Poor Drainage on a Commercial Flat Roof

Considerations when Replacing a Flat Roof

Pavel Gonchar Renoteck News, Roofing Resources

Many choices are available when replacing a flat roof. It is during the design period that costly mistakes can be avoided. Issues ranging from excessive maintenance costs to structural overload that causes damage to the building can be circumvented.

To avoid issues in the future 4 considerations must be dealt with.

The Structural Deck

The Structural Deck supports all the roofing components, roof top equipment, snow load and people.

Designed to carry a specific load, they are usually constructed of concrete or steel. Any changes in the load being carried must be considered.

Concrete roof decks are generally seen in high-rise buildings and can support a little more weight than specifically designed for, without creating structural strain.

The steel roofs normally found on industrial, retail and low rise residential buildings have very tight tolerances. As such, the initial design load specifications must be followed.

Roof renewal using overlays, or a ballasted system have a direct impact on load conditions.

Overlays are the installation of new membranes over the existing roof. This process may include new insulation and a coverboard. Proper installation can ensure effective performance, but greatly increase the load beyond original design specifications.

The application of a ballasted roof system involves adding weight (gravel or pavers for example) to the membrane and insulation. The result is a substantially heavier roof that most metal roof decks can not support. Simply put, a ballasted system should not be used to replace a non-ballasted system without a thorough structural analysis.

The Membrane

The Membrane keeps the water out. There is a wide range of membrane options that fall into one of three categories. They are built-up felt & asphalt systems, liquid applied coatings and single ply membranes.

Each system has its own list of pros and cons. A lengthy list of considerations will determine which one will provide the best performance and impact your final choice and the bottom line.

  • UV exposure
  • risk of damage and abrasion
  • thermal fluctuations
  • energy rating
  • servicing conditions
  • visibility to the public
  • access to the roofing area in question
  • regional availability
  • properly trained installers in the area


While you want a waterproof roof, it is critical that the water is taken away in a timely manner. Long standing water decreases the life of the membrane, causes bug infestations and can create safety hazards as in the case of freezing.

Effective drainage is the by product of a sufficient number of drains, and proper sloping to those drains. When a roofing system is changed an analysis of the drainage is critical. Switching from a conventional roofing system to a protected membrane system will provide the greatest challenge.

Poor Drainage on a Commercial Flat Roof

Poor Drainage on a Commercial Flat Roof

Wind Uplift Damage to a Flat Roof Apartment Building

Wind Uplift Damage to a Flat Roof Apartment Building

Wind Uplift

Wind Uplift results when the wind travels across a flat roof creating suction that can literally tear your roof off. Your membrane and flashing must be designed with this possibility in mind.

Many roofing installers do not fully grasp the implications of wind uplift. While manufacturers will provide generalized rules pertaining to this, factors such as local wind speed, building height, terrain exposure and our specialized Alberta building codes are not considered.

The wind pressures reach their highest levels along the perimeters and the corners of the roof. It is imperative that the edges are very secure. Flashing profile and the type and quantity of fasteners are critical in determining the direction you need to go.


So, remember, you need to work with a structural engineer or consultant familiar with the implications of re-roofing and the structural issues that will arise. Renoteck has extensive experience in commercial and residential roofing.  Projects in Calgary, Edmonton and points in between provides them with wide-ranging knowledge and problem solving for the unique Alberta climate and environment.

Whether or not a building permit is required you want to make sure that whoever designs the replacement is covered by professional liability insurance.

The replacement of a roof can protect and increase your investment value. Renoteck understands this and works to meet these goals.