Best Roofing Systems for Flat Roofs
Because flat and low-slope roofs collect moisture easily, they need to be built differently than other types of roofs. For example, shingles cannot be used for this type of roof. They are too vulnerable to leaks and instead you need a seamless protective covering.
Choosing the best roofing systems for flat roofs is a challenge for many homeowners and businesses owners. There are multiple types of roofing systems that will work for flat roofs, and when deciding on the right one, there are multiple factors to take into consideration. These factors include: durability, the building’s structure and roof weight limitations and the cost. Listed below are common flat roof systems, including their pros and cons. The characteristics of each roofing system should be reviewed carefully. As you try to choose what is the best roofing system for a flat roof?
Asphalt and gravel have long been the standard for flat roofs. However, in the past 20 years or so, elastomeric bitumen coatings which use a two-layer system have become popular because of their sturdiness and now make up a large portion of the residential market.
Asphalt and gravel System
An asphalt-and-gravel roof membrane consists of several overlapping layers of roofing material, three of which are laminated together using molten asphalt. Roofing felt is then used to smooth out uneven areas of the roof decking and form a stable base for the asphalt, which prevents water from seeping through.
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 will deteriorate quickly.
If you choose this type of coating, be aware that installing it is difficult. The roofers will be handling molten asphalt and working with a noisy machine called a tar kettle. There will most likely be unpleasant odours and toxic fumes to be careful of as well.
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 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 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 weather, tears, and UV radiation.
The top sheet is welded using a torch. Installation of an elastomeric membrane is much neater than laying an asphalt-and-gravel system. However, working with an open flame means a fire risk so if you choose this method, make sure your roofer has the right kind of insurance.
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 like polyvinyl chloride and thermoplastic polyolefin and ethylene propylene diene monomer are single layer membrane system. The roofer uses scissors to work around obstacles like vents and chimneys. Because of how they are applied, synthetic coverings are best used for large areas, free of obstacles.
Typically synthetic membranes are either mechanically applied or welded using a tool much like a hair dryer to soften the membrane.
Single-ply synthetic membranes, also known as 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.
PVC Roofing System
PVC roofing system is highly reflective and very durable. Moreover, PVC is one of the top flat roof options because it is easy to install and lacks water permeability. Meaning it can resist water ponding. It’s three main benefits are very comparable to number 5 on the list, spray-applied roof coatings. The problem with PVC, and the biggest difference from roof coatings, is that it is applied in rolls. And therefore it has seams that can lead to roof failure. When it is rolled out, these seams must be taped or glued together. While this may resist water for a while, it will not hold up for 10-15 years as a seamless roofing system would.
Ethylene propylene is a rubber compound rather than plastic. They are very durable and have built a reputation around durability, but they are not without their weak points. The seams in the EPDM system are treated with adhesives and this makes the seams vulnerable over long periods of time. This sealing system often leads to maintenance on the seams, or building owners living with leaks. This is one of the more maintenance-intensive flat roof options.
Is it a Good idea to Cover over an Existing Coating?
It is 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. Many roofers warn against doing this, however, because humidity rising from the home can be trapped in the old coating, between the framing and the new membrane, warping and damaging the roofing system. For more dependable results, it’s best to start from scratch.
Finding the Best Roofing Systems for Flat Roofs
Can’t decide on the best flat roofing system for you? Schedule a free consultation with Cambie Roofing. We are experienced roofers who will be able to weigh all your considerations. And recommend a roofing system that will work best for your Vancouver home or business.
Originally published Oct. 2017.
Updated and republished May 2023.