Two systems, one family
Two-ply elastomeric, or modified bitumen, is made of two membranes. The base sheet provides a first waterproof layer. The cap sheet, covered in granules, protects it from the sun and from wear. The membranes are torch-welded, cold-applied or self-adhered, depending on the product and the location.
Tar and gravel, or built-up roofing, is made of several felts laid one over the other in hot asphalt. A final coat of asphalt receives the gravel, which protects the whole assembly from the sun.
The logic of bituminous systems is the opposite of single-ply.
Toughness
On a roof with foot traffic, thickness counts. A dropped tool that would pierce a thin sheet rarely gets through two layers of modified bitumen.
Bituminous systems are often chosen for roofs loaded with equipment, apartment buildings and roofs with complicated shapes.
Depending on the manufacturer and the quality of installation, an elastomeric membrane lasts 25 to 35 years, and a tar and gravel roof 15 to 25 years.
Weight
A bituminous system weighs more than a single-ply membrane, and gravel adds a notable load. The question comes up when the system changes, when the old roof is covered over instead of torn off, or when insulation and equipment are added.
In those cases, ask a structural engineer for an opinion before the specification is set, especially on a light steel deck.
Installation and its constraints
Torch welding uses an open flame. Ask us how we manage the fire risk, and check whether your insurer has requirements for hot work.
Tar and gravel is installed with asphalt heated on site. The odour is strong and can enter through the building's air intakes.
Colour is the last constraint. A bituminous roof is traditionally dark. If your borough requires a reflective material when a roof is replaced, cap sheets with white granules exist. Compare the reflectance index on the manufacturer's data sheet with the required threshold.
Maintenance specific to these systems
On an elastomeric roof, the surface is visible. We watch the cap sheet seams, especially at the upstands, as well as blisters, wrinkles and areas where the granules are gone and the bitumen is bare.
On tar and gravel, the gravel hides the membrane. We look for patches scoured by the wind, exposed felts, cracks at the upstands and spongy areas underfoot. A leak takes longer to locate under gravel.
Frequently asked questions
Should a tar and gravel roof be replaced with an elastomeric membrane?
Replacing a tar and gravel roof with an elastomeric membrane is not mandatory. It is a frequent choice when a roof is replaced. The surface of an elastomeric roof is visible, so it is easier to inspect and repair.
Can you cover an old gravel roof without tearing it off?
An old gravel roof can sometimes be covered without tearing it off, but the added weight and the condition of the insulation must be checked first. Ask us for an inspection, and consult an engineer if the load is in question.
Why does my gravel roof have bare spots?
Wind moves the gravel, especially in corners and along parapets. The exposed asphalt then ages faster in the sun. Have the gravel put back and the membrane underneath checked.