Modified Bitumen vs Single-Ply for Burnaby Commercial Flat Roofs
Modified Bitumen vs Single-Ply for Burnaby Commercial Flat Roofs
For most Burnaby commercial and strata flat roofs, the honest answer is that both systems work when the drainage and the installation are right. Modified bitumen is a two-layer, asphalt-based membrane that suits roofs with foot traffic, lots of penetrations and complicated detailing. Single-ply (TPO, PVC or EPDM) is a factory-made sheet that goes down in one layer and suits large, open roofs where speed and a lighter system matter. The right pick depends on how your roof is used, how it drains and who is installing it.
Hey, it’s Harman here. Below is how we think about the choice on Lower Mainland flat roofs, with the Canadian standards that back it up.
TL;DR
- Modified bitumen (SBS or APP) is a two-ply system, a base sheet plus a cap sheet. Single-ply is one flexible synthetic sheet, either thermoset (EPDM) or thermoplastic (TPO, PVC).
- Mod-bit is a strong fit for roofs with HVAC curbs, regular service traffic, parapets and lots of flashing details. Single-ply is a strong fit for large, simple roofs.
- For SBS versus APP, SBS is the more flexible one in cold weather. APP is stiffer and stronger.
- Neither system forgives standing water or poor flashing. Slope, drains and detailing decide how a Burnaby roof ages.
- A mod-bit roof and a thermoplastic roof can’t simply be stacked together, so decide before the tear-off, not during it.
Table of Contents
- What actually separates the two systems
- Why Burnaby roofs put this choice under pressure
- Foot traffic, HVAC curbs and parapets: where mod-bit earns its keep
- Large, open roofs: where single-ply shines
- SBS vs APP: the second decision inside mod-bit
- Installation conditions that decide how the roof ages
- Drainage matters more than the membrane label
- How to choose: a practical decision guide
- Strata and multi-family roofs
- Talk to us about your flat roof
- FAQ
- Sources
What actually separates the two systems
The Canadian Roofing Contractors Association (ARCA) describes modified bituminous membranes as composite sheets made of modified asphalt, reinforcement and surfacing, typically installed as two plies: a base sheet and a cap sheet. The rolls are commonly about 1 m wide, and the two polymers used to modify the asphalt are SBS and APP.
Single-ply is a different idea. Construction Canada groups single-ply membranes into thermosets (rubber, with EPDM the most widely used) and thermoplastics (plastics such as PVC and TPO). The same article points out that single-ply is one layer of protection by nature, while mod-bit is built up in layers and is applied by hot mopping, cold adhesive or torching.
Think of it like this: mod-bit is a layered winter coat, and single-ply is a high-quality shell jacket. Both keep you dry. They get there differently, and they fail differently when someone cuts corners.
| Modified bitumen (SBS/APP) | Single-ply (TPO, PVC, EPDM) | |
|---|---|---|
| Construction | Two plies: base sheet and cap sheet | One sheet, factory-made |
| Material | Asphalt modified with polymers, reinforced | Synthetic polymer (plastic or rubber) |
| Typical attachment | Torch-on, cold adhesive, hot asphalt or mechanical fastening | Fully adhered, mechanically fastened or ballasted, depending on system |
| Seams | Overlapping, heat-welded or adhered laps | Heat-welded (TPO, PVC) or adhered/taped (EPDM) |
| Best suited to | Detailed roofs, service traffic, many penetrations | Large open roofs, fast installs |
Why Burnaby roofs put this choice under pressure
Burnaby has a lot of the roof types where this decision matters: low-slope commercial buildings, industrial units, strata podium decks and mixed-use blocks with rooftop mechanical equipment. Our wet winters mean a flat roof is under water load for months, and any weak laps, tired sealant at a curb or a half-blocked drain shows up quickly.
Temperature swings matter too, even on the coast. The National Research Council of Canada notes that in northern climates a dark roof membrane can reach about 100°C on a hot summer day and fall as low as -40°C in winter, so a membrane needs to perform at both extremes. Burnaby doesn’t see -40°C, but the principle holds: a membrane lives through summer heat on a dark surface and cold, wet winter nights, year after year.
Foot traffic, HVAC curbs and parapets: where mod-bit earns its keep
On roofs where technicians walk to rooftop units every quarter, a layered, granule-surfaced cap sheet gives a forgiving walking surface. It’s also a comfortable choice around HVAC curbs, drains, pipe penetrations and parapet walls, because flashing can be built up in matching plies and tied into the field membrane.
The ARCA modified bitumen manual spells out how much of the system is in the detailing. For example, base sheet side laps must be at least 64 mm wide, end laps at least 150 mm, and laps staggered at least 300 mm from adjacent laps. That’s the kind of detail we check on a Burnaby re-roof, because a roof with ten curbs and a parapet has hundreds of metres of laps and terminations.
The same manual also requires an SBS system to be a two-ply membrane with all components from the same manufacturer, and it calls for membranes that comply with CGSB 37-GP-56M / CSA A123.23-15. In practice, that means you can ask what’s going on your roof and get a straight answer in standards language.
Large, open roofs: where single-ply shines
A big, simple Burnaby warehouse roof with few penetrations is a natural home for single-ply. TPO is lighter and easier to handle than other membrane types, according to the National Research Council’s publication on TPO, and wide factory sheets mean fewer field seams and a quicker install. The ARCA thermoplastic manual requires that all TPO membranes be reinforced, at least 1.1 mm (45 mil) nominal thickness, and meet ASTM D6878.
EPDM, the rubber option, is the single-ply system most commonly specified in Canada among the thermosets, per the Construction Canada piece above. We use it on smaller, simpler low-slope roofs such as garages, canopies and townhouse roof sections, where large, uninterrupted sheets cut down on seams.
For a deeper look at the two thermoplastics, we’ve written a PVC vs TPO comparison for Vancouver roofs, and our overview of commercial roofing types explains how flat and low-slope assemblies differ.
SBS vs APP: the second decision inside mod-bit
If you land on modified bitumen, you still need to pick the modifier. The NRC’s review of low-slope membranes gives the numbers: SBS makes up an average of 12 to 15% of the formulation, and more SBS means greater low-temperature flexibility and fatigue resistance. APP makes up 25 to 35% of the modified compound, and APP membranes have higher strength and lower elongation than SBS types.
Here’s how that translates on a roof:
- SBS: rubber-like and flexible. A good match for roofs that move, such as buildings with thermal expansion at long runs or decks with some flex. It can be applied by cold adhesive, torching, hot asphalt or mechanical fastening.
- APP: plastic-like and stiffer. Strong and stable, and it’s usually torched on or applied with cold adhesive or fasteners.
The ARCA manual lists these attachment methods separately for each type, so what’s right depends on the building. A roof with a combustible deck or tight rooftop access might call for cold-applied or mechanically fastened options over an open flame, and that’s a conversation to have early, before a tear-off.
Installation conditions that decide how the roof ages
Both systems have temperature limits during installation. ARCA says SBS application must not be done when the roof-level air temperature is colder than -18°C with a wind over 14.5 km/h, and its thermoplastic manual sets the same limit for fully adhered systems. On the coast we rarely get near those numbers. Our real constraint is rain: a membrane laid over a damp deck or wet insulation traps moisture.
Construction Canada notes that heat-welding can evaporate surface moisture from the bonding area and softens the material, which helps in cold weather, whereas hot mopping is generally safer and faster than torching. On our jobs, wet-season scheduling and good tarping are as important as the membrane you picked.
One rule catches people out. The ARCA thermoplastic manual says thermoplastic membranes aren’t compatible with bitumen, so any bitumen-based materials have to be removed or covered with an acceptable underlayment. If you’re replacing an old torch-on roof with TPO, the plan has to include that step.
Drainage matters more than the membrane label
A lot of Burnaby leak calls we see trace back to ponding, blocked drains or weak flashing, not the membrane brand. ARCA’s thermoplastic manual sets minimum drainage slopes of 1:100 (1%) for conventional designs and 1:50 (2%) for protected membrane and combination designs, and requires emergency or overflow drainage when flow-control drains are used. It also says membranes should be laid so water doesn’t flow against the laps.
A quick drainage check before you choose a system:
- Do you see standing water 48 hours after rain? That’s a slope or drain problem to fix in the re-roof design with tapered insulation, not a reason to blame the membrane.
- Are drains and scuppers clear, with overflow routes in place?
- Are curbs, parapet bases and pipe penetrations flashed to the same standard as the field of the roof?
- Is there a service route that keeps foot traffic off seams and edges?
How to choose: a practical decision guide
| Your Burnaby roof | Often the better fit |
|---|---|
| Many HVAC curbs, regular service traffic, parapets | Modified bitumen (SBS) |
| Large, open roof with few penetrations | TPO or PVC single-ply |
| Small, simple low-slope section such as a garage or canopy | EPDM |
| Existing torch-on roof being replaced with a thermoplastic | Plan full removal or approved underlayment |
| Cold-weather flexibility is the priority | SBS-modified membrane |
This is a starting point, not a verdict. Roof decks, insulation, access and budget all play in, which is why we walk the roof first.
Strata and multi-family roofs
Strata councils in Burnaby usually need more than a membrane choice. They need a reroofing plan that fits depreciation reports, phasing and owner notices. A layered mod-bit system can be repaired in sections, while a single-ply roof is typically straightforward to extend across big areas. We cover both on strata and multifamily roofs, including repair programs that keep a roof going until the replacement is funded.
Our perspective
We don’t push one membrane. On a Burnaby flat roof, the system that wins is the one matched to how the building is used: torch-on and SBS-modified systems for detailed, trafficked roofs, EPDM or other single-ply for simpler ones. We care about the parts nobody sees from the street: laps, curb flashings, drains, overflows and the way the membrane ties into the parapet.
Our approach is to inspect first, explain the options in plain language, and put the reasoning in writing so your board or ownership group can sign off with confidence. A good flat roof is a system, and we build it that way.
Harman
Talk to us about your flat roof
We install, repair, inspect and replace flat and low-slope roofs for Burnaby and Metro Vancouver commercial owners, property managers and strata councils. If you’re weighing torch-on against single-ply, we’ll look at your drainage, penetrations and access, then recommend the system that fits. See our commercial roofing services or get in touch through our homepage for an inspection.
For an overview of other systems, our guide to industrial roofing types in BC is a good next read.
FAQ
Is modified bitumen better than TPO?
Neither is better across the board. Modified bitumen is a two-ply asphalt system that handles foot traffic and detailed roofs well, and TPO is a lighter, single-layer sheet that suits large, open roofs. The right choice depends on your roof’s use, drainage and penetrations.
What is the difference between SBS and APP modified bitumen?
SBS is rubber-modified and more flexible in cold weather and under movement. APP is plastic-modified and stronger with less elongation. The NRC reports SBS content around 12 to 15% and APP around 25 to 35% of the modified compound.
Can I put a TPO roof over an old torch-on roof?
Not directly. ARCA notes that thermoplastic membranes aren’t compatible with bitumen, so bitumen-based materials must be removed or covered with an acceptable underlayment. We plan this in the scope before work starts.
How is modified bitumen installed?
It’s typically applied in two plies, a base sheet and a cap sheet. Depending on the system, installers use torching, cold-applied adhesive, hot asphalt or mechanical fastening. Details such as lap widths and staggering are set out in the ARCA manual.
Sources
- ARCA Canadian Roofing Reference Manual, Modified Bituminous Membranes introduction
- Construction Canada, Designing Modified Bitumen
- NRC, Characteristics of membranes and insulations used for low-sloped roofs
- NRC, Thermoplastic Polyolefin Roofing Membranes
- ARCA Modified Bitumen Manual
- ARCA Thermoplastic Roofing Manual
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