Choosing the membrane for a commercial roof

Under the surface, a whole assembly

From the deck up, there is a vapour barrier, insulation, often a cover board, then the membrane. Add to that the way everything is held down. Wind resistance, thermal performance and long-term durability depend on it.

For quotes to be comparable, require each one to state:

• the list of layers, from the deck to the top,
• how the insulation and the membrane are held in place,
• the thickness of the insulation and of the membrane,
• the details planned at parapets, drains and around equipment,
• the warranties, who gives them and the maintenance they require.

Layers of bitumen or a single sheet

In a commercial roofing project, the choice between layers of bitumen and a single sheet comes early. It determines the load added to the building, the installation method and the maintenance for decades.

Bituminous systems are stacked. Modified bitumen, or elastomeric membrane, goes on in two plies: a base sheet, then a cap sheet with granules. Built-up roofing, known as tar and gravel, layers felts in hot bitumen and finishes with a bed of gravel.

Single-ply membranes do the job with one sheet. TPO, a thermoplastic that is often white, has seams welded with hot air. EPDM, a synthetic rubber, comes in large sheets with glued seams. PVC, another thermoplastic, is welded the same way.

What the building dictates

• The structure. A concrete slab accepts loads that a light steel deck cannot take. Any heavy or ballasted system goes to a structural engineer first.
• The foot traffic. If technicians go up often, puncture resistance comes first.
• The exhaust. Kitchen grease, industrial products: some membranes tolerate them less well than others.
• The penetrations. Lots of equipment means lots of details to form.
• The installation method. Torch, hot bitumen, adhesive, screws or hot air do not disturb occupants in the same way.

What Laval's Code de l'urbanisme says

Section 379 of the Code de l'urbanisme (Laval's urban planning code) lists the permitted roof coverings. TPO and EPDM membranes are on it, as are the elastomeric membrane, the built-up tar membrane and gravel covering an authorized membrane. PVC is not named. Before you choose it, ask the City's Service de l'urbanisme (urban planning department) whether it fits the list.

On a flat roof, section 380 also applies. The roof must be green, white (or covered with a reflective coating or white ballast), or made of a material with a solar reflectance index of at least 78, attested by the manufacturer's specifications or by a professional's specification. The Code does not, however, say what applies to an existing roof redone identically. Ask the Service de l'urbanisme before you settle on the system.

Keep the system or switch to another

Keeping the system in place often makes things simpler: the structure already carries it, and its details are known. People switch families for one of these reasons: a new use of the building, a required reflective surface, or an old roof that aged badly for a known cause.

Frequently asked questions