Tar and gravel roof replacement usually means stripping the gravel and the layers of felt and asphalt down to the deck, repairing the deck and drainage, and installing a modern low-slope system: modified bitumen, a white TPO or PVC single-ply membrane, a new built-up roof, or spray foam with a coating. A cover-over is allowed only when the old roof is dry and sound. This guide explains how to tell when the old roof is finished and how the modern options compare. For an inspection, call NEMA Roofing at 866-631-3366.
What is a tar and gravel roof?
“Tar and gravel” is the everyday name for an aggregate-surfaced built-up roof (BUR): several plies of roofing felt laid in hot asphalt or coal tar, finished with a flood coat of bitumen with gravel pressed into it. The gravel is not decoration. The U.S. Department of Defense’s roofing criteria (UFC 3-110-03) note that it keeps the membrane cooler than a granule-surfaced cap sheet and protects it, which is why aggregate-surfaced built-up roofs tend to outlast cap-sheet versions. The same document puts typical gravel surfacing at about 400 pounds per 100 square feet, which matters when you plan what goes back on.
How do you know a tar and gravel roof needs replacing?
- Bare patches where wind, foot traffic or water has moved the gravel and the bitumen underneath is exposed to sun.
- Blisters and ridges in the membrane, or cracked, alligatored bitumen in exposed areas.
- Standing water that stays long after rain; the Department of Defense criteria define ponding as water still there 48 hours after rain stops, in drying conditions.
- Soft or spongy spots underfoot, which point to wet insulation or a damaged deck.
- Leaks at edges, walls and pipes. The metal gravel stop at the roof edge and the flashings at walls and penetrations usually fail before the field of the roof.
- Repeated patching. Several generations of patches and mastic on the same areas mean the membrane is at the end of its life.
More warning signs, and what each one means, are in our list of common flat roof problems.
Tear off or cover over?
California’s building and residential codes allow a new roof over an existing one only when the old roof is an adequate base. A recover is not allowed when the existing roof is water-soaked or deteriorated, or when it already has two or more roof coverings (CBC 1512.3, CRC R908.4). A full replacement removes all layers down to the deck (CBC 1512.2, CRC R908.3).
A gravel roof adds its own steps if a recover is chosen. Los Angeles County’s reroofing manual (Building Code Manual 1512, for areas where the county issues permits) requires an existing aggregate built-up roof to be brushed clean of loose gravel and of embedded aggregate sticking up more than 1/8 inch, and blisters and buckles to be cut and flattened, before the new roof goes on. The Defense Department’s criteria, written for federal buildings, go further and do not recommend recovering low-slope roofs unless a moisture survey and cost analysis show it is the best option.
In practice, most old tar and gravel roofs are torn off: the weight of the gravel comes off with them, the deck can be inspected and repaired, and drainage can be corrected with tapered insulation.
What replaces a tar and gravel roof today?
| System | How it relates to the old roof | Points to weigh |
|---|---|---|
| Modified bitumen | The closest modern relative: asphalt improved with polymers, in fewer, heavier plies, often with a granulated or coated cap sheet | Familiar multi-ply redundancy; installed with heat, cold adhesive or self-adhering sheets; surface must meet cool-roof values on most reroofs |
| TPO or PVC single-ply | A single sheet with heat-welded seams, white from the factory | Light, reflective, quick to install; seams and flashings need careful welding; the U.S. criteria say properly built thermoplastic systems may last 15 years or more |
| New built-up roof | The same family as the old roof, rebuilt with a cap sheet or a reflective surfacing | Heavy and labor-intensive; the surfacing must be rated to today’s cool-roof values or fall under an exception |
| Spray polyurethane foam with coating | Foam sprayed onto a prepared deck, then coated | Adds insulation and can build slope; the coating is the weather surface and needs periodic recoating |
Every system and its layers are compared in more depth in our flat roofing systems guide, and service life by material is in how long each flat roof material lasts.
Why the new roof will probably be light-colored
Under California’s 2025 energy code, when more than half of a home’s low-slope roof is replaced in climate zones 4 and 6 through 15, the replacement has to test at 0.63 or better for aged reflectance and 0.75 or better for emittance (an SRI of 75 or more also qualifies), unless insulation is added at the roof deck under the exception. Commercial roofs have matching values when more than half the roof, or more than 2,000 square feet, whichever is less, is replaced, with an insulation trade-off. Unless the chosen surfacing is rated to those values, a traditional gray gravel finish will not qualify, so the changeover to a modern system is also a change to a white or light surface for most owners.
What happens when a gravel roof is replaced?
- Permit and plan: the system, the insulation and the drainage fix are decided and permitted.
- Removal: gravel and plies come off down to the deck and are hauled away.
- Deck repair: soft or rotted sheathing is replaced; this is only known once the deck is open.
- Drainage: tapered insulation or crickets correct low spots. For a reroof, California’s codes waive the 1/4-inch-per-foot minimum slope only where the roof still drains positively.
- New edge metal and flashings: the old gravel stop is replaced with edge metal suited to the new system, and walls, curbs and drains are reflashed.
- Membrane and final check: the new system goes on, then seams, flashings and drains are checked.
The day-by-day view of a flat roof job is in flat roof installation: what to expect.
Replacing your tar and gravel roof with NEMA Roofing
NEMA Roofing replaces built-up and other flat roofs on homes and commercial buildings through our flat roofing services. With five locations (Santa Barbara, Oxnard, Woodland Hills, Tarzana and Downey), our crews serve every county from Santa Barbara and Ventura through Los Angeles to Riverside and San Bernardino; see our service areas. To find out if your gravel roof can be recovered or needs a tear-off, call 866-631-3366.
