Flat Roof Covering Options

Although asphalt shingles are still commonly used, they are not ideal for flat roofs. Consider other roofing materials should you opt for this type of roof.

Here are the available options to prevent your flat roof from damage caused by harsh weather.

How to Choose Flat Roof Materials

Flat Asphalt and Gravel Roof 

Source: Canva

An asphalt and gravel roof will be made up of several layers of asphalt felt (four or five) with overlapping folds to completely cover the surface of the roof. Between each layer, liquid asphalt is applied. Then, a layer of white or gray gravel is spread to cover and protect the asphalt from UV rays. Without this protective layer, the asphalt would rapidly wear over time.

Although asphalt and gravel roofing has on average a durability of 10 to 15 years (more in some cases depending on the quality of the installation), it will nevertheless be necessary to add gravel to the surface as needed. Note that temperature fluctuations and bad weather may cause some gravel to be stripped from certain sections of the roof.

This type of roofing material is known to be resistant to water seepage, which means that even small accumulations of water on the surface are unlikely to cause damage. For gravel and asphalt roofing, expect costs between $7 and $12 per square foot.

Lastly, note that coating a flat roof requires the expertise of a professional, especially since handling molten asphalt is rather perilous.

Elastomeric Membrane

Elastomeric membranes consist of two layers: the base (screwed or glued) and a gravel layer (white, light gray, maroon, or green); this second layer obviously being the finishing layer. 

The importance of adding a granulated finish is to ensure that the whole installation is highly resistant to sunlight, impacts, and tears. The joints are fused with a blowtorch, which forms a tight seal for the entire installation.

Note that the elastomeric membrane can also be cold-applied using self-adhesive layers. The membrane, which is particularly resistant to impact and temperature variations due to its high elasticity, has the great advantage of requiring few repairs throughout its years of durability. 

While the membrane will remain water-resistant over its estimated lifespan, it is still vulnerable to the effects of stagnant water, which can shave at least 10 years off its expected durability. Also, note that if the algae and/or ice found on the surface of the membrane is not removed within a reasonable amount of time, it can damage the finishing layer of the membrane.

Ideally, the roof drain should be at least 4 inches in diameter in order to properly drain the accumulated water. If you are thinking of investing in a membrane, be prepared to pay between $9.50 and $15 per square foot for this type of roof covering. 

The cost is clearly higher than asphalt and gravel covering (which is between $7 and $12 per square foot). However, it is worth noting that asphalt and gravel covering is far less durable than elastomeric covering, which lasts 25 to 40 years. 

Single-Ply Flat Roof Membrane

TPO Membrane

Source: PC Qualité Toits Inc

TPO (thermoplastic polyolefin) is a single-ply reflective roofing membrane made of layers of polypropylene and ethylene-propylene rubber. The edges are usually fusion-welded like the elastomeric membrane, although the TPO membrane can also be mechanically fixed.

Furthermore, this type of membrane has a glossy finish, making it easy to maintain. However, due to its multiple advantages, it is widely used in the construction and renovation industry.

When it comes to installation costs for a TPO membrane, it will ultimately depend on the square footage needing to be covered. Expect to pay between $7 and $8 per square foot for small surfaces and up to $12 for larger surfaces, as the cutouts and overall work will be more extensive. 

Additional fees may be charged to obtain a contractor's warranty for labour and materials.

* This price includes materials and labour.

Benefits of TPO Membrane for a Flat Roof 

• Easy installation (material is light);

• Flexible to adapt to the building’s motions;

• Extremely durable and resilient to tears, wind, impact, bacteria, moisture, and punctures;

• When the joints are thermally welded, the fusion of the two membranes creates a homogeneous bond, which makes the joint even stronger than the membrane itself;

• Large panels significantly reduce the number of joints (which automatically reduces labour costs);

• Eco-friendly product (TPO membranes are toxic- and hazardous-free ingredients and fully recyclable).

EPDM Membrane

Source: PC Qualité Toits Inc

The EPDM membrane (thermoset rubber) is made of ethylene, propylene, diene, and monomer. Installation-wise, it is similar to the TPO membrane.

The only difference between the two is that the joints are not made in exactly the same way, i.e. by hot air welding. Instead, the joints are sealed with tape or contact cement. This is why EPDM cannot be recycled in the same way as TPO roofing. However, its durability, being a minimum of 50 years, makes it a particularly popular choice. 

In terms of its unique characteristics, it is important to note that EPDM roofing is rot-resistant, which often makes it the preferred option for green roofs.

Expect costs to be between $9 and $12 per square foot for an EPDM membrane (including materials and labour).

Benefits of an EPDM Membrane

• UV, fire, cold, steam, and temperature resistant;

• Low maintenance, aside from occasional repairs to rubber membrane seals and bi-yearly inspections (spring and fall);

• High elasticity and flexibility, making it less prone to splits or cracks;

• Excellent long-term investment that saves energy and reduces heating costs during the winter.

Benefits of a Single-Ply Flat Roof Membrane

Source: Canva

Single-ply membranes offer several advantages including the fact that they are recyclable and easy to install. Moreover, they are competitively priced and represent a 10% to 15% saving compared to roofing materials combining asphalt and gravel. More precisely, these cost between $7 and $12 per square foot for expected durability between 15 to 35 years.

Having these membranes available in white means that heat islands can be reduced, which makes them an environmentally friendly product. Generally speaking, these two single-ply membranes are well suited to withstand the effects of UV rays, as well as snow or rain accumulation, and temperature variations. Lastly, these types of coverings are also used to host green and blue roofs.

Re-roofing your house? Check out our article Renovation Guide: Fixing Your Roof.

Drawbacks of Single-Ply Membranes

Single-ply membranes have some drawbacks, the first of which is the method of installation: as it is a single-ply membrane (single sheet covering the roof), it is more suitable for commercial buildings.

Otherwise, it will require cutting into the fabric to allow for obstructions naturally present, such as vents, on the roof of a house. In addition, sealing the joints calls for a skillful hand, as failure to do so will impair the roof’s overall waterproofing properties.

These two types of coverings are also at a loss during the cold seasons. In fact, since the TPO membrane is welded with hot air, it is a little more difficult to maintain the normally resulting adhesion during the cold season.

EPDM membranes are also affected by our harsh climate, as the glue or tape used to seal the joints can be negatively impacted by the cold.

Why Should You Never Shingle a Flat Roof? 

Unfortunately, traditional asphalt shingles cannot be installed on a flat roof. The reason behind this stems directly from the presence of gaps between the shingles that allow for possible water infiltration.

However, this risk is rather limited on sloped roofs, as water can flow down normally. But, on a flat roof, the water will accumulate on the surface and will eventually seep through the gaps.

To learn more about flat roof maintenance, check out our article: Flat Roof Maintenance: What You Need to Know.

Thinking of building a deck on your flat roof? Check out the article Cost of Building a Rooftop Terrace in Canada.