Plies, Felts and the Bitumen Between Them
The quality of a built-up roof lives in its bitumen. Asphalt heated to the right temperature wets the felt and bonds it, while asphalt that is too cool fails to adhere and asphalt that is too hot degrades. We control the heat in the kettle, stagger the laps so no two joints line up, and push a full mopping of bitumen ahead of each roll so the layers fuse instead of sitting on one another. Cold-applied systems avoid the kettle for sites where hot work is a problem.
Flashing at walls and curbs is built in step with the field, with modified bitumen or reinforced membrane flashing set into a reglet or fastened to a termination bar, and counterflashing over the top. Insulation and a cover board under the plies protect against foot traffic and rooftop equipment, and a moisture check before a roof recover tells us whether the old build-up can stay.
Terrace Decks and Large Civic Roofs
Some roofs are also floors. Plaza Las Fuentes, the six-acre complex across from City Hall, has roof terraces and gardens, and any terrace puts planters, paving and foot traffic on top of the waterproofing, so a deck like that can only be reached by removing everything above it. A multi-ply system with a protection layer is a common choice for such a deck because redundancy matters when a leak is expensive to find, and drain details have to be right from the start.
Scale matters too. The Pasadena Convention Center on East Green Street has three buildings, including a 50,000 SF exhibition hall and the 2,997-seat Civic Auditorium, and with several buildings under one owner, a small drainage error becomes a big wet area. We survey the drains, the decks and the insulation before pricing any scope, because on a large roof the failure points are usually a few drains, a few terminations and a few old patches, and finding them early saves the budget.
Hot Asphalt, Heat and Fire Rules
A kettle on a roof is an open fire with a tank of hot asphalt attached, and it asks for discipline: a watcher staying on after the last mop, extinguishers within reach, and no work on days when the Santa Ana wind is blowing. In the days before the Eaton Fire, wind forecasts put widespread gusts at 60 to 80 mph, and no roofing task is worth a flying ember. We plan hot work around weather and neighbors, and where the site cannot support it, we switch to a cold-applied or self-adhered system.
The Eaton Fire, which began near Altadena, destroyed 9,419 structures in January 2025, which gives owners good reason to ask about Class A assemblies. A mineral-surfaced cap sheet may be one component of a listed roof assembly; the Class A rating applies to the tested combination, not to that layer alone. We review the applicable assembly alongside the 2025 California Energy Code requirements. We check how the cool roof rule applies to each surface, whether gravel, cap sheet or coating, and write the scope to match.
When an Aging Roof Can Be Kept
Many built-up roofs reach a stage where the surface is tired but the plies below are still dry. At that point a restoration coating or a recover with another system can save the owner a tear-off, provided an infrared moisture survey and core cuts confirm the insulation is dry. A roof like that needs repairs at the drains, edges and flashings before anything is added on top.
Conversion is the other path. Owners sometimes move from built-up to modified bitumen or to a single-ply membrane such as TPO, usually because they want a lighter roof, a different attachment method or a reflective surface. We lay out those options with their drawbacks and give the owner the facts, whether the building is a flex warehouse on East Foothill Boulevard or an older block in Old Pasadena.
Commercial Roofing Questions
Is built-up roofing outdated, or is there still a reason to choose it?
Redundancy is the reason. A thick, multi-ply roof can absorb a small flaw without leaking, and gravel surfacing protects the bitumen from sun and foot traffic. It is heavier and slower to install than a single-ply sheet, and hot asphalt brings safety demands, so we recommend it where a building can carry the load and the owner wants a proven build-up.
How many plies does my roof need?
The count comes from the deck, the insulation, the slope and the exposure, not from a number we pull from habit. A roof that sees heavy traffic or sits beneath a terrace may need more plies or a heavier cap than one over a quiet warehouse. We state the build-up in the scope with every layer named, so the owner can compare it line by line.
Can I recover a gravel roof with single-ply, and what happens to the gravel?
The loose stone has to be swept or removed, because a membrane laid over gravel is punctured by it. We also check whether the old plies are dry, add a cover board if the surface is uneven, and make sure the structure can carry the combined weight. When those conditions are met, a recover saves the tear-off, but it only works over a roof that is still sound.
Why is the gravel piled up at my drains or missing from my edges?
Wind and runoff push stone toward the drains and off the perimeter, and the bare bitumen left behind ages in a single season of strong sun. It is one of the earliest warning signs we look for on a gravel roof. We reset the stone in a fresh flood coat where it is needed and add gravel stops or edge metal so it stays put.
How do you handle a kettle on a roof next to occupied floors?
Carefully and in the open air. The kettle sits where fumes cannot reach air intakes, work is timed for calm weather, and someone stays on the roof after the last pass. Where intakes, tenants or nearby buildings make that impractical, we use cold-applied materials and keep the hot asphalt off the site entirely.
