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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).
