Elastomeric and tar and gravel
Two-ply elastomeric, also called modified bitumen, is made of two membranes laid one over the other. The bottom one forms a first barrier. The top one carries granules that protect it from sun and wear. Depending on the product and the part of the roof, they are torch-applied, cold-applied with adhesive or chosen in self-adhering form.
Built-up roofing, or tar and gravel, layers several felts in hot bitumen. A final flood coat of bitumen holds the gravel, which shields it from the sun.
Among commercial roofing membranes, these two systems choose thickness and redundancy over light weight.
Section 379 of the Code de l'urbanisme (Laval's urban planning code) lists both among the permitted roof coverings, as well as gravel laid on an authorized membrane.
Where they are chosen, and for how long
These systems are often chosen where there is a lot of foot traffic: roofs covered in equipment, apartment buildings, roofs with a complicated layout.
According to manufacturers and depending on the quality of installation, an elastomeric membrane lasts 25 to 35 years and tar and gravel 15 to 25 years.
The load on the structure
A bituminous roof is heavier than a single-ply, and gravel adds more. Three situations raise the question of weight:
- switching from one system to another,
- laying the new roof over the old one without tearing it off,
- adding insulation or equipment.
In those cases, bring in a structural engineer before settling the specifications, especially on a light steel deck.
Flame, hot bitumen and colour
The torch works with an open flame. Ask the roofer what fire precautions they take, and ask your insurer whether it sets conditions for hot work.
For tar and gravel, the bitumen is heated on site. It smells strong, and the odour can be drawn in through the building's air intakes.
Then there is the colour, traditionally dark. In Laval, section 380 of the Code de l'urbanisme requires a flat roof to be a green roof, a white covering or a material with a solar reflectance index of at least 78, without addressing the case of an existing roof redone identically. Cap sheets with white granules exist. The index is shown on the manufacturer's data sheet, and the City's Service de l'urbanisme (urban planning department) can tell you what applies to your roof.
What we watch for
On elastomeric, everything is visible: the seams of the cap sheet, especially at the upturns, blisters, wrinkles and spots where the bitumen is bare because the granules are gone.
On tar and gravel, the membrane is hidden. We look for areas the wind has stripped, exposed felts, cracks at the upturns and spots that give underfoot. Finding a leak under gravel takes longer.
Frequently asked questions
Should elastomeric replace my tar and gravel?
Nothing requires replacing tar and gravel with elastomeric, even though it is often done during a roof replacement. The advantage of elastomeric is its exposed surface, which is easier to inspect and repair.
Is it possible to lay a new roof over the old gravel?
A new roof can be laid over the old gravel in some cases. First you need to know the condition of the insulation and the load the structure can still take, through an inspection and, if the weight is in doubt, an engineer's opinion.
Where do the bare patches without gravel on my roof come from?
The wind pushes the gravel, especially at corners and along parapets. Once exposed, the bitumen ages faster in the sun. We can re-gravel these patches and check the membrane.
Official sources, consulted on October 8, 2026: