Roofing systems
Modified bitumen.
Asphalt-based and reinforced, usually in two plies. Thick and tough under foot traffic.
What it is
Two plies of reinforced asphalt.
Modified bitumen is asphalt blended with polymers and built around a polyester or fiberglass reinforcement. A typical system is two plies: a base sheet and a cap sheet, the cap usually surfaced with granules or a reflective coating.
SBS sheets stay flexible and are set in hot asphalt, in cold adhesive, by heat welding or as self-adhered sheets, depending on the product. APP sheets are usually torch-applied. Torch work brings fire-safety rules with it: a fire watch, extinguishers on the roof and no torch near combustible edges.
The assembly
What goes on the deck.
At a glance
Modified bitumen in numbers.
- Product standards
- By reinforcement, e.g. ASTM D6164 (SBS) and D6222 (APP) for polyester
- Plies
- Usually two: base sheet and cap sheet
- Application
- Hot asphalt, cold adhesive, heat weld, torch or self-adhered
- Surface
- Granulated or coated cap sheet
- Strength
- Thick, redundant, good under foot traffic
- Typical buildings
- Schools, civic, retail, roofs with heavy equipment
Where it fits
Best for traffic and complex details.
- Roofs with frequent service traffic to rooftop equipment
- Roofs crowded with curbs and penetrations
- Some re-roofs over an existing built-up roof, where the spec allows it
- Owners who want two layers of waterproofing instead of one
It is heavier and slower to install than single-ply, and the application method affects the schedule and the site safety plan. We plan both before mobilization.
How we price it
Plies, laps and flashing, all on the takeoff.
Base and cap sheets are priced by area with lap and waste allowances. Flashing at curbs, walls and drains is priced per item or linear foot, along with the application method the spec names. If the spec allows a choice, we can price an alternate method on its own line.
In the field
A roof built for traffic
Walkway pads to equipment 
Curb flashing 
Drains and scuppers
FAQ
Questions GCs and owners ask.
SBS or APP?
SBS stays more flexible in cold and can be applied without a torch. APP is usually torch-applied and resists heat well. Most North Texas specs name one; we price the one named and list an alternate if you ask.
Is torch work allowed on every job?
No. Some owners, insurers and building types limit open flame. Cold-applied or self-adhered SBS products avoid the torch. The spec and the site rules decide.
Can modified bitumen go over an old built-up roof?
Sometimes, if there is only one existing roof, the insulation is dry and the spec allows a recover. Otherwise the old roof comes off. See re-roof and tear-off.
Related
Compare systems, or plan the work around them.
Have a modified bitumen roof out for bid?
Send the invitation. Link the drawings and spec, give the bid date, and we'll confirm we have them and whether we're bidding.