Primary and Secondary Roof Drainage on a Parapet Warehouse
A new 18,000 sq ft warehouse has a flat roof surrounded on all four sides by a 42-inch parapet. The authority having jurisdiction recognizes a design rainfall rate of 3 in./hr for the site, in northern Indiana, where about 30 in. of snow accumulates on roofs each winter. The contractor has proposed one 5-inch roof drain with a 5-inch leader discharging to the site storm lateral, plus a single 4-inch scupper through the parapet that spills onto the concrete apron at the loading dock where trucks back in daily. The owner says the roof drains at their previous building clogged with leaves every fall, and asks whether one drain is 'enough, since the whole roof is pitched to it.'
1. Is one roof drain acceptable for this roof? Explain the code basis for secondary drainage and what the secondary system may and may not do.
Read the worked answer
One drain is not acceptable. The roof is enclosed by parapet walls on all sides, so if the primary drain blocks, water has nowhere to go until it reaches a structural limit - the roof becomes a pond with a parapet as the dam. That is exactly the condition the code addresses by requiring a secondary drainage system.
Acceptable secondary means: additional roof drains, or scuppers/openings through the parapet, sized for the design flow, and arranged so overflow is visible from outside or from a normally occupied area, alerting someone before the roof loads up.
Not acceptable:
- A secondary system that ties into the primary leader or shares the primary's strainer and clog point.
- A secondary that discharges where it creates a hazard or nuisance - the loading dock apron, where trucks and pedestrians are, is the wrong place.
- Relying on the parapet itself as the overflow.
For an 18,000 sq ft roof in a snow region, the secondary must also be located so ice and drifted snow at the low point do not disable both systems at once; typically the secondary inlets sit slightly higher than the primary strainers and away from the primary.
2. Compute the design flow and describe how you would verify that the proposed drain and leader are adequate - and what you would tell the owner about redundancy.
Read the worked answer
Design flow: use Q (gpm) = roof area (sq ft) × rainfall rate (in./hr) ÷ 96.
Q = 18,000 × 3 ÷ 96 = 562.5 gpm for the primary system.
Verify capacity. Look up the code table of roof-drain and vertical-leader capacities at 3 in./hr for a 5-inch drain and leader and confirm the listed capacity exceeds 562.5 gpm with margin for the strainer and for the leader's developed length and fittings. Also confirm the horizontal storm lateral and the site storm main can carry the combined flow from this roof plus everything else tributary to it.
Redundancy. Even where a single 5-inch drain has the table capacity, capacity is not reliability. One strainer is one point of failure on a 540 sq ft-per-drain imbalance, in a building where the owner already reports annual leaf clogging. The practical recommendation:
- Two or more primary drains, divided so no area is more than the code's tributary limit per drain, plus
- a separate secondary system (drains or scuppers) sized independently for the design flow, discharging to a visible location.
Add sump-type roof drains with strainers, cleanouts on the leaders, and a maintenance schedule; tell the owner in writing that capacity compliance and blockage tolerance are different things.
3. The proposed scupper dumps onto the loading dock apron. Address that, and address winter performance for both systems.
Read the worked answer
Discharge location. Storm water from roof drainage must discharge to an approved point of disposal - a storm sewer, a splash block/leader draining to grade that slopes away, or an approved dry well - and must not create a nuisance, erode the surface, or create a hazard where people and vehicles move. Dumping the scupper onto the apron at a truck dock fails on all three: sheet flow across a working surface, ice in winter, and no positive disposal. Correct it by piping the secondary to a storm leader or to a graded area away from the dock, and keep the overflow point visible.
Winter performance in northern Indiana:
- Roof drainage must be designed for snow and ice, not just rain. Locate secondary inlets and scuppers so drifting snow and ice dams at the parapet do not block them; a scupper set at the parapet base is the first thing to freeze.
- Keep the secondary inlet elevation above the primary strainer so routine flows go through the primary and the secondary stays dry and unfrozen.
- Confirm the leaders are protected from freezing and that the storm lateral has adequate cover or insulation; a shallow leader under a truck apron is both a freeze and a load problem.
- Confirm roof structure and ponding: with the primary blocked, the secondary must start relieving before the ponded depth approaches the structural allowance.
Document all of this on the drawings, because the inspector will ask where the overflow goes and how it stays open in February.