Creating A Strong Foundation: Why We Use Tapered Insulation

Why We Use Tapered Insulation

Part One: Building Code requirements 

We’ve written at length about the damage that ponding water can cause to your roof. Ponding water, when left untreated, makes its way through the roofing membrane, causing leaks that can lead to severe damage over time.

According to Garland Canada:

“40% of all building-related problems are due to water intrusion. Even worse is that water intrusion accounts for more than 70% of construction litigation.”

Tapered roof insulation, where insulation is cut and fit in a way that moves ponding water towards existing drainage points, is a great solution to help maintain the overall health of any low-sloped roofing system.

There are various building codes that have addressed the issues of positive drainage on low-sloped roofing systems.  To date, the Ontario Building Code has not formally adopted this requirement, however, the Canadian Roofing Contractors Association does endorse this recommendation..

Building Code Jurisdictions: 

Minimum and maximum roof slope conditions may be limited based on specific roofing systems and the type of project that is being undertaken by your contractor, however, these standard guidelines help ensure that the roofing slope will work effectively for your structure and achieve the desired positive drainage.

International Building Code: According to the International Building Code 2018 edition, the International Building Code,® 2018 Edition (IBC® 2018) prescribes a 1/4:12 minimum roof slope [2%] for asphalt built-up, polymer-modified bitumen, thermoset and thermoplastic single-ply, and liquid-applied membranes in new construction. This 1/4:12 minimum roof slope requirement also applies to spray polyurethane foam (SPF) roof systems in new construction. However, if the roof in question is a replacement or re-cover of an existing structure, the 1/4:12 minimum slope requirement is waived in favor of a performance-based measurement that allows for positive drainage within 48 hours of precipitation.

Canadian Building Codes: 
Canadian roofing codes echo most of the same stipulations as international codes with a key exception. The ¼:12 specifications are not required by building codes in Ontario, which could be one reason why only 10-15% of projects (rough estimate) have fully tapered roof insulation installed.

The object of roof drainage by sloping roof surfaces is to keep vulnerable areas at high points and encourage water to drain away from them. In this way the chances of penetration through any imperfections are reduced. Any slope is
usually better than none, but a practical minimum accepted by most for the main roof areas is about ¼ inch per foot. 2% slope is suggested as the minimum required to compensate for deflections of the deck and structure and for inaccuracies in building elevations. [Canadian Roofing Contractors Association, Technical Bulletins #18 and #29]

According to a March 2015 article in
“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. A qualified roofer will calculate correctly the amount of drainage required for the given roof area.”

Specifications for ponding water duration after a rainfall vary, as evidenced by the chart below, but as a rule of thumb, if after 48 hours your roof is still very wet, it may be time to consider installing protective measures to make sure you don’t run into problems down the road.


The Asphalt Roofing Manufacturers Association bulletin “Effects of Ponding Water on Low Slope Roof Systems” mentions that if ponding water does occur, efforts should be taken to eliminate or reduce the accumulation and persistence of water on the roof surface. Failing to address ponding water may shorten the effective life of the roof membrane system.

No matter what your roofing needs are, our experts are up to date on the latest technology and regulatory standards in order to help serve you better. If you’re looking for more information regarding tapered roofing insulation, contact us here, or chat with us using the chat box on the right during working hours. Don’t forget to check back soon for part two of this series, where we tackle the downsides of tapered design.

Thank you to Andrew Zammit at Posi-Slope and Marc Allaire of North 49 Sales Agency for collaborating on this article with us.