Positive Drainage: The Secret To Roof Longevity

Positive Drainage: The Secret To Roof Longevity

When it comes to commercial roofing systems, one of the most important elements your roofing contractor should put forth is positive drainage. Commercial buildings most often have low-sloped or nearly flat roofs, which makes them more vulnerable to standing water issues. When there is rain or snow pile on a structure with a sloped roof, there is a clear direction for the rainwater to go, and it runs off of the roof into the drain outlets and off the roof structure. Most commercial buildings built before the mid 1980’s do not have positive drainage, which is why measures need to be taken to ensure that standing rainwater does not damage the integrity of the roof.

So What Exactly Is Positive Drainage?

The OIRCA and the CRCA (Canadian Roofing Contractors Association) define positive drainage as “the drainage condition in which consideration has been made during the design for all loading deflections of the deck and additional roof slope has been provided to ensure drainage of the roof area within 48-72 hours of rain during conditions conducive to drying.”

What this means is, positive drainage is the construction of systems that slope in a way that allows for water to be directed away from the structure. This does not mean that all water that remains after rain will be drained, but that “most” of the water that remains will disappear within 48-72 hours with normal exposure to sunlight. Technically speaking, this means that an ideal roof should allow for a slope of 2% or 0.25 inch per 1.00 foot [0.635 cm per 30.48 cm].

What Happens If My Building Doesn’t Have Positive Drainage?

Insufficient roof drainage can lead to a multitude of problems with your structure. Water that stands on a roof might cause structural issues over time, and increases the likelihood that your building will need more frequent and larger repairs. Ponding water accelerates the deterioration of most roofing materials, and can leave the roof membrane brittle and subject to fracturing. The fracturing of the membrane can lead to leaks at the ponding location, which has the potential to damage the components of the roof assembly (including insulation layers) and the interior of your building. Ponding water may also even void your roof warranty, costing you even more money.

How Do I Know If Drainage Is An Issue?

The easiest way to see if ponding water is an issue is to examine your roof 2-3 days after a significant rainfall, at a time when post-rainfall conditions are conducive to drying. For example, if there was significant rain on Monday, and sunny Tuesday-Thursday, your roof should be dry if there are no ponding issues. In winter months, due to the short number of daylight hours and colder temperatures it is not as easy for standing water to evaporate so it may take longer for the roof to dry relative to the other three seasons. The OIRCA refers to the pools of water after rainfalls as “birdbaths”, which are random and inconsequential amounts of water after a rainfall.

These collections are completely normal and are most pronounced on smooth-surface systems. If you have water accumulation like this, but none after a few days, it’s likely that your roof has positive drainage, and that the drainage is working effectively. If it’s been a few days and your roof still looks like the image below, you most likely have a ponding water issue.

If you do notice that there is ponding water, it doesn’t necessarily mean that your roof is not sloped to drain. A lot of the time, we notice that our clients have had equipment installed that impedes the drainage of rainwater. Roof tops that are cluttered with equipment sleepers, HVAC units, and skylights could impede the ability of the roof to drain effectively, causing damage to the roof with prolonged exposure. Previous roof repairs, especially near or around existing roof drains, can also affect the positive slope to drain of an in-place roof assembly.

What Solutions Are There For Existing Structures?

If you have an existing structure without positive drainage, there’s no need to panic! Positive drainage is possible with strategically placed drainage systems. At Elite Roofing, we specialize in assessing your drainage issues and adding effective drains where needed. We use this protocol as our first line of defence, because of how much more cost-effective it is than replacing the roof complete with the use of tapered insulation. It is important to note that even if you do have a roof drain, it may not be working 100% effectively, which could cause long term damage to your roof.
If you’re having ponding water issues and aren’t sure how to move forward, Elite Roofing offers inspection services to help you assess the best steps to maintain the health of your roofing system.

What Are The Benefits Of Installing A Positive Drainage System?

Positive drainage systems not only allow for a roof that is much lower maintenance, but a sloping structure will also give you peace of mind. With the weather in Canada being largely unpredictable, having a drainage system that can handle water accumulation effectively is an important asset for your building.

Many building owners who are thinking about a new roofing system don’t consider positive drainage to be a necessary component of their structure and become frustrated with the inevitable damage that occurs when water ponds. Sloping roofs are especially important in Canada, where snow and ice that has accumulated over the winter melts, and spring-summer rains can be unpredictable and quite severe. If your roof is in need of repair, or you have questions about what can be done to maintain the health of your building; chat with us! We’re available Monday-Friday, 9AM-5PM through the chat box on the right.