Canadian Contractor

John Bleasby   

The right materials for flat and low-sloped roofs

Canadian Contractor Commercial Service

Some residential builders are leery of flatter roof profiles common in modern architecture. But flat or low-slope roofing options are well-established in commercial construction - and you can use them too.

Once a roof design reaches a certain low level of slope, the main issue moves from ‘water shedding’ to ‘water proofing.’ This means a shift in roofing techniques and materials. Brian Audia of A&G Roofing in Orillia, Ont. believes ‘water shedding’ materials like shingles and steel become susceptible to water backflow below a pitch of 4:12. At that point he will direct his customers to one of several roll-on ‘waterproofing’ products.

However, some residential builders remain leery of low-slope profiles common in modern architecture, often from a structural standpoint. Consequently, they might be unfamiliar with the advanced roofing choices now available in the commercial world, where flat roofs have been a staple design for decades.

Gone are the days of heavy hot-tar and gravel as the default flat roof treatment. Manufacturers now offer a choice of roll-on materials via accredited roofers, including single or 2-ply lightweight synthetics, rubber, and asphalt/granule products. These can be hot-air welded, glued, peel-and-stick, or torched together at seams as the case may be. Each delivers a watertight surface. “There is a lot of design finesse that goes into a flat and low sloped roof” explains Audia. Material decisions are based on the substructure underneath, anticipated foot traffic due to rooftop mechanical service requirements, detailing issues, and esthetic considerations. However, cost is actually not the biggest factor, continues Audia. “They all price out within 10% to 20% of each other.”

The life expectancy on a correctly installed, properly maintained flat roof is 20 to 25 years. Nevertheless, an engineered drainage plan is vital. A 2% slope is considered adequate in most instances, with roof drainage either via eaves, scuppers, or drainage outlets in the roof itself. The chemical makeup of standing water can degrade some roof materials or allow plant growth. All of this is bad news for longevity. In fact, many extended warranties are voided if standing water exists more than 48 hours after a rainfall. A qualified roofer will calculate correctly the amount of drainage required for the given roof area.


Audia advises residential builders not to be shy. “Like everything else, if you don’t know, ask!” A qualified, accredited roofer can bring flat roofs into your world of doable building projects.

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2 Comments » for The right materials for flat and low-sloped roofs
  1. Marlene says:

    where can I find a contractor or a supplier for this roofing?

  2. chris gabriel says:

    looking for new build of flat roof that doubles as deck above living area
    Size 12′ x 16′

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