ageDo you know the benefits of interior versus external roof drainage systems? Inadequate drainage can wreak havoc on your roof, making it vulnerable to leaks, weather conditions, mould, fungus, wear and tear, and in extreme cases structural issues:
- A roof drainage system must remove water within 48 hours after rainfall, according to the Canadian Roofing Contractors Association (CRCA).
- After the 48 hours, any ponding water that remains is an indication of drainage failure and can cause membrane and structural issues, leading to costly repairs.
- Minor leaks can slash a roof’s lifespan by nearly 40%, and can account for approximately 90% of roof failures.
Learn what you need to know to help protect your commercial, industrial or multi-residential property. Â
Commonalities, advantages, and disadvantages of roof drainage systems
The primary purpose of flat roof drainage systems is to manage water flow and prevent water from pooling on the roof’s surface, which is the source of many roof leaks and other issues. The appropriate drainage system can also help address issues that can cause leaks, including accumulation of debris, damage to caulking sealants, premature breakdown of the roof membrane, extra stress from ice in the winter and leaks near roof projections such as chimneys, HVAC equipment or skylights.
The basic concept of interior (also called internal) and external drainage systems is simple enough. Interior drainage systems have openings in the roof structure or the parapet wall which lead to an interior piping [rainwater leader] system. External drainage systems rely on scuppers, gutters and downpipes attached to the outside of the roof. On some buildings we have observed internal drains that do not drain into the underground storm system for the building but are directed back outside through openings in an exterior wall at or near grade level.
Read part two in our Roof Drainage Series
How interior and exterior roof drainage systems work
The first step in deciding which type of drainage system to use is understanding each type. The common denominator is the system’s ability to manage water flow and direct it away from the building. Both drainage systems do this in different ways.
Interior roof drainage systems
Interior drainage systems start with a drain, known as a collection basin, placed in areas of the roof that collect the most water. Strainers or gates are placed over the drain to keep debris out. The water then flows down through a system of connected pipes installed beneath the roof. The rainwater leaders typically channel the water into an underground storm sewer system for the building.
Exterior roof drainage systems
Exterior systems use either scuppers or eavestroughs installed along the edges of the roof to drain water to grade through connected downpipes. Sometimes chutes or overflow scuppers are used in which case there are no downpipes installed.
Pros and cons of each type of drainage system
Internal and external drainage systems have distinct advantages and disadvantages, discussed in the sections below.
Internal roof drainage advantages
One of the main advantages of an interior drainage system is its natural protection against Toronto’s harsh weather and other external conditions. Unlike external drainage, internal systems won’t fail from external sources. Interior drainage systems also have a reduced risk of damaging the foundation, disrupting the landscaping and soil erosion. From a design perspective, interior drainage systems offer greater flexibility and customization, more efficient use of outdoor space and improved aesthetics.
Internal roof drainage disadvantages
Along with the advantages, there are downsides to interior drainage systems. High on the list of disadvantages is higher upfront costs. System design is more intricate, installation is more complex, and installation may cause disruptions especially over finished interior spaces. Maintenance also presents challenges. Problems like mould and mildew can be harder to detect and often more complicated (and expensive) to repair. If leaks or other issues remain hidden or unattended to, they can escalate and also damage the interior.
On the rooftop, strainers can get clogged and cause leaks if not checked and cleaned regularly. We often experience problems in warehouses where forklifts impact rainwater leaders causing extensive leaks. We also find occasional issues with backfall on rainwater leader piping that can result in leaks from drain couplings over time.
How internal roof drainage systems work
- Buildings that have underground storm sewers
- Factories or warehouses that deal with processes generating moisture or water runoff
- Multi-unit properties with shared basement space or storage areas
- Properties in flood-prone areas or those with higher water tables or uneven terrain
- Older buildings with outdated drainage systems
- Structures with limited space for external drainage
- Buildings with lower-level functionality, such as garages or storage areas
The Advantages and Disadvantages of External Roof Drainage Systems
Now that we understand some of the advantages and disadvantages of internal drainage systems, we’ll switch focus to external systems.
 External roof drainage advantages
Compared to internal systems, easier design and installation cuts down on design and installation costs. Exterior drainage reduces the risk of water intrusion into the interior (although scuppers can occasionally leak). In terms of design, while internal systems are highly flexible, external systems can be adapted to property layouts, landscaping, and runoff management.
External roof drainage disadvantages
Many consider gutters unsightly, but potential damage is the bigger issue. While internal systems have maintenance challenges, external systems can also rack up repair costs. Gutters can easily be damaged by snow, sleet, wind, fallen limbs or wind-blown debris. Ice formation can mean frozen or cracked gutters, ice dams and other leakage issues. If not checked regularly, clogged gutters can overflow, damage the foundation, leak into insulation or the interior and create mould or mildew problems. With less control over the water flow, exterior systems also have a greater risk of foundation issues, soil erosion, damage to landscaping and poor performance with uneven terrain and high water tables.Â
Common usages of external roof drainage systems
- Buildings that do not have underground storm sewers
- Buildings in flood-prone areas
- Commercial properties with large parking lots or outdoor spaces
- New multi-unit residential developments
- Industrial facilities with large, paved areas
Factors to consider for exterior versus interior drainage systems
Exterior and internal systems each have their pros and cons. Your professional mechanical engineer, roofer and/or plumbing contractor will help make an informed decision based on a combination of factors, including the property’s layout and building structure, soil and site conditions, local climate and weather patterns and water table level or flood risk. As always, installation and maintenance costs will be part of the equation.
Consult with your professional roofer about your roof drainage system
This article has provided just some of the factors relevant to external versus internal drainage systems. A qualified professional roofer has the knowledge to help you make the best decision based on your specific needs. Elite Roofers, proudly serving Toronto since 1956, has the in-depth experience to guide you on which drainage system is best for your property.You can trust Toronto’s Elite Roofing for all your roofing needs, with superior workmanship backed by our 24/7 emergency service. Contact us today and get the Elite Advantage.
