Flat roofs are used on commercial buildings, shopping centres, warehouses, schools, apartment buildings and many other properties throughout British Columbia.
But despite their name, flat roofs are rarely completely flat.
They are designed to provide waterproofing, accommodate building equipment and direct rainwater towards roof drains, scuppers or gutters.
At Flat Roofing Insights, we explain how these systems work through short educational videos, practical advice and real photographs from commercial roofing projects.
Whether you’re a building owner, property manager or simply interested in understanding your roof, this section is designed to make flat roofing easier to understand.
Watch: What Is a Flat Roof?
Our introductory video explains what makes a roof a flat roof, why drainage is important and introduces some of the different roofing systems used on commercial buildings.
What Is a Flat Roof?
A flat roof is a roofing system constructed with a relatively low slope rather than the steeper pitch normally associated with residential roofing.
Although the surface may appear level from the ground, most flat roofs are designed with a slight slope to encourage water to move towards drainage outlets.
This slope can be created through the building structure itself or by installing tapered roofing insulation.
Without effective drainage, water can remain on the roof for extended periods, placing additional demands on the waterproofing system.
How Does a Commercial Flat Roof Work?
A commercial flat roof is more than just the waterproof membrane visible from above.
Depending on the building and roofing design, the complete assembly may include:
- A structural roof deck, commonly steel, concrete or wood.
- A vapour barrier or air and vapour control layer, where required.
- Thermal insulation to help control heat loss and condensation risks.
- A cover board or suitable substrate.
- A waterproof roofing membrane.
- Flashings and waterproofing details around walls, penetrations, curbs and drains.
Each component has a purpose, and the assembly needs to be designed to work as a complete system.
Why Is Drainage So Important?
Drainage is one of the most important considerations in commercial flat roofing.
Rainwater needs a reliable route off the roof through drains, scuppers or other designed outlets.
Blocked drains, inadequate slopes and changes in roof levels can allow water to accumulate.
Standing water does not automatically mean that a roof is leaking, but prolonged ponding can contribute to deterioration and may reveal drainage deficiencies that need attention.
Keeping drains clear and checking the condition of the surrounding waterproofing are important parts of roof maintenance.
What Types of Flat Roofing Systems Are Available?
Commercial flat roofs can be constructed using several different waterproofing systems. Each has its own installation methods, detailing requirements and maintenance considerations.
SBS Modified Bitumen Roofing
SBS modified bitumen is commonly installed as a two-ply roofing system, consisting of a base sheet and cap sheet.
It is widely used on commercial buildings and is particularly suited to applications where a robust, multi-layer waterproofing system is required.
Learn more in our SBS Roofing Guide.
TPO Roofing
Thermoplastic polyolefin (TPO) is a single-ply roofing membrane with heat-welded seams.
It is commonly used on large commercial roofs and is available in reflective colours.
Learn more in our TPO Roofing Guide.
PVC Roofing
Polyvinyl chloride (PVC) is another thermoplastic single-ply membrane with heat-welded seams.
Certain PVC roofing systems offer specific chemical and grease resistance characteristics, making system selection important around commercial kitchen exhaust equipment.
Learn more in our PVC Roofing Guide.
EPDM Roofing
Ethylene propylene diene monomer (EPDM) is a synthetic rubber roofing membrane.
It is commonly used on commercial buildings and may be installed using adhered, mechanically attached or ballasted methods, depending on the system design.
Seams, perimeter details and penetrations require appropriate installation and ongoing attention.
Learn more in our EPDM Roofing Guide.
What Should Building Owners Know About Their Flat Roof?
One of the most important things to understand is that a commercial roof requires attention throughout its service life.
Even a relatively new roofing system can develop problems if rooftop equipment is altered, drains become blocked or waterproofing details are damaged.
Some common concerns include:
- Standing water caused by drainage problems.
- Deteriorating flashings around walls and roof penetrations.
- Damage caused by rooftop mechanical work.
- Open seams or membrane deterioration.
- Debris accumulation around drains.
- Previous repairs that have not been properly integrated into the existing waterproofing.
Identifying these concerns early can help building owners make more informed maintenance and repair decisions.
Learn From Real Roof Observations
Understanding how flat roofs are designed is only part of the picture.
Seeing what happens when roofing systems deteriorate can be equally valuable.
Our Flat Roof Problems section uses original photographs and professional roof observations to explain real roofing defects, their possible implications and the options building owners may need to consider.
Explore our Flat Roof Problems section.
More Flat Roofing Explained Videos
This is the beginning of our Flat Roofing Explained educational series.
Future videos will explore subjects such as:
- Why flat roofs need drainage.
- How roof insulation works.
- What a vapour barrier does.
- The differences between single-ply and two-ply roofing.
- Why roof flashings are important.
- What happens when new equipment is installed on an existing roof.
Our aim is to explain commercial roofing in straightforward language, using practical examples and real project photographs.
Need Advice About Your Commercial Flat Roof?
If you’re responsible for a commercial building in British Columbia and have questions about its roofing system, Flat Roofing Insights provides practical information to help you understand your options.
For building-specific concerns, you can also enquire about a professional commercial roof observation report.
