Roofing
Modified bitumen roof installation field guide for low-slope crews
Installing a mod-bit low-slope roof: SBS vs APP, the four application methods, torch fire watch, the plies, the seam bleed-out, the flashings, and what to document.
Direct answer
Modified bitumen is an asphalt-based sheet roofing membrane reinforced with polyester or fiberglass and modified with SBS rubber or APP plastic, installed in multiple plies for redundancy. SBS is usually torched, mopped, cold-applied, or self-adhered; APP is torched. The membrane manufacturer's specification and the project documents govern the buildup.
Key takeaways
- Modified bitumen is an asphalt sheet membrane reinforced with polyester or fiberglass and modified with SBS rubber or APP plastic, installed in multiple plies.
- SBS can be torched, hot-mopped, cold-applied, or self-adhered and suits cold climates; APP is almost always torch-applied and suits hot, high-UV climates.
- A bead of bitumen squeezed out at the lap proves the seam fused; for SBS the common target is roughly 3/8 in of flow, less means too little heat.
- Torch jobs need a CERTA-trained applicator and a post-job fire watch, commonly about two hours, since torch fires usually start after work stops; never torch to a combustible substrate.
- Hot-asphalt mopping holds the asphalt within the EVT window, commonly EVT plus or minus 25 degrees F, measured at the point of contact, with a floor of EVT or about 400 degrees F.
What modified bitumen is, and where it still beats single-ply
Modified bitumen, mod-bit on the roof, is an asphalt-based sheet membrane reinforced with a polyester or fiberglass mat and modified with a polymer that gives the asphalt elasticity and a wider service temperature. It is the modern descendant of built-up roofing. Where old BUR was hot asphalt mopped between plies of felt and then flood-coated and graveled, mod-bit moves the asphalt into a factory-made roll so you set a controlled, uniform sheet instead of building the membrane by hand on the deck.
The reason it survives in a market that single-ply mostly took over is redundancy and toughness. A mod-bit roof is built in layers, commonly a base or ply sheet under a cap sheet, so the waterproofing does not live in one thin membrane and one seam. The granule-surfaced cap takes foot traffic, hail, and dropped tools better than a 60 mil single-ply, which is why it keeps winning on roofs that get walked, on plaza decks, and on buildings with constant rooftop service. If you are choosing between mod-bit and a single-ply system in the first place, that decision belongs in the membrane-selection guide. This guide assumes mod-bit is the call and walks the install.
The tradeoff is labor and, in the torch and hot-asphalt versions, fire and fumes. A single-ply crew unrolls and welds. A mod-bit crew is running open flame or a hot kettle, which is more skill, more safety overhead, and more that can go wrong on the deck.
SBS vs APP: the two modifiers and how each one goes down
The polymer in the asphalt is the whole story, and there are two. SBS is styrene-butadiene-styrene, a synthetic rubber. It makes the asphalt elastic, so the membrane stretches and recovers and stays flexible in the cold. APP is atactic polypropylene, a plastic. It makes the asphalt behave like a tougher, more heat-tolerant plastic that resists UV and high roof temperatures but has less give when it is cold.
The modifier also drives how the sheet gets installed. SBS is the versatile one. It can be torched, mopped in hot asphalt, set in cold adhesive, or made as a self-adhered peel-and-stick sheet, which is why most crews reach for SBS first. APP is almost always torch-applied. When you torch APP, the asphalt on the back flows like candle wax once it passes roughly 300°F, and that molten layer is what bonds the sheet down. SBS under the torch gets sticky and gooey rather than running, which is a different feel on the roll.
Pick by climate and by method. SBS for cold-weather flexibility and for jobs where you want a no-flame option. APP for hot climates and high UV where its plastic surface holds up, accepting that it commits you to the torch. The membrane manufacturer's data sheet specifies which modifier and which method is approved for the product, and that data sheet, not a rule of thumb, governs the warranty.
| Modifier | Behavior | Typical methods | Leans toward |
|---|---|---|---|
| SBS (rubber) | Elastic, flexible, gets sticky under heat | Torch, hot-mopped, cold-applied, self-adhered | Cold climates, no-flame options |
| APP (plastic) | Stiffer, heat and UV tolerant, flows under torch | Torch-applied almost exclusively | Hot climates, high UV exposure |
How is modified bitumen applied?
There are four application methods, and the method changes the crew, the safety plan, and the conditions you can work in. Torch-applied uses a propane torch to melt the asphalt on the back of the roll as it unwinds, bonding it to the substrate. It builds a strong monolithic bond and works in cold weather, but it is open flame on a roof, which is the highest fire risk in the trade.
Hot-asphalt mopped is the BUR method carried into mod-bit. A kettle heats roofing asphalt, the crew mops it onto the substrate, and the sheet is rolled into the hot asphalt before it skins over. It is proven and strong, but it means a kettle, hot fumes, and asphalt temperature control. Cold-applied uses a brushed or squeegeed cold adhesive to bond the sheet. No flame, no kettle, lower fumes, and it lets you set membrane in conditions where fire or hot work is not allowed, at the cost of cure time and adhesive coverage that has to be right.
Self-adhered is peel-and-stick. The sheet has a factory adhesive backing under a release film; you pull the film and roll the membrane into place, pressing it down. It is the safest and often the fastest method, with no flame and minimal fumes, which is why it is favored on occupied buildings, schools, and hospitals where torching is a non-starter. The catch is temperature: self-adhered sheets need a warm enough surface to bond, so cold mornings and primer choice matter. Match the method to the substrate, the weather, and the building's fire restrictions, and confirm the manufacturer approves it for that sheet.
| Method | How it bonds | Tradeoff |
|---|---|---|
| Torch-applied | Propane torch melts back of sheet | Strongest bond, highest fire risk, needs fire watch |
| Hot-asphalt mopped | Sheet rolled into mopped hot asphalt | Proven, but kettle, fumes, temperature control |
| Cold-applied | Brushed or squeegeed cold adhesive | No flame, lower fumes, slower cure |
| Self-adhered | Factory peel-and-stick adhesive | Safest and fast, but needs warm surface to bond |
Is torch-down roofing safe?
Torch-down is safe when it is done by trained applicators with a real fire watch, and it is dangerous when it is not. The open propane flame is hot enough to ignite anything combustible it touches or that hides under the deck, and the worst fires start where you cannot see them: flame drawn into a wall cavity, a parapet, a wood nailer, or insulation, smoldering for an hour or two after the crew has packed up. Torch fires that burn a building down usually start after the torching stops, not during.
The trade answer to this is the NRCA CERTA program, Certified Roofing Torch Applicator. CERTA trains the safe handling of the torch and, importantly, the post-job fire watch. The standard practice is a dedicated fire watch after torching ceases, and CERTA best practice commonly calls for a fire watch of about two hours after the work stops, with many crews also running a thermal-imaging sweep to find hidden heat that the eye and hand miss. Confirm the duration against the CERTA guidance, the project safety plan, and any insurer or jurisdiction requirement, which can be stricter.
Never torch directly to a combustible substrate. The flame goes to a non-combustible base or an approved torch-grade base sheet, not straight to wood, not into a parapet you cannot see behind. Keep an extinguisher and water within reach, do not torch into laps you cannot watch, and treat the building corners, edges, and penetrations as the places a fire hides. If a substrate or detail cannot be torched safely, that detail gets a no-flame method, full stop.
How many plies does a mod-bit roof have?
A mod-bit roof is usually two plies: a base or ply sheet bonded to the substrate, and a cap sheet bonded on top of it. That is the standard buildup and it is what gives mod-bit its redundancy advantage over a single-ply. Two independent waterproofing layers with staggered laps mean a flaw in one ply is backed by the other. Some assemblies run a third ply for added redundancy or to meet a fire or wind listing, and some use two base or ply sheets under one cap. The exact number comes from the specified assembly, not a default.
The cap sheet is the one that faces the weather, so it is granule-surfaced for UV protection and is the layer you see and walk. The base and ply sheets below it are usually smooth-surfaced, built to bond well to the substrate and to the sheet above. Below the membrane sits the rest of the assembly: a cover board over the insulation, the insulation itself, and the deck. The membrane is only as good as what it is bonded to.
Do not count the plies and assume the roof is right. Each ply has to be fully bonded with proper laps, and the cap has to be detailed at every edge and penetration. Two plies set badly leak faster than one ply set well. The redundancy is real only if both layers are actually bonded and the laps are sealed.
Substrate and prep: cover board, insulation, and primer
The membrane bonds to whatever is under it, so the substrate prep is where a mod-bit roof is won or lost before the first roll comes off the truck. Most assemblies set the membrane over a cover board, a thin dense board such as a high-density polyiso, gypsum, or perlite cover board, laid over the insulation. The cover board gives a sound, uniform, and in torch work non-combustible or torch-rated surface for the membrane, and it protects the insulation from foot traffic and the heat of the torch or hot asphalt.
Whatever the substrate is, it has to be clean and dry. Trapped moisture under the membrane has nowhere to go and shows up later as blisters and bond failure. On a concrete or gypsum deck, and on many cover boards, the spec calls for an asphalt primer to give the membrane or the adhesive something to grab. The primer has to flash off and be dry before the membrane goes down, not tacky, not wet.
The detail on the drawing assumes a clean dry deck. On a re-roof you rarely get fully dry, so you schedule around the dew point and the morning damp instead of pretending the drawing is true, and you check the substrate with a moisture meter rather than the back of your hand. Set membrane on a damp or dusty substrate and you own the bond failure months later when someone else is on the roof.
How do you know a mod-bit seam bonded?
You know the seam bonded when you see a bead of bitumen squeezed out along the lap. That bleed-out is the proof. On a torched or mopped mod-bit roof, a properly fused side or end lap pushes a visible line of melted bitumen out past the edge of the upper sheet, and that bead is the visual confirmation that the lap reached fusion and sealed. No bleed-out means not enough heat and not enough bond, and that lap is a future leak whether it looks fine today or not.
There is a target on the bleed. For SBS sheets a common expectation is roughly 3/8 in of bitumen flow at the seam, and less than that signals the lap was not heated enough to bond. Confirm the figure against the manufacturer's instructions, since it varies by product. The seam is the leak path on this roof the same way it is on a single-ply, so the laps get the attention. Stagger the end laps so they do not stack into one continuous joint, and treat the T-joints, where three sheets meet, as the spot most likely to leave a void.
After the membrane cools, the laps get probed. A dull seam probe drawn along the edge of the lap finds the spots that did not bond, the same probe-walk discipline used for single-ply seam QC. Probe the laps, mark what fails, and repair it before it goes under the cap or under a warranty inspection.
Flashings and details: walls, curbs, and the cant strip
Most leaks start at the flashings and penetrations, not in the field of the roof, so the base flashings at walls and curbs are where the roof is really tested. At an inside angle where the roof meets a wall or curb, the membrane wants to bridge the 90 degree corner and crack or void. A cant strip, the angled fillet set in that corner, eases the transition so the flashing turns up the wall at an angle instead of a hard right angle. Skip the cant and the flashing fails at the corner.
Mod-bit flashings are built up in plies the same as the field. The base flashing turns up the wall or curb, lapped and bonded over the field membrane and run up to the proper height, and on a finished system a granule-surfaced cap flashing covers it for UV protection just as the cap sheet does in the field. The flashing is multi-ply because that is where water concentrates and where movement works the membrane hardest.
Carry the same QC mindset onto the flashings that the single-ply seam QA guide brings to welds. Watch the bleed-out on the flashing laps, watch the termination at the top of the flashing, and watch the corners and the T-joints where flashing meets field. A roof can have a perfect field and still leak at one bad inside corner, and that corner is exactly what the inspector checks first.
The cap sheet, surfacing, and granule loss
The cap sheet is the wear surface and the UV shield, and the granules embedded in its top are what do that work. Asphalt degrades fast under direct sun, so the mineral granules block UV, add fire resistance, and give the cap a walkable, abrasion-resistant surface. The standard granule color is a dark mineral, but a white or light reflective granule, or a factory reflective coating, makes a cool cap that cuts roof temperature and can help meet an energy code or a cool-roof requirement. If the project needs a reflective roof, the cool cap or a field-applied reflective coating is how mod-bit gets there.
Granules come off over time, and where they thin out the asphalt below is exposed and starts to weather. Some granule loss right after install is normal, the loose ones that were never fully embedded. Steady loss into the drains years later, or bald patches over the laps and high-traffic paths, is the sign the cap is aging and the membrane needs attention before the asphalt cracks.
Embed the granules while the cap is fresh. On torched and mopped caps, rolling or brooming the granules back into the warm asphalt at the laps keeps the seam covered and protected. A bald lap is an unprotected lap, and an unprotected lap is the first place the roof checks and cracks.
Cold-weather work and the SBS advantage
Cold is where SBS earns its place. The rubber modifier keeps the membrane flexible at low temperature, so an SBS sheet bends and bonds in cold conditions that make an APP sheet stiff and prone to cracking when you work it. For a roof that will live through hard winters and thermal cycling, that cold flexibility is the reason SBS is the common pick in northern climates.
Cold changes the install too, not just the material choice. Self-adhered and cold-applied membranes need a warm enough surface to bond, so on a cold morning the adhesive may not grab until the deck warms, and the manufacturer sets a minimum application temperature you have to respect. Some crews keep self-adhered rolls warm before use so the adhesive is active when it hits the deck. Cold adhesive thickens in the cold and goes on heavy and slow. Hot asphalt loses heat fast on a cold deck and can skin over before the sheet is in it.
Check the manufacturer's minimum temperature for the product and the method, and check it for the surface, not the air. Below that line the bond is a gamble, and a sheet that looks stuck on a cold day can let go when it warms and moves.
Hot asphalt and the EVT window
If you are mopping the membrane in hot asphalt, the asphalt temperature is not a detail, it is the bond. Asphalt that is too cool goes on thick, skins over before the sheet is set, and does not wet the surfaces. Asphalt that is too hot is thin, runs, and starves the mopping of the mass it needs between the plies. The target is the equiviscous temperature, the EVT, the temperature at which that specific asphalt has the right viscosity to be mopped evenly.
EVT is a property of the asphalt and comes from the supplier, with the application window commonly given as the EVT plus or minus 25°F. Applying within that band is what puts the right mass of asphalt between the plies. A common floor for the asphalt at the point of contact with the sheet is the EVT or about 400°F, whichever is higher, with the manufacturer's instruction controlling. Below that the bond suffers.
Watch the kettle, not just the mop cart. Asphalt loses heat between the kettle and the roof, more on a cold or windy day, so the temperature that matters is at the point of contact with the membrane, not in the kettle. Keep a thermometer on it and do not let the asphalt cook above its flash point or sit overheated for hours, which damages it and is a fire hazard in its own right.
Self-adhered and cold-applied: the no-flame options
When the building cannot take a flame or a kettle, self-adhered and cold-applied are the way mod-bit still goes on. Occupied schools, hospitals, food plants, and many institutional roofs ban hot work or restrict it so tightly that torching is not practical. Self-adhered peel-and-stick and cold-applied adhesive both bond the membrane with no open flame and far lower fumes, which keeps the crew and the building out of the fire and air-quality problem that torch and kettle work create.
Self-adhered is the cleanest of the four methods and often the fastest. Pull the release film, roll the sheet into place, and roll it down to set the adhesive. The whole system, base and cap, can be self-adhered. The constraint is temperature and surface prep: the adhesive needs a primed, warm, clean surface to grab, so cold and dust are the enemies, and a missed primer means a sheet that peels.
Cold-applied trades speed for working room. The adhesive is brushed or squeegeed at a specified coverage rate, then the sheet is rolled in, and it cures over time rather than bonding instantly. Get the coverage right, because too little adhesive starves the bond and too much can stay soft and slip. Both methods cost more in material than torch and ask for care on surface and temperature, and both are the right call when fire risk or fumes rule the flame out.
Attachment and wind uplift
A mod-bit roof resists wind by being bonded down, and how the bottom layer attaches to the structure is what holds the whole assembly in a storm. In a fully adhered system the base sheet is bonded to the substrate, by torch, hot asphalt, cold adhesive, or self-adhered, and the cap is bonded to it, so the membrane and the structure act as one. In a mechanically attached approach the base sheet is fastened to the deck with plates and fasteners and the cap is bonded over it, which puts the uplift resistance in the fasteners.
Wind drives the edges and corners hardest. Uplift pressure is highest at the perimeter and at the corners of the roof, which is why the attachment pattern and the edge metal there are heavier than in the field, and why those are the zones that peel first when the attachment is light. The metal edge is part of the wind system, not just trim, and a loose edge gives the wind a place to start.
The wind and fire ratings come from tested assemblies, not from picking good parts. FM Global approvals and UL listings rate a specific assembly, the deck, insulation, cover board, attachment, and membrane together, for wind uplift and fire. Build the assembly that was tested and approved, and confirm the rating required for the building against the project documents and the wind design. Substitute one component and the rating no longer applies.
Penetrations: pipes, drains, and curbs
Every pipe, drain, and curb is a hole in the roof, and the membrane has to be returned and sealed around each one. Pipe penetrations get a flashing, commonly a multi-ply mod-bit boot or a metal sleeve flashed in, that wraps the pipe and laps onto the field so water running down the pipe is carried out onto the roof rather than behind the membrane. The boot is built up and bonded the same as a flashing, because it lives where movement and water both concentrate.
Drains get special attention because that is where the water goes on purpose. The membrane is run into the drain bowl, set in the clamping ring, and the field around the drain is sloped or sumped so water reaches it. A drain flashed high or with a poor seal under the ring backs water up under the membrane. Curbs for equipment and skylights get base flashings with a cant, run up to height, and capped, same as a wall.
Penetrations are where the field crew's good roof gets undone by a rushed detail. Group and box small penetrations where you can so each one is not its own weak point, and give the boots and the drain the same probed, bled-out laps the field gets. The inspector counts the penetrations and looks at every boot, because that is where the leaks are.
What does an inspector check on a mod-bit roof?
The first thing an inspector looks at is the bleed-out at the laps, because that bead of bitumen is the visible proof the seams fused and bonded. A continuous, consistent bleed along the side and end laps says the heat or the asphalt was right. Missing or starved bleed says it was not, and that is a flag whether the field looks clean or not. The laps are then probed for spots that did not bond, the same probe walk used on single-ply seams.
After the laps, the inspection moves to the places that actually leak. The base flashings at walls and curbs, the cant strips, the terminations at the top of the flashings, the pipe boots, the drains, and the corners and T-joints where sheets and flashings meet. Each gets looked at for full bond, proper height, and a sealed lap.
Then the field of the membrane: no fishmouths, no blisters, no voids. A fishmouth is a lap that lifted open at the edge instead of lying flat and bonding, an obvious open leak path. A blister is trapped air or moisture under or between plies, a void that grows with heat and eventually cracks the cap. Both come from a substrate that was not clean and dry or a lap that was not pressed in while the asphalt was live. The inspector marks them, and they get cut, patched, and re-bonded before the roof is accepted.
Recover vs tear-off, and compatibility over BUR
Mod-bit is a common recover membrane because it is compatible with the asphalt systems already on a lot of low-slope roofs. A recover sets a new membrane over the existing roof instead of tearing it off, which saves the demolition cost and disposal and keeps the building dry through the work. Mod-bit over an aged built-up roof is a natural pairing, since both are asphalt-based and a new mod-bit cap can be bonded to a sound, prepared BUR surface.
Recover only works if the roof under it is sound and dry. Wet insulation does not dry out under a new membrane, it stays wet and rots the deck and grows the leak you covered. Probe and cut test the existing roof for trapped moisture first, and pull a core if you are unsure. Codes also limit how many roofing layers a building can carry, commonly to two total, so a building already on its second roof is a tear-off whether you want one or not. Confirm the layer count and the moisture survey before you commit to a recover.
When the existing roof is wet, deteriorated, or already doubled up, tear it off. A recover over a bad roof saves money on the bid and spends it twice on the callback. The honest read of the existing roof, by core and moisture survey, is what decides recover or tear-off, not the number that makes the bid look better.
Ponding and slope
Mod-bit is a low-slope membrane, not a flat one, and it needs to drain. Water that sits on the roof, ponding that does not drain within a couple of days of rain, works against any membrane: it accelerates aging, it finds the weak lap, it grows algae that holds more water, and it loads the deck. NRCA has long recommended positive drainage, and many specs call for a minimum design slope, commonly around 1/4 in per ft, to move water to the drains.
Ponding is also a warranty problem. Many membrane warranties exclude or limit areas of standing water, so a roof that ponds can void the coverage you installed it to get. Check the warranty language on ponding before you accept a design that does not drain, and raise it as a design issue, not a field one, because the field crew cannot fix a deck that has no slope to the drains.
Where the deck is dead flat or sags between drains, the fix is in the buildup, tapered insulation that builds slope and crickets that divert water around curbs and into the drains. Set the slope before the membrane, because no membrane drains a roof that was built to hold water.
What to document
Built up ply by ply and then covered over, a mod-bit roof is judged at the warranty inspection and the first leak against records that have to exist before the cap hides the work. Capture the assembly as built, not as drawn, because the warranty inspection runs against what is actually on the deck. Record each ply, the sheet and its modifier, the application method, how the laps were made and checked, and the surfacing, plus the substrate prep and the daily conditions.
The record is also what proves the safety side on a torch job. Log the fire watch, who held it and for how long, and the thermal sweep if one was run. Photograph the bleed-out at the laps and the flashings before the cap covers them, because that proof disappears under the next layer.
| What to record | Why it matters |
|---|---|
| Ply (base, ply, cap) | Confirms the specified buildup was installed |
| Sheet and modifier (SBS or APP) | Selects the method, climate fit, and warranty |
| Application method | Torch, mopped, cold, or self-adhered drives the QC |
| Lap type and bleed-out checked | The bead of bitumen is the proof of bond |
| Surfacing (granule, cool cap, coating) | UV protection and cool-roof or energy compliance |
| Substrate prep and primer | Bond failure starts at a dirty or damp deck |
| Fire watch and thermal sweep | Proof the torch job was closed out safely |
Common mistakes
- A cold lap with no bleed-out: not enough heat or asphalt, so the seam never fused and will leak.
- Torching with no fire watch, or torching to a combustible substrate or into a parapet you cannot see behind.
- Fishmouths and blisters from laps not pressed in while the asphalt was live, or a substrate that was not clean and dry.
- Wrong method for the temperature or substrate: self-adhered on a cold deck, torch where no flame is allowed, cold adhesive at the wrong coverage.
- Thin or short flashing: no cant strip, base flashing run too low, or a termination left unsealed at the top.
- Setting a recover over wet insulation or as a third roof layer the code does not allow.
- Leaving laps and flashings bald where granules were never embedded, so the asphalt weathers and cracks first there.
Field checklist
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Standards and references
The framework lives across a few bodies, and the membrane manufacturer's published instructions and warranty govern the install over any general reference. The NRCA roofing manual is the trade reference for the assembly and the details, and NRCA's CERTA program is the standard for torch safety and the post-job fire watch. Build to the manufacturer's data sheet first, then the project spec, then the manual.
For the sheets themselves, the ASTM specifications cover SBS modified bituminous sheets by their reinforcement: D6162 for a combination of polyester and glass fiber, D6163 for glass fiber, and D6164 for polyester reinforcement. In those specs a Grade G classification designates a mineral or granule-surfaced product and Grade S a smooth-surfaced one, which lines up with cap versus base and ply sheets. APP sheets fall under their own ASTM specifications rather than these SBS standards, so confirm the correct designation for an APP product. Verify the exact specification, type, and grade against the product submittal, since ASTM standards are revised on a cycle.
Wind and fire performance come from tested assemblies: FM Global approvals and UL listings rate the specific deck, insulation, attachment, and membrane together for uplift and fire class. Confirm the rating the building requires against the wind design, the building code edition the jurisdiction has adopted, and the project documents before you treat any figure as the requirement.
Units and terms
Modified bitumen goes by several names on a drawing set and a data sheet, so the same product can read differently across the documents. Mod-bit and modified bitumen are the same thing, sometimes written as a polymer-modified bituminous membrane. Torch-applied is also called torch-down or torch-on. The sheet is supplied in rolls, the buildup is counted in plies, and the surfacing is granule or smooth.
Keep the abbreviations straight, because SBS and APP are the whole material decision. SBS is the rubber modifier and the versatile, cold-flexible one; APP is the plastic modifier and the torch-applied, heat-tolerant one. EVT is the asphalt temperature target on a mopped job. Bleed-out is the bead of bitumen at the lap that proves the bond.
- Mod-bit
- Modified bitumen, an asphalt sheet membrane reinforced with mat and modified with SBS or APP polymer
- SBS
- Styrene-butadiene-styrene, the rubber modifier; elastic, cold-flexible, torch, mopped, cold, or self-adhered
- APP
- Atactic polypropylene, the plastic modifier; heat and UV tolerant, almost always torch-applied
- Cap sheet
- The top, granule-surfaced ply that faces the weather and takes UV and traffic
- Base / ply sheet
- The lower ply bonded to the substrate beneath the cap, usually smooth-surfaced
- Bleed-out
- The bead of melted bitumen squeezed out at a lap, the visual proof the seam fused
- EVT
- Equiviscous temperature, the asphalt temperature for proper mopping viscosity, applied within plus or minus 25°F
- CERTA
- NRCA Certified Roofing Torch Applicator, the torch-safety and fire-watch program
FAQ
What is modified bitumen roofing?
Modified bitumen, or mod-bit, is an asphalt-based sheet roofing membrane reinforced with a polyester or fiberglass mat and modified with SBS rubber or APP plastic. It is the roll-applied successor to built-up roofing, installed in multiple plies on low-slope roofs for redundancy and foot-traffic durability.
What is the difference between SBS and APP modified bitumen?
SBS is styrene-butadiene-styrene, a rubber modifier that makes the asphalt elastic and cold-flexible; it can be torched, mopped, cold-applied, or self-adhered. APP is atactic polypropylene, a plastic modifier that is heat and UV tolerant and almost always torch-applied. SBS suits cold climates, APP suits hot ones.
Is torch-down roofing safe?
Torch-down is safe with trained applicators and a real fire watch, and dangerous without. The open flame can ignite hidden combustibles that smolder for hours after work stops. NRCA's CERTA program trains safe torching and a post-job fire watch, commonly about two hours, often with a thermal sweep to find hidden heat.
How many plies does a mod-bit roof have?
A mod-bit roof is usually two plies: a base or ply sheet bonded to the substrate and a granule-surfaced cap sheet on top. Some assemblies run a third ply for redundancy or to meet a fire or wind listing. The specified assembly, not a default, sets the number.
What are the four ways to install modified bitumen?
Torch-applied melts the back of the sheet with a propane flame. Hot-asphalt mopped rolls the sheet into mopped asphalt. Cold-applied uses a brushed or squeegeed adhesive with no flame. Self-adhered is peel-and-stick. SBS works with any of the four; APP is almost always torch-applied.
How do you know a modified bitumen seam bonded correctly?
You see a bead of melted bitumen squeezed out along the lap. That bleed-out is the proof the seam fused. For SBS a common expectation is roughly 3/8 in of flow, with less signaling not enough heat. After cooling, probe the laps for spots that did not bond and repair them.
Can you install mod-bit over an existing roof?
Yes, mod-bit is a common recover over a sound, dry asphalt roof such as an aged built-up roof, since both are asphalt-based. But wet insulation will not dry under it, and codes commonly limit a building to two roof layers. Survey for moisture and confirm the layer count before recovering instead of tearing off.
Torch-applied vs cold-applied modified bitumen: which should I use?
Torch-applied gives the strongest bond and works in cold weather but is the highest fire risk and needs a fire watch. Cold-applied uses adhesive with no flame and lower fumes, which suits occupied buildings and fire-restricted sites, at the cost of cure time. The building's fire restrictions usually drive the call.
Why do mod-bit cap sheets have granules?
The mineral granules on a mod-bit cap block UV, which otherwise degrades the asphalt quickly, and they add fire resistance and a walkable, abrasion-resistant surface. A white or reflective granule makes a cool cap that cuts roof temperature for energy compliance. Granules thinning over laps and traffic paths signal the cap is aging.
What temperature should roofing asphalt be when mopping mod-bit?
Hold the asphalt within its equiviscous temperature window, the EVT, commonly the EVT plus or minus 25°F, measured at the point of contact with the sheet, not just in the kettle. A common floor is the EVT or about 400°F, whichever is higher, with the manufacturer's instructions controlling.