It’s well known that flat and low-slope roof structures have specific needs. For example, shingles are never used for this type of roof: they are vulnerable to leaks, so they require a seamless protective covering.
Asphalt and gravel have long been the covering of choice for roofs of this type. In the past 20 years or so, however, elastomeric bitumen coatings, which use a two-layer system, have become popular, and now make up a large share of the residential market. Another, more recent option is single-ply synthetic membranes, but these are mostly used on commercial buildings.
Asphalt and gravel: the traditional choice
An asphalt-and-gravel roof membrane (also called a built-up roof) consists of several overlapping layers of roofing felt, at least three of which are laminated together using molten asphalt (bitumen). The felt smoothes out uneven areas of the roof decking and forms a stable base for the asphalt, which is the waterproofing element of the membrane.
A layer of gravel is then added on top of the membrane to shield it from the sun’s UV radiation—without that protection, the asphalt would deteriorate due to evaporation of its constituent oils.
If you choose this type of coating, be aware that installing it is no picnic: the workers will be handling molten asphalt, and you will have to deal with a noisy tar kettle, unpleasant odours and toxic fumes. Once the roof is covered, however, its lifespan can be maximized by simple seasonal maintenance, which consists of adding gravel over any bare spots.
An asphalt-and-gravel roof covering should last for 20 to 25 years, depending on the quality of the installation job and of seasonal maintenance.
Elastomeric bitumen: the newer-generation coating
Elastomeric membranes come in rolls, and are used to create roof coverings typically made up of two layers: a bottom membrane and a granule-covered top sheet. The granules make the covering resistant to impacts, tears and UV radiation.
The top sheet is hot-applied; i.e., the seams are heat-welded using a torch. Installation of an elastomeric membrane is a cleaner operation than laying an asphalt-and-gravel system. Working with a naked flame, however, means a fire risk, so if you choose this method, make sure the contractor has the right insurance coverage.
Some types of dual-layer elastomeric membrane can also be cold-applied. In this case the strips of each layer are self-adhesive, and all the installer has to do is remove a film covering the adhesive as the strips are unrolled. This product is also a good substitute for shingles on a low-sloping roof that lacks a drain to collect rainwater.
An elastomeric membrane results in a lighter coating with greater impact resistance than the classic method, due to its elasticity—an advantage that is even more apparent in cold weather. This type of roof covering is also easier to inspect and repair, as there is no layer of gravel masking the surface.
The cost of a hot-applied elastomeric membrane is 10 to 15% higher than for asphalt-and-gravel coating. It lasts much longer, however: an average of 30 to 35 years.
Synthetic coverings and their variants
Synthetic coverings like TPO (thermoplastic polyolefin), PVC (polyvinyl chloride) and EPDM (ethylene propylene diene monomer, a type of thermosetting rubber) are a lot like swimming pool liners. They are all single-ply membranes. To work around obstacles like vents, the material is simply cut with scissors.
Because of how they are applied, synthetic coverings are best for commercial roofs with expansive areas free of obstacles.
Typically, TPO and PVC membranes are either mechanically applied or hot-air-welded (using a tool much like a hair dryer that softens the membrane). With EPDM, however, hot-melt techniques cannot be used; the joints are sealed using contact cement or adhesive strips. A top layer of gravel can also be added.
Single-ply synthetic membranes (called monocouche coverings in Quebec) are light, recyclable, can be installed quickly, and cost 10 to 15% less than asphalt-and-gravel coatings. They are also easy to maintain and repair—except during the winter months, when hot-air gluing methods are less effective.
In our climate, the lifespan of single-ply synthetic membranes is around 15 to 17 years, according to statistics compiled over the past 50 years by the Association des maîtres couvreurs du Québec.
When a white roof material is a must …
In some Montreal neighbourhoods, so-called white roofs (also known as cool roofs) are now mandatory to reduce the urban heat island effect. Light-coloured membranes reflect sunlight and heat up much less.
If you live in one of those neighbourhoods, complying with the regulation will not be a problem. All of the membrane types described above are available in whitish colours, or can be covered in light-coloured gravel. Either way, the new coating must have a solar reflectance value of at least 78 (which basically means that it reflects 78% of sunlight during the day).
Is it a good idea to cover over an existing coating?
Technically, it’s possible to cover an asphalt-and-gravel coating with a single-ply membrane, after first removing the gravel and laying down sheets of high-density fiberboard (HDF). Many roofers warn against doing this, however, because humidity rising from the home can be trapped in the old coating, “sandwiched” between the framing and the new membrane. For more dependable results, it’s best to start from scratch!
Our thanks to Pascal Hogue of Toitures PME Inc. in Mirabel, a CAA-Quebec Approved Supplier, for his contributions to this instalment of Tips & Tricks.