Allan Avis Architects Inc | The Low-Down on Low-Slope Roofing

The Low-Down on Low-Slope Roofing

Published: May 18, 2018

Flat Roofing for Commercial Buildings


The roof of a building is arguably the most important system of all the building enclosure assemblies. The roofing system is the first line of defense in keeping out precipitation, and at the same time keeping conditioned air inside the building. The roofing system selected can have a significant impact on the energy efficiency of a building, as well as the cost of owning, operating and maintaining the building. At this time of year, with April showers (hopefully more showers than snow), the importance of your roofing system should be at the forefront.

Choosing the correct flat roofing material and system can become a difficult task, with dozens of options and combinations of materials. The roof is one of the most susceptible areas of a building envelope for water intrusion. Selecting the best roofing system for your building will reduce long-term operating and maintenance costs, as well as reduce the risk of damage to the interior of the building. Flat roofing generally consists of UV protection, a membrane, or layers of membrane, insulation (tapered and flat), air and vapour control layers and the structure. Flat roofing options include: ballasted, inverted, fully adhered, self-adhered, mechanically fastened or liquid applied.

Flat roofs, when done properly, are not perfectly flat, but are low-sloped, generally with a pitch of around 2% (1/4″:12″), although some roofs have a steeper pitch, depending on the design. Finding the best roofing system for your application can be challenging, so we always recommend contacting a professional for assistance before making such an important decision.

These are some considerations when selecting a flat roofing system:

Cost: How much capital funding is available for the roofing replacement versus the maintenance that will be required over the lifespan of the roof? Generally, single-ply membranes are less expensive than multi-ply roofing systems, both in material cost and in the labour costs to install. However, based on our experience, single-ply membranes require increased maintenance versus, a two-ply modified bitumen roofing system.

Schedule: When will the roof be installed? Many roofing systems have a temperature threshold that determines when installation can occur. Snow, wind and rain will affect the temperature at the roof surface and, therefore, effect when a roofing system can be installed.

Life Cycle: What is the required life of the roofing? Depending on the building, a 10 or 20-year life expectancy for the roofing may be enough, but typically Institutional and Commercial buildings are constructed to last much longer and, therefore, materials should be specified that have the lowest cost over the building’s life-cycle. Again, in general, two-ply modified bitumen roofing systems will provide a lower cost over the life of the building, as they will need to be replaced less frequently than other systems.

Maintenance: Will the roofing be regularly maintained? Is there a maintenance program in place? Is the roof in a location where it is accessible and apparent issues can be addressed as they come up, before they get worse?

Accessibility: Is the primary intention of the roofing system to keep out water, or is there equipment on the roof, such as an HVAC unit? Is there access to the roof, by means of stairs, elevator or roof hatch? Is there currently, or the potential for a future green space, patio or photovoltaic system?

All of these considerations, and many more, are reviewed and ultimately help guide roofing strategies and selections. There are many aspects in the design of flat roofing systems that can have a major impact on reducing long-term building costs, energy efficiency and performance of the roofing. That’s why it’s important to engage an Architect, or Building Science Engineer, who has the experience in flat roofing systems and can help guide you through the replacement process.

Remember, it’s technically low-sloped and not flat, but nevertheless is one of the most important aspects of your building. Give us a call to see how we can help design a “flat” roofing system that suits your requirements.

The opinions expressed herein are those of Allan Avis Architects Inc.. This Bulletin is circulated for the purpose of bringing architectural information to the attention of the reader. The data, commentary, opinions and conclusions, if any, are not intended to provide the reader with conclusive technical advice and the reader should not act only on the information contained in this Bulletin without seeking specific professional, engineering or architectural advice. Neither Allan Avis Architects Inc. nor any of its officers, directors, members or employees assume any responsibility for any of the information contained herein or the consequences of any interpretation, which the reader may take from such information.