Is 2-Ply SBS Modified Bitumen Right for My Flat Roof?

An aerial view of a 2-ply SBS modified bitumen roof in Toronto, Canada.

Your roofer has suggested a bitumen roof, whether it is 3-ply or 2-ply SBS modified bitumen. Why? Most likely, your building has a low-slope roof, for which this type of roof was designed. Its top benefits include outstanding durability and tensile strength, for high resistance to tears and punctures, harsh weather conditions, and high foot traffic. 

But what type of SMB modified bitumen (mod-bit or MB) roof assembly will be best for your project? There are many options to choose from in the mod-bit market. The primary considerations are the method of installation, membrane colour and number of plies.

What more do you need to know? Let’s take a deeper dive.

Not all Modified Bitumen Systems are Equal

Different Substance Combinations

Bitumen is a type of standard roofing asphalt that can be modified with APP (atactic polypropylene) or SBS (styrene butadiene styrene). However, only SBS-modified bitumen is sold in Canada because of its capacity to work appropriately with our climate and its resulting temperature swings, high winds, and wind-blown debris.

Different Thicknesses and Weights

Modified bitumen membranes can have two or three layers, with most installed mod-bit roofing membranes having two plies. The first ply is a base sheet that is applied directly to the roofing substrate or over top of either a layer of insulation or a cover board, while the second ply is a cap sheet that is often torch applied, but can be installed with cold adhesive, self-adhesive primer, hot air welding equipment or hot asphalt. 

Individual rolls of mod-bit are typically 87-158 mils thick (2.2 – 4.0 mm) and weigh 1.1 lbs per sq. ft. (5.3 kg per sq. m.). The average 2-ply MB roof membrane is about 300 mils thick and weighs around 2.2 pounds per square foot. The more plies, the thicker and heavier the membrane will be.

A side view of a 2-ply SBS modified bitumen roof membrane.
A rooftop view of an in progress new commercial plaza with 2-ply SBS modified bitumen roof membrane.

Two-Ply SBS Mod-Bit May be the Best Option

While 3 ply modified bitumen membranes have become increasingly popular, two plies are often more than enough in Toronto, and sometimes preferable to thicker, heavier mod-bit membranes. In use since the 1970s, 2-ply has stood the test of time.

Here are some of the benefits of 2-ply SBS modified bitumen assemblies:

  • Faster Installation: When time is an issue, installing two plies will take less time than installing 3-5 plies. A choice of hot and cold installation methods provides flexibility.
  • Lower Maintenance: 2-ply MB assemblies often require lower maintenance than a 4-ply roof with gravel ballast. Retrofitting is fairly straightforward, often eliminating tear-off costs and reducing disposal expenses.
  • Lighter and UV Resistant: Fewer plies means less weight, a critical consideration for some structures and lower cost recovery options.  Also, the cap sheet is ultra violet resistant so the gravel ballast is not a requirement as a weathering surface. For protection against hotter weather and less energy consumption, white reflective options are available.
  • Lower life cycle costs: Since there is no ballast cover and these systems are relatively light weight, they can be recovered fairly easily provided the insulation layer(s) are still dry.
  • More Maneuverable: 2-ply membranes are easier to work with in tight spaces, such as under rooftop patios and decks.
  • Tear-resistant: SBS Mod-Bit systems, regardless of how many plies, offer outstanding durability and tensile strength, making them resistant to punctures, tears from storm damage, and wind-driven debris.
  • Impact Resistant: The granulated cap sheet in a 2-ply MB membrane stands up well against consistent foot traffic. The 7mm thickness of the combined 2 plies provides high puncture resistance.
  • Reduced Odour: Low odour asphalt products are available to reduce issues during installation.
  • Variety of installation methods: There are many different types of membrane that can be installed with asphalt, cold adhesive, self adhesive, mechanical fasteners, hot-air welding and thermofusible etc. to suit many building conditions.
An Elite Roofing team member working on a roof membrane in Toronto, Canada.
An Elite Roofing team member cutting mod-bit membrane for roof parapet walls and upturns at HVAC curbs in Toronto, Canada.

Have an Expert Install Your 2-Ply SBS Modified Bitumen Roof Assembly.

A side view of a 2-ply SBS modified bitumen roof membrane.
A completed SBS modified bitumen roofing project

Many Toronto building owners will enjoy the advantages of a 2-ply SBS-modified bitumen membrane. Talk to a qualified roofing contractor to see if this is the right option for you. Proper installation and ongoing maintenance by expert roofing professionals will ensure that your roof has a long life, protecting your building investment. Contact Elite Roofing today.