Ponding water, defined as water that remains standing on a roof more than 48 hours after rainfall, is one of the biggest issues that come with having a flat roof. Flat roofs are advantageous for a number of reasons and are also often the only choice for commercial structures. These roofs offer exceptional protection from the elements if the membrane and insulation layers are installed correctly and efficiently, and this by definition means that rainwater is directed into appropriate drainage systems such as drains, scuppers, and/or gutters.
There are several reasons for ponding water that we have found over the years, including but not limited to:
1. Structural Issues With The Deck Substrate
a. Most buildings designed prior to the early 1980s did not have positively sloped deck substrates. The thinking was that having some water collected on the roof surface [referred to as rainwater buffering] was a desirable condition and would assist with heating and cooling of the building. Actually, quite the opposite is true. Ponding water has the potential to damage roof membranes, which could in turn cause leakage into the interior of the building.
b. On older buildings, often including those with wood deck substrate, there can be substrate deflection over time, which can cause issues with ponding water. We have seen this in many older buildings in the city centre areas.
c. Roof projections that were either not adequately reinforced or where loads were transferred in whole or in part onto the structure might also cause unintended deflection issues on the deck substrate and affect the positive flow of water to existing drains.
2. Improper Location
Improper location of roof drains (drains that are not located on a low part of the roof), can cause ponding water issues.
3. Construction Over Deck Substrate
Flat roofs constructed over wood deck substrate with block firewall separation between units can lead to drainage issues. The firewalls usually become high points due to deck substrate deflection. Drain locations inside demising walls between the units could lead to water ponding issues on the roof.
There could be undetected errors in concrete footings and/or steel fabrication that result in deck substrate that does not have a positive slope towards existing roof drains.
So How Is Ponding Water Fixed?
Andrew Zammit at Posi-Slope advised that tapered insulation is a great solution for the problems listed above and others. A 2% sloped insulation will usually suffice on most projects. There are some site-specific conditions or other considerations that could lead to a lesser slope being recommended. On an as-needed basis, the tapered insulation manufacturer can send a representative to the site to survey the existing roof condition.
The assessor will utilize a self-leveling laser to take elevation grades to aid in determining lower-lying roof areas, deck slopes, and pre-existing site conditions. Based on their findings, they will come up with recommendations to resolve ponding water issues.
What Are Some Other Tapered Insulation Design Considerations?
Tapered insulation isn’t a one-size-fits-all system. Depending on the needs of your roof, there may be certain considerations that would affect what method we use for specific tapered insulation use cases.
In some cases the deck substrate is uneven and there are undulations. In such cases, the tapered insulation manufacturer might recommend the use of insulation as a deck leveler prior to the installation of tapered insulation. Deck leveler is a tapered insulation solution designed to achieve a near flat substrate.
Height & Slope Specifications
As we wrote about in Part 1 of this series, a 2% sloped insulation solution is the preferred approach. This might not work in all situations. Here are a few reasons why not:
There might be height restrictions for the use of tapered insulation due to the presence of existing windows, doors, roof projections, and/or parapet walls. Sometimes the height of these items can be modified to accommodate thicker roof insulation and other times it is too difficult.
The roof structure might be sloped in a single direction with a 2% slope and simply require a back slope insulation on the low end to guide water to the roof drains. In order to achieve a backslope with tapered insulation, the insulation slope needs to be double the roof slope. For example, 4% back slope insulation is required to offset a 2% sloped roof condition.
There are customer considerations to contend with in the real world. Some customers do not want to pay for a completely dry roof and are prepared to accept that their roof will hold some water for an extended period of time albeit that this could lead to premature deterioration of the roof membrane. As in all cases, a creative solution requires consideration of all factors and compromise.
We have found that, in practice, tapered insulation is an underutilized solution and could be implemented as an effective drainage method in many instances where life cycle costing and long-term performance matter almost as much as the price of the roofing project. Sometimes there are “compromise” options that are less expensive than a 2% fully tapered roof. These should be explored with much more regularity.
No matter what your building specifications, there is always an ideal combination of materials that will protect your roof over time, and ensure that ponding water does not damage the integrity of your structure. If you’re looking for more information regarding tapered insulation, click here to contact us, or chat with us using the box on the right during working hours. Protecting your building from ponding water can be a daunting task, but that’s what the Elite team is here for. That’s the Elite Advantage.