Why the layer count comes first
An old asphalt roof is usually not one roof. Downtown pre-war brick and masonry buildings behind parapets have been recovered over the years, and once two or more applications of any covering sit on the deck no further recover is permitted. The next job is then a replacement, which removes every existing layer down to the deck. Counting the layers is what the survey does first.
Built-up and modified bitumen are separate systems in the code, with their own slopes. Chapter 15 sets a design slope of not less than 1/4 unit vertical in 12 for built-up, modified bitumen, single-ply and liquid-applied roofs, and 1/8 in 12 for coal-tar built-up. A reroof over an existing low-slope roof with positive drainage is treated differently and is not required to reach that minimum.
Aggregate does not go back on. Where a built-up roof is stripped, its aggregate is not reinstated, and existing flashings, metal edgings, drain outlets and counterflashings are not reinstalled where they are rusted, damaged or deteriorated. New flashings are reconstructed to the manufacturer's instructions. On a masonry parapet that usually means the counterflashing and the cast-iron outlet are replaced as part of the roof rather than argued about afterwards.
A tear-off almost always crosses the energy-code line. Once more than 50 percent of the roof area or more than 2,000 square feet, whichever is less, is altered, the new surface has to reach a 0.63 aged solar reflectance, 0.75 thermal emittance and SRI 75. Exceptions cover a U-factor alternative, roof area under solar panels, and constructions of at least 25 pounds per square foot.
What built-up and modified bitumen roofing includes
The roof plan is marked with outlets, seams, curbs, penetrations and wet areas, and cores are taken where the layer count is in question, because how many coverings sit on the deck decides whether this is a repair, a recover or a strip.
Chapter 15 sets the route: two or more existing applications rule out a recover, and the altered area decides whether the energy-code values come with the job.
Line items, with tear-off and wet-material removal separate from the new plies, and a statement of what can move once the deck is exposed.
Every existing layer off to the deck where a replacement is called for, aggregate and wet insulation out, and the deck checked before anything new is laid.
The new system built up in its own sequence, with the perimeter and the parapet base flashing set as the field goes down.
Counterflashing, edge metal and outlets renewed to the manufacturer's instructions, the outlets watched under water, and the product label filed with the photographs.
Count the coverings before pricing the roof. A deck already carrying two applications cannot take a third, so an estimate written as a recover on that building is an estimate for work the code does not allow.
Where asphalt systems remain
Downtown brick and masonry
Flat roofs behind parapets, recovered more than once over the decades, where the layer count and the counterflashing decide the scope.
Schools and institutions
Large roof areas built in eras when asphalt systems were standard, now worked in sections around a break schedule.
Churches and civic buildings
Long-lived roofs with parapets and old outlets, where a strip exposes decking that has not been seen since it was built.
Packing sheds and processing
Large-span agricultural roofs where dust and chaff load the outlets and the surfacing takes a beating every season.
Older industrial buildings
Roofs with heavy rooftop plant, where the weight of the construction and the state of the deck shape what can be laid.
Self-storage and long runs
Roofs over rows of units, rarely inspected between re-roofs, where one failed lap wets a long stretch of the assembly.
Questions about built-up and modified bitumen roofing
Two. Where two or more applications of any covering are already on the deck, no further recover is permitted, so the next job is a replacement that removes every existing layer. A core or a cut at the edge is how the count is confirmed, rather than a guess from the surface.
No. Aggregate from an existing built-up roof is not reinstalled, and the same rule catches flashings, metal edgings, drain outlets and counterflashings that are rusted, damaged or deteriorated. They are replaced rather than refitted.
New work is designed to at least 1/4 unit vertical in 12 units horizontal, and coal-tar built-up to 1/8 in 12. Reroofing over a roof that already drains is judged on drainage instead.
Almost always, because it passes the altered-area threshold. The new surface then has to meet the aged reflectance, emittance and SRI values, unless one of the code's exceptions applies.
They are separate systems with separate sections in the code, though both are asphalt-based and both take the same minimum design slope on new work.
Built-up and modified bitumen roofing by city
Asphalt roof work is requested through the commercial roofing page for your city, which adds the local stock and the permit authority for a re-roof.
Related commercial roofing services
