Making Good Better: How to Ensure Your New Flat Roof is Code Compliant

August 2022

When a flat roof membrane reaches the end of its service life you might be tempted to hire your local roofing contractor to replace like for like. Before you make that call you should be beware of new code requirements that will impact the design of your roof assembly as well as the installation methods.

A growing emphasis on climate change and the need to conserve energy and optimize performance has led to more stringent requirements being set by both industry bodies and the government on the installation of new roofs. Life safety measures may also be required to protect the inhabitants of a building as well as workers on a roof.
Here we review some common code requirements that impact a flat roof replacement project.

Building Permit

Roof replacements generally do not require a building permit unless the project includes a structural alteration to an existing component. This includes replacement of guards (for example on terrace roofs) or structural repairs (to the roof deck). However, different municipalities might have slightly different requirements. It is always a good idea to check with your municipality prior to undertaking a project of this nature.

Roof Structure

Structural analysis is not typically performed when replacing a flat roof. The exception is if the proposed system increases the dead load (weight) imposed on the structure, such as an overlay or ballasted roofing system. Often found in commercial or industrial buildings, metal roof decks are not typically designed to carry higher roof loads associated with installation of a new membrane over an existing assembly (i.e., roof overlay) or with heavy stone ballast used to weigh down membrane and insulation (i.e., ballasted roof system). If installing a ballasted roof system or a roof overlay over an existing metal roof deck, a structural analysis should be performed.


While the existing roof insulation likely met all the applicable code requirements at the time of its construction, it likely falls short of the current code. In particular, energy efficiency requirements have continually increased with every iteration of the Ontario Building Code (OBC).

The good news is that roof replacements do not need to meet all the requirements of the current code. When replacing a roof, the new assembly should meet or exceed the performance level of the existing assembly. While in theory, it is acceptable to install like-for-like insulation, replacing a roof provides a valuable opportunity to optimize its performance. Designers will often consider adding or replacing the insulation in the roof system to reduce heat loss through the roof and reduce overall energy costs.

The standard used to discuss the performance of a roof insulation is the R-value (imperial) or the RSI value (metric). The R-value is a measurement of the effectiveness of insulating materials. Materials with a high R-value will be better insulators than materials with a low R-value. The required type and thickness of new insulation will depend on a variety of factors including roof type (conventional built-up roof, protected membrane, single ply, etc.) and building use (residential vs. commercial).
Another important consideration when increasing insulation thickness is the impact of the finished roof assembly height. For example, the height of existing parapets, door thresholds, mechanical equipment and pipes may limit the insulation thickness or need to be raised accordingly to fit the new insulation. This will add additional costs to the construction project and should be planned for during the design stage.

Wind Uplift

Fallen trees, power lines and broken fences are not the only victims of high wind events. During storms with heavy winds, pressure builds up inside a building as air infiltrates through various openings. As wind moves around a building, the pressure above a flat roof surface decreases. When the interior pressure exceeds the exterior pressure, a roof system is subject to wind uplift. If not properly designed, the various components of a roof system will detach from each other or the structure potentially resulting blowoff and leaks. Building corners and edges are especially susceptible to the effects of wind uplift.

All new roofing systems must be designed to withstand the wind loads specified in the OBC. The wind loads depend on various factors including building height, area, and location and may be different for various parts of a roof.
Manufacturers will test their roof assemblies to CSA A123.21 which is a normalized test method to determine an assembly’s resistance to dynamic wind uplift. Based on an analysis of the wind loading, an appropriate assembly can be selected. This will affect not only the type of roofing membrane, but also whether its adhered, partially adhered or mechanically fixed to the structure.

Figure 1: Wind uplift


Unlike a sloped roof which drains to a large trough, a flat roof requires internal drains and/or scuppers. Replacing a roof provides a valuable opportunity to review and improve existing drainage. The OBC sets out minimum requirements for drainage which is based on the slope, roof area, and average rainfall intensity. However, an owner may wish to add more drains, if feasible, to improve drainage. This is especially relevant considering the increased frequency and intensity of rain and snow that is occurring as a result of climate change.

For roofs with parapets higher than 150mm, emergency roof overflows or scuppers should be considered to reduce the risk of structural overloading and leaks should the main roof drains become clogged.

Figure 2: Typical internal drain on a flat roof

Figure 3: Typical scupper drain on a flat roof

Fire Ratings & Safety

The roof assembly is a critical barrier in preventing the spread a fire from beyond its zone of origin. The OBC (Section requires every roof covering to have a rating of Class A, B or C as determined by CAN/ULC-S107. These ratings indicate how well a roof assembly performed in a lab when exposed to various fire tests, with Class A being the highest level of fire protection. Since the OBC does not provide guidelines for selecting the classification of the roof, the decision is the responsibility of the designer.

Roof manufacturers will have a classification for each of their roof assemblies. When selecting a roof, it is important to note the classification applies to the entire assembly, not on individual components. This means changes to an individual component within an assembly will change its original classification.

In terms of fire safety, a roof can also be exposed to a potential fire risk during its construction. Modified bitumen membranes are sometimes directly adhered to the substrate and to each other using a torch-applied process. Although torching is allowed on roofs in Ontario, precautions need to be taken to mitigate the risk of a fire. Torching should not be performed on combustible substrates, such as wood decks or wood-fibre roof insulation. Some building jurisdictions or Owners require a fire watch to be in place during or following constriction. The Canadian Roofing Contractors Association (CRCA) details safety tips and best practices for using a torch on a roof.

Figure 4: Torching in progress on a flat roof

Guard Requirements

Falls from heights is one of the leading causes of workplace injuries in Ontario. Owners may be held liable for not providing adequate protection on a roof. For example, you will sometimes see mechanical equipment situated near the edge of a roof. If maintenance of these units presents a risk of falling, a guard rail system is required per the Occupational Health and Safety Act. Another guard consideration is around openings in a roof, including at roof hatches or skylights. The guards should be constructed to meet the requirements specified in the OBC.

More Than Just a Roof

Roof replacement represents a significant investment in a building’s envelope system and not all roofing systems are created equally. To ensure maximum value, be sure to retain an experienced professional to design a new roof system that will not only prevent leaks but will also meet all applicable codes and regulations.