House Design, Passive House resources

Why a low slope roof makes sense

Southern elevation of Ottawa passive house by EkoBuilt

EkoModel Home – southern elevation

The EkoModel Home was designed with a low slope roof, a smart feature shared by all 13 of EkoBuilt’s passive house plans.

This style of roof greatly reduces construction costs, and also introduces great strength and stability, which is a tremendous benefit in our part of Canada where snow loads in the winter can be incredibly deep and heavy.

While a standard cathedral ceiling roof rafter is required by current code to be 12 inches deep to carry snow loads, our passive house rafters are 30 inches deep. The depth originally designed for increased insulation (typically three times that for a conventional build) lends amazing strength for those snow loads.

As Europeans – who have lived with high utility costs for many years – understand innately, a low slope roof offers tremendous cost savings in the long run. North Americans are only just starting to come around to the wisdom of this roof-style, as they realize how important reducing energy costs will be the further we move into the 21st century.

A number of key factors go into an effective low slope roof, including the following:

Roofing underlayment

As part of our standard PassiveHouse package, we include the installation of a high temperature ice/water membrane which completely waterproofs the roof and is guaranteed for 50 years. It is specifically designed for low slope roofs as low as 0.5 inch (2 1/2°), and is able to withstand temperatures of up to 121°C!

The membrane that we use is WinterGuard HT Advanced Waterproofing Underlayment by CertainTeed. For more detail:

See product details on CertainTeed’s website.
Review the WinterGuard HT brochure [pdf]

Roof cover

Another important factor is to reduce the heat island effect by making use of cool roof technology. The term “heat island”describes built areas that are hotter than nearby rural areas. The annual mean temperature of a city with 1 million people can be 1 – 3°C warmer than its surroundings. By the evening this difference can rise to around 12°C.

Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat related illness and mortality, as well as water quality.

High solar reflectivity [albedo] is the most important characteristic of a cool roof as it helps to reflect sunlight and heat away from a building. Cool roof technology can reduce a roof’s temperature by 28 to 33°C during summer weather.

More information

Information on Heat Islands (US Environmental Protection Agency)
Using Cool Roofs to Reduce Heat Islands (EPA)

Wakefield Bridge steel shingles

Click to enlarge image

We chose to use Ideal Steel’s Wakefield Bridge Steel Shingles because they have the most advanced cool metal roof technology we could find. The special resin paint system used results in a high albedo factor; there are a range of colours available.

Added benefits of Wakefield Bridge Steel Shingle roofing:

  • Manufactured from recycled and new steel
  • 50 year warranty
  • Cost-effective at approximately $2 per square foot

More information

Product brochure for Wakefield Bridge Steel Shingles [pdf]

Soffit and fascia

passive-house-roof-soffit

Click to enlarge image

Finally, additional roof finishes include aluminum soffit and fascia, chosen for being inexpensive, clean lines and excellent ventilation properties for a steel roof.

Together, all of these elements work together to make for a high performance, low cost roof that will help to keep more money in your pocket now, and in the future.

EkoBuilt is here to help those who are thinking a little ahead of the curve to maximize their home’s beauty while making the most of its energy efficiency and comfort. Get in touch to find out more.

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