Flat Roof Membranes: Asphalt-Gravel vs. Elastomeric Membrane

Gone are the days when all flat roofs were covered with asphalt and gravel. Today, a variety of flat roof membranes are available to property owners looking for a new roof.

The most popular of these options is the elastomeric membrane.

It can be overwhelming to choose between flat roof membranes.

Each flat roofing technology has its own advantages and cost. To make the right choice for your needs and means, it’s best to be well informed.

Here’s everything you need to know about traditional roofing (asphalt-gravel) and today’s most popular roofing technology, the elastomeric membrane.

The asphalt-gravel roof

Asphalt and gravel roofing is not new: it has been used for over a century! If you smelled an unpleasant tar odour when your neighbor had his roof replaced, it’s because he opted for this type of covering.

The asphalt-gravel roof is composed of 4 to 5 layers of felt, between which a layer of molten asphalt is coated. The number of layers is what we call folds. It is this asphalt that makes the roof waterproof. A layer of gravel is then spread to protect the asphalt from UV rays and the outside elements.

Durability and price of an asphalt-gravel roof

The thicker the asphalt layer, the more durable the roof. Asphalt-gravel roofs are not very technological, but they have proven themselves. A well-maintained, top-quality roof will generally last 15 to 20 years.

The price of an asphalt-gravel roof is the lowest of all the options. Since it also has the shortest life span, its cost/year is one of the highest, which does not make it the most cost-effective choice in the long run.

What you need to know about gravel asphalt

The main drawback of asphalt-gravel roofing is its weight. It is by far the heaviest roofing material, weighing an average of 6.3 lbs per square foot (for a 4-ply roof).

This is more than three times the weight of an elastomeric membrane roof. On an average size duplex, asphalt gravel weighs 5200 lbs more than elastomeric membrane. Or the weight of a Ford F-150 pickup truck.

The elastomeric membrane for flat roofs

The elastomeric (or modified bitumen) membrane is also a proven flat roof membrane that has been installed for almost fifty years. This system is very different from asphalt-gravel roofing. It is a more sophisticated two-ply system consisting of an underlayment and a finishing membrane.

The underlayment is screwed or glued to the deck while the top layer is welded to the underlayment. All joints are torch-welded to fuse the membrane to itself. The granular layer on the top coat protects the system from UV rays and the elements.

Elastomeric membrane durability and price

A top-quality, well-maintained elastomeric membrane roof will last 30 to 35 years.

One of the factors that can influence the life of the membrane is the accumulation of stagnant water. To optimize its life span, the roof slope must be adequate to avoid accumulation.

As with all types of roofing, the attic must be well ventilated to avoid overheating and problems caused by humidity.

The price of the elastomeric membrane is 10 to 15% higher than that of asphalt-gravel.

Considering that its life span is about twice that of gravel asphalt, the cost/year is much more advantageous.

What you need to know about membranes

Much more refined than gravel asphalt, elastomeric membrane is delivered in rolls. Forget the kettle, the unpleasant odors, the traces of tar and the toxic fumes.

The membrane is spread on the roof and then welded. It is recyclable at the end of its life and has a low heat absorption index. In the city, it contributes to the fight against heat islands.