Granules collecting in a scupper box, a gritty surface that sheds a little more each winter, and a lap line that has opened where one sheet overlaps the next: these are the signs that a modified bitumen roof is wearing from the top down. Modified bitumen is an asphalt-based sheet whose asphalt has been blended with a polymer to make it more flexible and more durable than the older built-up roofs it grew out.
In West Jordan, UT, a roof has to take snow load, freeze-thaw movement and a hot summer sun in the same year, and modified bitumen's layered construction handles that mix well on the right building. We install it, repair it and judge when a worn cap sheet can be restored and when it needs to come off.
SBS and APP: two different polymers
The two common modifiers are styrene-butadiene-styrene, called SBS, and atactic polypropylene, called APP. SBS is a rubber-like additive that gives the sheet elasticity, so it stretches and recovers through temperature swings, which is useful where winter freezes are common. APP is a plastic additive that raises the sheet's heat resistance and stiffness and improves UV performance, so it suits sun-exposed roofs.
Both come reinforced with polyester or fiberglass mats and are produced in rolls with a smooth or granule-covered surface. The choice depends on the roof's exposure, the application method and what the manufacturer's specification allows. We read the details of the existing roof before recommending either one, as a new sheet must bond to what is already there or to a new base.
Plies, base sheets and the granule cap
A modified bitumen roof is built in layers. A base sheet goes on first, fastened or adhered to the deck or insulation. One or more plies follow, each sealed to the layer below, and a cap sheet finishes the system, with mineral granules that protect the asphalt from sunlight and give a measure of reflectivity. The layers give the roof redundancy: a defect in one ply is covered by the next.
That layering is what makes the roof tough underfoot and forgiving of the occasional dropped tool, but it also hides trouble. Water entering at a lap can travel between plies and appear far from the entry point. When we inspect, we trace granule loss, exposed reinforcement and soft spots to their source, using the commercial roof inspection record to separate cosmetic wear from real exposure.
Torch, mop, cold adhesive and self-adhered application
Modified bitumen is installed in four ways. A torch-applied system melts the underside of the sheet with a propane flame as it is rolled out. A hot-mopped system sets the sheet in heated asphalt. Cold-applied systems use adhesive and no heat. Self-adhered sheets have a peel-and-stick backing. Each affects speed, odor, fire risk and what the deck can tolerate.
Open flame and hot asphalt call for rigorous fire protection: watch periods, extinguishers, protected edges and a check of any combustible material near the work. On an occupied building, or one with a wood deck or a combustible wall detail, we frequently choose a cold or self-adhered method to reduce the hazard. The method also sets the temperature limits: adhesives and peel-and-stick sheets need warm, dry surfaces, and cold weather slows or stops the work.
Edges and penetrations get the same care as the field. Base flashings are built from reinforced sheet set in compatible adhesive and lapped over the field plies, and the top edge is terminated with a reglet or bar and sealed. Each stage is inspected before the next ply covers it, because a void in a base flashing is almost impossible to reach once the cap is on.
Laps, blisters and ridging
Common modified bitumen problems follow a pattern. A lap that was not fully sealed lifts and takes in water. A blister forms where moisture or air trapped under the sheet expands in the sun. A ridge shows where a seam in the layer below has telegraphed through the cap. Granule loss exposes the asphalt, which then oxidizes and cracks. Each leaves a path for water to the insulation below.
The weather record at Salt Lake City International Airport shows cold-season freezing and warm summers, and the swing between them works every one of these defects. We open blisters only when we have to, cut out and replace damaged cap, and re-seal laps with compatible material. If standing water is behind the damage, our ponding water repair comes first, as the sheet will keep failing otherwise.
Foot traffic, equipment and surface protection
A granule surface wears quickly under repeated walking, and a path from the access hatch to a rooftop unit becomes a bare strip in a few seasons. Dragged tools, dropped fasteners and equipment feet do the rest. Walk pads along the service route, protection under pipe stands and a change to the unit supports cut that wear sharply.
Surface restoration is an option when the sheet is sound and dry. A coating over the cap restores the reflective surface and covers worn granules, and a silicone coating or an acrylic one can bridge small defects. We test adhesion on the old surface, because a coating applied over loose granules peels away. Where equipment is dense, our curb and support work goes in first.
A restored cap also gives the next owner a clearer picture of the roof. We record the area coated, the date, the product type and the adhesion tests, so a later inspection can tell which parts of the roof are original and which were restored, and a coating that is wearing at its expected rate is not mistaken for a new defect.
Older buildings and adaptive reuse
Salt Lake City describes the Granary District as an industrial and railroad corridor where adaptive reuse brings older warehouse buildings into new business use, and its redevelopment program supports rehabilitation of existing buildings. The North Temple corridor connects downtown toward the airport, with transit-oriented infill and street improvements planned. Rehabilitated and infill buildings in settings like these raise roof questions that begin with what the existing roof is.
Our service area includes Salt Lake City and West Jordan, where the general plan supports commercial, service and office buildings. For any of them, we compare modified bitumen against a single-ply such as TPO by building use, roof access and the cost of the work.
Commercial Roofing Questions
Which is better for a valley roof, SBS or APP?
SBS stretches through temperature swings, which helps on a roof that freezes and thaws repeatedly. APP resists heat and sun, which helps on exposed roofs. The existing roof and specification often decide it.
Is a torch-applied roof safe on an occupied building?
It can be done with strict fire controls, but a cold-applied or self-adhered system avoids open flame. We discuss the choice with the property manager and the insurer.
What does granule loss mean?
The protective mineral surface is wearing away and exposing the asphalt to sunlight. A little is normal, and a bare area signals aging that a coating or cap repair may address.
Can I put a coating on a modified bitumen roof?
If the roof is dry and the surface is clean and firmly bonded, yes. We test adhesion and scan for moisture before choosing a coating.
How do laps get repaired?
We clean the lap, heat or prime it as the system allows, set compatible material into it and press it. A lap that keeps lifting usually points to movement or ponding nearby.
Share what the cap sheet looks like
Tell us the roof age, where granules collect and where water has shown up. We will arrange a roof walk, trace the laps and blisters, and confirm the repair boundary before work is priced.