flashing takes more movement and more sun than the field does, and it is where the termination bar, the counterflashing and the coping all interact.
• Ponding. Standing water on an asphaltic roof accelerates every one of the above and voids most warranties beyond a stated number of hours after rainfall.

Recover, tear off, or restore an existing built-up or mod bit roof

Most of our modified bitumen work in this market is not new construction; it is deciding what to do with an assembly that has already been re-roofed once or twice. Three questions decide it, and none of them can be answered from the ground.

How many roof coverings are already in place? Building codes limit the number of roof coverings that may remain, and beyond code there is a load question. Older wood plank and lightweight concrete decks were not designed to carry three or four layers of asphaltic roofing plus modern mechanical equipment. Core cuts tell us how deep the assembly goes and what is in it.

Is the insulation dry? An infrared or capacitance moisture survey verified by cores answers this. Wet insulation cannot be recovered over and cannot be coated; it has to come out. This single fact is the difference between a modest project and a large one, and any bid produced without it is a guess.

Is the surface sound enough to restore? Where the plies are intact, the laps are closed and the substrate is dry, a fluid-applied restoration over a mod bit or built-up roof is frequently the highest-return option available — it addresses the ultraviolet exposure that is actually killing the roof and adds a reflective surface, at a fraction of replacement cost. Where the sheet has lost its granules and cracked through, it is not.

How a modified bitumen project runs

1. Assessment with core cuts. We open the assembly to find out how many coverings are present, what the deck is made of, and whether the insulation is dry. On an asphaltic roof that has been re-roofed before, this is not optional and no honest number exists without it.
2. Method selection before price. SBS or APP, and torch, hot, cold-adhesive or self-adhered, chosen against your deck type, occupancy, insurer requirements, access and season. The proposal states the method and why, so you can compare bids that are actually comparable.
3. Installation with hot work control. Where flame is used: written hot work procedure, extinguishers and charged hose at the work area, and a fire watch that remains after the torches shut down. Daily dry-in, controlled debris and managed odor throughout, whichever method is used.
4. Manufacturer inspection and closeout. Manufacturer field inspection where the warranty term requires it, then a closeout package with the warranty, the as-built assembly description and the roof file, so the next contractor is not guessing at what is up there.

Modified bitumen scope

• SBS
• APP
• Two-ply base and cap
• Granulated cap sheet
• Torch-applied
• Hot asphalt
• Cold adhesive
• Self-adhered
• Built-up roof tie-in
• Base flashing at parapets
• Hot work permit
• Fire watch
• Core cuts
• Tapered polyiso

Hot work, fire watch and the questions to ask a torch contractor

Torch-applied modified bitumen is the only common commercial roofing method that involves an open flame held against the building, and it deserves to be treated as the hot work it is. On our torch work that means a written hot work permit for the day, a fire extinguisher and a charged hose at the work area, a fire watch that stays on the roof after the torches are shut down rather than leaving with the crew, and a hard rule that we do not torch directly to a combustible deck. Ask any bidder those four questions. A contractor who has no answer to the fire watch question is a contractor whose insurance you are relying on rather than their process.

Where a building's construction, its insurer or its occupancy makes flame unacceptable — and on a wood-deck rowhouse, a church with an open timber roof structure or an occupied school it usually should be — the same system installs in cold adhesive or as a self-adhered sheet with no flame on the roof at all. That option exists, it performs, and a contractor who only offers torch is offering you their preference rather than your building's requirement.

The ordinary commercial requirements are in place before we mobilize: certificate of insurance naming your ownership entity and managing agent as additional insureds, general liability and workers' compensation limits confirmed against your lease and lender requirements, bonding where a contract calls for it, and licensure held where the building is — MHIC #111971 in Maryland. One named project contact runs the job start to finish, and odor from asphalt and adhesive is planned around your air intakes rather than apologized for afterwards.

Where modified bitumen belongs in this market

The concentration is exactly where you would expect: the older masonry stock. District rowhouse commercial buildings, Baltimore mill and warehouse conversions, church and school property across Maryland, municipal buildings, and the low-slope sections tucked behind parapets on buildings that are otherwise steep-slope. On those buildings modified bitumen is frequently not a legacy system to be replaced with something modern; it is the correct specification, because two plies over an old plank deck with unpredictable movement is a more robust answer than one sheet.

It is also the system most likely to be sitting under whatever the last contractor installed. When we take core cuts on a District or Baltimore roof, finding three or four generations of asphaltic roofing is routine, and that discovery changes the scope. We would rather find it during the assessment than during the tear-off.

Related commercial pages: commercial roofing systems, TPO, EPDM, modified bitumen, flat roof repair, coating and restoration, maintenance programs, roof asset management, emergency response. By audience: property managers, HOA and condominium associations, multifamily and apartments, retail and office buildings, churches and nonprofits. Residential and adjacent work: roofing, aluminum siding.

Compare with TPO and EPDM, or read the full systems overview. Buildings where this system is most common: Washington DC, Baltimore, churches and nonprofits.

Modified bitumen roofing questions