8.4 Side Walls Draining Onto a Roof, Secondary Roof Drainage & Controlled-Flow Roofs
Key Takeaways
- UPC Section 1103.4 adds a percentage of adjacent vertical wall area to the projected roof area when walls drain onto the roof below.
- For one wall add 50 percent of the wall area; for two adjacent walls of equal height add 35 percent of the total wall areas.
- Two opposite walls of the same height add no area at all, because water shed toward the roof from one is offset by wind and by the geometry of the pair.
- Secondary or overflow roof drainage protects the structure when the primary drains block, and is provided by separate overflow drains, by scuppers through parapet walls, or by an open roof edge.
- Controlled-flow roof drainage deliberately ponds water on a structurally designed roof to reduce peak discharge, and requires secondary drainage at a higher elevation.
Side Walls Draining Onto a Roof, Secondary Roof Drainage & Controlled-Flow Roofs
1. Vertical walls that drain onto a roof (UPC Section 1103.4)
A blank wall rising above a lower roof is a rain-collecting surface. Wind drives rain against it, the water sheets down the face, and every drop lands on the roof below. Section 1103.4 handles this with a set of percentages rather than a formula — and it is one of the most quotable rule sets in Chapter 11, because the numbers are short and the geometry cases are distinct.
UPC Section 1103.4 — Where vertical walls project above a roof to permit storm water to drain into the roof area below, the adjacent roof area shall be permitted to be computed from Table 1103.1 as follows:
| Case | What to add to the roof area |
|---|---|
| 1. One wall | Add 50 percent of the wall area |
| 2. Two adjacent walls of equal height | Add 35 percent of the total wall areas |
| 3. Two adjacent walls of unequal height | Add 35 percent of the total common height, plus 50 percent of the remaining height of the highest wall |
| 4. Two opposite walls of the same height | Add no additional area |
| 5. Two opposite walls of differing heights | Add 50 percent of the wall area above the top of the lower wall |
| 6. Walls on three sides | Add 50 percent of the area of the inner wall below the top of the lowest wall, plus an allowance for wall area above the top of the lowest wall per cases 3 and 5 |
| 7. Walls on four sides | No allowance for wall areas below the top of the lowest wall; add for areas above the top of the lowest wall per cases 1, 3, 5, and 6 |
Why the percentages differ
Case 4 is the one that surprises people: two opposite walls of the same height add nothing. The logic is meteorological. Wind-driven rain strikes one face of a pair of opposing walls at a time; whichever wall is on the windward side is shedding onto the roof, and the leeward wall is in the rain shadow. The pair never both contribute at once, and the code accounts for that by treating the pair as neutral. The same reasoning explains case 7 — a well surrounded on four sides has no net directional gain below the lowest wall top.
Worked example
A lower roof measures 40 feet by 80 feet (3,200 square feet). Two adjacent walls of equal height rise above it: one is 40 feet long by 12 feet high (480 square feet) and the other is 80 feet long by 12 feet high (960 square feet).
- Total wall area = 480 + 960 = 1,440 square feet.
- Case 2 applies: add 35 percent → 1,440 × 0.35 = 504 square feet.
- Design area = 3,200 + 504 = 3,704 square feet, and that is the number you take to Table 1103.1.
Skip the wall allowance and you would have sized the leader for 3,200 square feet — potentially a size too small, and a roof that ponds in the storm the system was supposed to handle.
2. Secondary (overflow) roof drainage
Why it exists
A flat roof is a shallow tank. Its primary drains are the only opening in it, and their strainers sit in the same debris field as the roof itself. One windstorm's worth of leaves, a torn membrane patch, or an ice plug can seal a primary drain completely. When that happens the roof begins to fill, and water is heavy: one inch of standing water weighs about 5.2 pounds per square foot, so six inches of ponding over a 10,000 square foot roof adds roughly 156 tons. Roof collapses from blocked drains are not hypothetical; they are one of the classic structural failure modes.
Secondary drainage is the pressure-relief valve for that scenario.
The three acceptable arrangements
- Separate secondary roof drain system. Independent overflow drains, each with its own leader and its own point of discharge, set at a higher elevation than the primary drains so they only receive flow when the primary system is failing. The discharge is placed where it will be conspicuous — the whole point is that the building owner sees water pouring out somewhere it normally does not.
- Scuppers through parapet walls. Openings through the parapet, sized and located so that water reaching a set depth leaves the roof over the edge. Simple, no moving parts, nothing to clog beyond the opening itself.
- An open roof edge. Where a roof has no parapet and simply drains over its perimeter, the edge itself is the overflow path and no separate system is needed.
Sizing and elevation
Secondary drainage is sized for the same design rainfall rate as the primary system, using the same Chapter 11 tables — it must be able to carry the entire storm alone, because when it is doing its job the primary system is contributing nothing. Its inlet or invert is set above the primary drain level by the amount of ponding the structure is designed to tolerate, and the roof structure must be designed for that ponding depth. Secondary drainage may not discharge into the sanitary drainage system.
3. Controlled-flow roof drainage
A controlled-flow roof does deliberately what a blocked roof does accidentally: it holds water. Flow-restricting roof drains meter the discharge, so a roof designed for controlled flow ponds to a designed depth during a storm and releases slowly afterward. The benefit is downstream — peak discharge to the storm sewer is cut sharply, which lets a site develop where the public storm system has no spare capacity, and it reduces or eliminates a separate detention basin.
The trade-offs are structural and procedural:
- The roof structure must be designed for the ponded load, including the rain load itself, and the design maximum water depth is a controlled value, not an incidental one.
- A secondary drainage system at a higher elevation is required, and it must limit the depth of water on the roof above the controlled level.
- Controlled-flow drains are maintenance-critical. A restricting weir that silts up turns a designed pond into an undesigned one.
- Controlled-flow roofs are not appropriate everywhere — in a climate with hard freezes, a designed pond is also a designed ice sheet, and the AHJ's judgment governs.
4. What the exam asks here
Expect one question from this section, and expect it to be one of three:
- A percentage question. "Two adjacent walls of equal height rise above a lower roof — what fraction of the wall area is added?" Answer: 35 percent. The most common wrong answer is 50 percent, which is the one wall case.
- A "what if the drains block" question. Answer with the three acceptable overflow arrangements and the fact that secondary drainage is sized for the same rainfall rate and set at a higher elevation.
- A controlled-flow question. Answer that the roof structure must be designed for the ponded water and that secondary drainage at a higher elevation is required.
Two adjacent vertical walls of equal height project above a lower roof and shed water onto it. Under UPC Section 1103.4, how much of the wall area is added to the projected roof area?
Two opposite vertical walls of the same height rise above a lower roof. What does UPC Section 1103.4 require you to add to the roof area?
A roof is designed as a controlled-flow roof drainage system. What does that require in addition to the flow-restricting drains?