Avoiding pitfalls and navigating retrofits in roof drainage design

by arslan_ahmed | December 8, 2023 11:00 am

The specified slider does not exist.

By Allen Lyte, B.Tech., C.E.T., RRO and Barb Kemp, B.Tech., C.E.T., RRO

Appropriate drainage is critical to ensure durable performance of any roof, but this is especially critical for large industrial and commercial roofs (i.e. low-sloped roofs). During a roof repair or replacement project, it is important to consider having options to remove water from the roof assembly to ensure the roof slopes and drain locations achieve positive drainage. Numerous complications can occur if water is able to stand or pond on the roof’s surface.

Standing water on the roof

If the roof membrane has started to deteriorate, standing water could enter the assembly risking leakage. This leakage can cause concealed moisture damage that can go unnoticed for years before being readily noticeable. In the worst-case scenario, leaks could be catastrophic by allowing large amounts of water to enter the building by rising above roof curbs or breaking the roof membrane from the increased hydrostatic load.

Roof structures are typically designed to hold a set amount of water, but given the weight of water, it is best to minimize standing water and avoid any excess. Just 1 L (0.2 gal) of water is equal to 1 kg (2.2 lb). Where there is standing water, there is an increase in the roof live load. If 1 mm (0.3 in.) of water does not drain from the roof surface, an extra kilogram per square metre (kg/m²) is added to the load.

• An overloaded roof deck can cause deflection or even collapse, most often at the middle section of the roof area.
• In cold climates, standing water can freeze and cause hazardous surfaces when accessing the roof for maintenance and repairs to rooftop equipment. Thick layers of ice can cause disconcerting ice quakes, which are load booming sounds that happen with sudden temperature changes as the ice expands and contracts, for the tenants below.
• Standing water can also attract birds, and where there are birds, there is feces and debris, which can result in blocked drains leading to additional water build-up.
• Illnesses such as Legionnaires’ disease (caused by inhaling bacteria-infected water), West Nile virus (WNV) (contracted by being bitten by an infected mosquito) and inhaling mould spores causing allergic reactions, can all arise from standing water.

Buildings such sensitive occupants such as hospitals or long-term care homes can be at high risk from these situations. Where building supply air intakes have compromised filters, the problem can be quickly spread through the structure.

How to improve roof drainage

There are proactive design options to assist in promoting drainage and avoiding water problems on a low-sloped roof. Designing the drainage system to meet code requirements is always the first step. The National Plumbing Code (NPC) of Canada contains roof drainage capacity calculation procedures, including hydraulic load tables to identify the drain quantity, size, and minimum slope required for low-sloped roofs (Table 1 and 2, page 16). Roof drainage requirements, in respect to drain quantity, location, and the need for control flows or scuppers, are summarized in the NPC under “Hydraulic Loads from Roofs or Paved Surfaces.”

Design load checks are calculations to confirm both how much water the roof deck can hold, as well as the capacity of the hydraulic load on each roof drain. This is essential to prevent a roof collapse by allowing a specified drain down time, which is the amount of time it takes for the water to drain off the roof. The structural loads need to be calculated by a structural engineer during the design stage of new construction, and if not documented on as-built drawings, can be recalculated from review of the existing structure during a retrofit project.The roof decks or supporting elements can be structurally designed to have a slope prior to the roof membrane construction. This can also be done with sloped concrete or by installing a concrete topping that provides a positive gradient toward drains, to avoid any ponding water. Alternatively, a roof assembly that includes sloped or tapered insulation, even localized tapered insulation, provides slopes to drains and assists with draining water from the roof. Perimeter back-sloped insulation (tapered insulation along the roof perimeter that directs water into the field of the roof), drain sumps (tapered insulation reducing the insulation thickness around drains), and crickets are used for local tapered insulation locations and can result in a 20 to 30 per cent savings in tapered insulation over full tapered systems (sloped insulation installed over the entire field of the roof).

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To understand how roof drains work, one should not only consider pipe size and plumbing piping slopes, but also understand drainage anatomy. Flow control devices slow the rainwater runoff into the storm system, leaving a higher water load on the roof for a longer period. Ballast guards are used to keep stone/debris from blocking drains and entering the plumbing piping.

In a retrofit project, depending on local bylaws and codes, rubber compression seals or watertight mechanical compression seal (Figure 1) might be installed in retrofit drains, but these can decrease the size of the drain and reduce the effective drain capacity.

Redundant drains are another option to protect a roof from ponding water. If the main drains get blocked, or heavy rainfall causes water to back up on the roof, overflow scuppers can be installed to relieve the roof from water after it reaches a certain level/depth. Overflow drains can also be placed beside the primary drains, or evenly spaced along the roof perimeter for added protection.

Common pitfalls for roof drainage

Poor location of drains

When drains are located at the high point of the roof deck, the result will be standing water. Depending on the depth of the water and if the weather is favourable, evaporation can occur quickly or water may remain there for a long time. Water is typically considered ponding if it remains standing on the roof for more than 48 hours after weather is conducive to drying. In a conventional roof assembly, where the membrane is exposed, the membrane manufacturer may void a new roof warranty.

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Rooftop equipment obstacles

Rooftop equipment can block drainage paths by acting as a barrier for water flow. Concrete pavers used for walkways or as setting surfaces for equipment can also block drainage paths. The result of these obstacles is water that cannot drain efficiently and locally pool at critical locations.

Dirty snowballs

When dirt and debris build around drains or gain access to the drainpipes, blockage can allow water to back-up on the roof. The presence of water can encourage unwanted wildlife and vegetation contributing to the debris, creating a snowball effect.

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Mechanical parts of the drain assembly

High drain perimeters guards, mesh covers, flow control devices, and retrofit drain inserts are examples of mechanical parts that can obstruct drainage. When used as intended, they will enhance the roof’s drainage. However, flow control with no overflow drainage or drain screens/membrane clamping rings not having clear drainage can hold back water to the point that things turn ugly. In worst case scenarios, excessive water impacts the roof’s performance and safety.

Retrofit options

There are retrofit options available to improve the roofs drainage after the roof has been installed.

 Maintenance

Ongoing maintenance is required to keep the drains clear of debris. Any building operator on a regular basis can check the roof drainage by removing leaves, dirt, and garbage from around the roof drain surface. Ballast guards and drain screens prevent debris from entering the drainpipes themselves, but debris can build up around the guards, and if not cleared regularly, can cause blockage. This can be outsourced to a contractor, and if modifications need to be completed which the contractor can implement.

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Prevent or remove mechanical obstructions

There are drain fittings that can be installed on the inside of the drainpipes, which are an option when installing new insert drains where interior plumbing is not accessible. However, these reduce the flow rate from the original drain due to the reduction in plumbing pipe diameter and may result in the same roof being under designed for the hydraulic load required by the code. These devices can be avoided by accessing the drains from the underside and installing fittings on the exterior side of the pipes.

While new buildings require flow controls, these devices should only be installed if the roof was designed for them, including overflow drains. On the same note, they should not be removed if they are already incorporated into the roof system, as the increased flow rate can overwhelm the municipal sewers.

Drain relocation

As mentioned, existing drains set at high points of the structural deck can cause standing water. Relocating the drains to the low points of the roof deck will promote drainage. This would require plumbing repairs and additional or repositioning of the drainpipes, and localized repairs to the entire roof assembly.

Adding drains or scuppers

Additional drains can be added to the roof area and, if feasible, new drainpipes can connect into the existing pipes. Again, plumbing repairs by a professional would be required. Note that installing extra drains beyond the original hydraulic load capacity should not be completed unless all the plumbing lines to the sewers are checked to be able to take the increased load. If not, then the interior plumbing pipes can burst and/or backup into the building.

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Scupper drains can be installed along the exterior perimeter of the roof and used as overflow scuppers for emergencies, or at roof level to act as a consistent drainage outlet. Downspout connections from scupper drains can be either connected directly to storm drains or over splash pads, depending on local by-laws and the existing site drainage.

   Add sloped infill

A slope can be integrated during construction of the roof or as surface retrofits. Lightweight concrete with built-in sloped insulation can be installed during a roof retrofit before the new roof membrane is installed. This can remain intact in a single building over many roof membrane replacement projects as the concrete will protect the insulation from deterioration.

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As previously mentioned, common practice is to use tapered materials integrated into the roof during construction.

In situations where the rooftop equipment blocks drainage paths, crickets created by sloped insulation can help direct water around the equipment to allow for proper drainage. For blockages caused by concrete pavers with insulation bases that are used for walkways or as setting surfaces, drainage slots can be cut into insulation, or using smaller pieces of insulation,  to let water flow below the pavers and toward the drains. There is also the option of having walkways with pedestals to allow water to flow beneath.

Other types of surface retrofits use material infills with cementitious materials, rubber polymer, or additional asphalt flood coats with gravel to build up the surfaces of bituminous conventional roof assemblies at problematic low points of the roof.  Where pooling water is desired to be reduced prior to the next roofing replacement cycle, these materials can be placed at the low spots to displace the water and promote better drainage. The long-term performance of these products will vary on the materials used and the harshness of the climate.

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Mechanical pump drains

Another option to remove water from roof areas is by using a pump. A power-operated or solar-powered pump can be used to siphon water off a roof and direct it into the drains or over the edge. While power-operated pumps are typically only used to remove water from a roof in emergencies, passive pumps, including solar-powered, can be permanently placed at poor draining roof locations.

Conclusion

Standing water can be detrimental to the health and longevity of a building’s roof and its occupants. Water remaining on a roof 48 hours after rainfall can cause many complications, from increased maintenance, reduced roof service life, and an increased risk of leaks. Following the governing plumbing code and using smart and practical drainage design for a new roof are all methods to having quick a drying roof. A roof that is already constructed and struggling with standing water still has a number of retrofit options to help with problem drainage as well.

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For best results, building owners should obtain the advice of experienced professionals who will assist with determining the best solution. Do not get sucked into reusing a poorly functioning drainage system or being sold ideas that will not hold water—or rather—end up holding too much water. A structural engineer should be engaged when the roof design load is unknown, and an experienced roofing professional can help navigate serviceability and durability concerns to ensure the most appropriate drainage design for a building and the owner’s budget.

Authors

[9]Allen Lyte, B.Tech., C.E.T., RRO, is a principal partner in the building science consulting firm, W. Allen Partners, and specializes in roofing materials, and their application, with more than 27 years of consulting experience and more than 10 years of hands-on construction experience. Lyte is a two-term past president of the Ontario chapter of the International Institute of Building Enclosure Consultants (IIBEC). He can be reached at alyte@wapeng.ca.

[10]Barb Kemp, B.Tech., C.E.T., RRO, is a principal in the building science consulting firm W. Allen Partners, and has experience working from the east to west coast of Canada. With more than 24 years of experience in the industry, Kemp has worked in various construction sectors ranging from residential, commercial/industrial, to institutional. She can be reached at bkemp@wapeng.ca.

Source URL: https://www.constructioncanada.net/avoiding-pitfalls-and-navigating-retrofits-in-roof-drainage-design/

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