Parkade Flooding After a Storm - Roof Drainage Capacity and Backwater Protection
A 1,200 m² low-slope roof drains through two 100 mm primary roof drains with dome strainers into a 150 mm storm building drain that also carries foundation drainage and connects to a combined municipal sewer. A 150 × 100 mm secondary scupper serves as overflow. During a 25-minute storm recorded at 100 mm/h, water ponded about 100 mm deep, entered the stairwell and flooded the lower parkade to 150 mm. Investigation found the strainers about 70 % blocked with leaves, a bird's nest in the scupper, roof slope at one drain flattened to less than 1 %, and the backwater valve on the building drain stuck open. The building is 40 years old, the storm system has never been rodded, and the insurer is asking for a written assessment.
1. Calculate the storm load the roof had to handle and decide whether the drainage system was adequate. Show the arithmetic.
Read the worked answer
Runoff: Q = area × intensity = 1,200 m² × 0.100 m/h = 120 m³/h = 2,000 L/min = 33 L/s.
Per drain: with both primaries flowing, about 1,000 L/min each. That must be compared with what a 100 mm roof drain can actually pass at the available head. Treat the dome strainer roughly as an orifice: at 100 mm of head the ideal velocity is about 1.4 m/s, and with a discharge coefficient of about 0.6 a 100 mm opening passes only on the order of 400 L/min. Even allowing for a generous rating, two 100 mm drains are far short of 2,000 L/min. The roof was never capable of this storm with both drains clear.
Conclusion: the system is inadequate on three counts.
- Capacity - primary drainage is grossly undersized for the roof area at design intensity; more or larger drains are required, sized from the code rainfall intensity for the site and the actual roof area.
- Redundancy - the secondary scupper is 150 × 100 mm, cannot carry the full overflow, and was blocked; a single blocked primary instantly overloaded the other.
- Condition - 70 % blocked strainers, flattened slope and never-rodded leaders mean the as-built capacity could not be delivered even in a moderate storm.
Note the storm intensity used should be the site's design value for the applicable return period - confirm it against the local code rather than assuming 100 mm/h is the design figure.
2. Explain how primary and secondary roof drainage should be arranged and list the corrective work you would specify.
Read the worked answer
Principle: the secondary (overflow) system must be independent of the primary, must sit at a level above the primary so it only operates when the primaries are blocked or overwhelmed, and its discharge must be to a location where overflow is visible - not to the same below-grade leader where nobody sees it until the parkade floods.
Specify:
- Recalculate primary and secondary drainage for the roof area and the site design intensity. Add drains or increase sizes so the primaries alone carry the design flow.
- Provide a genuine secondary system - scuppers or overflow drains sized for the design flow, set above the primary strainer level, discharging to a visible exterior location (or to a separate leader with an alarm), and never interconnected with the primary below grade.
- Correct the roof - restore slope to the drains so no area ponds, and add a tapered cricket where the slope has flattened below about 1 %.
- Strainers - replace with correctly sized dome strainers and add access so they can be cleaned; blocked strainers were a direct cause.
- Building drain - verify capacity for the combined roof, foundation and any other inflow; separate the foundation drainage from the sanitary/storm building drain where the code requires it.
- Flood mitigation - raise or curb the stairwell entry, add a trench drain and an alarm at the low point, and protect the parkade openings.
- Maintenance - a written twice-yearly roof and leader cleaning program, with records.
3. What is the significance of the stuck-open backwater valve on a combined sewer, and what would you recommend for both immediate and long-term protection?
Read the worked answer
The backwater valve was the last line of defence and it was not working. On a combined sewer, storm flows and sanitary flows share one pipe. When the municipal main surcharges - which is exactly what happens during an intense storm - the hydraulic grade line rises above the basement floor, and without a functioning backwater valve, sewage and storm water enter the building through the lowest openings. A valve stuck open is equivalent to no valve at all, and in many codes a backwater valve is mandatory on a combined or storm building drain serving a building with fixtures or drainage below the level of the street.
Immediate: clean and repair or replace the valve with a listed, properly oriented backwater valve of adequate size, with an accessible cover and a cleanout. Rod the building drain and storm leaders, and document the condition found.
Long-term:
- Add an annual inspection and exercise of the valve to the maintenance program - they are found stuck open precisely because nobody looks at them.
- Consider a normally open, motorized valve with a level sensor on the lower fixtures, or a sump-and-pump arrangement for the parkade drainage.
- Keep parkade floor drains and any area drains on a separate system discharging through the backwater valve, not upstream of it.
- Advise the owner that the combined sewer belongs to the municipality; report the surcharge history so the utility can address system capacity, and keep the assessment letter on file for the insurer.