Design Roof Drainage

The plumbing engineer sizes the drains. Everything else is architectural and it gets decided by default: where the falls come from, what they do to the build-up and the parapet, where the overflow discharges, and what the roof looks like when the primary drains are blocked and the secondary system is doing its job.

Workflow

Step 1. Establish the roof and what is under it

Ask, in one message:

1. What is the roof: area, shape, level changes, and what is on it?
2. What is the structure, which way does it span, and is it level or already sloping?
3. What is the assembly, or is it open? Point at design-roof-assembly.
4. What is directly below each part of the roof? A drain drops through the building and lands somewhere.
5. Is there a parapet on every edge, and how high?
6. Quote the code's secondary drainage provisions: what is required, and does it permit scuppers, a separate piped system, or either?
7. Is there a roof warranty in prospect with its own requirements?

Step 2. Decide where the falls come from

Three ways to get slope, and the choice changes the whole assembly:

Source	What it costs	What it constrains
The structure	Cheapest, and the ceiling below follows the fall	Only works where the space below tolerates it
Tapered insulation	Common, and the build-up varies across the roof	Thickness at the high point, and the parapet height
A fill or topping	Weight, and it is dead load somebody has to carry	Structure

Say which, and say what it does to: the thinnest point of the assembly, the parapet height at the low corner, the height at every upstand and curb, and the door threshold at any roof access.

A tapered layout has a thinnest point, and the assembly's performance at that point is the assembly's performance. Name it and say the R-value there is what governs, not the average.

Step 3. Place drains against three plans at once

A drain location has to work in the roof plan, the reflected ceiling plan and the plan of the space below.

Drain	Roof position	Structure it passes	Space below	Conflict

• Drains want to be at the low point; structure wants them beside a beam, not through it.
• The pipe drops through every floor below and needs a route and a shaft. Point at /size-shaft.
• A drain over an occupied space is a leak in that space. Over an electrical room it is worse; say so.
• Drains at a parapet or at a level change have their own conditions.
• Keep drains away from where equipment will be set, and from the fall arrest and access routes.

Step 4. Work the secondary system as a visible thing

Secondary drainage exists for when the primary blocks, and it is designed as an afterthought because nobody expects to see it work.

• What kind: overflow scuppers through the parapet, a separate piped system with its own drains at a higher inlet, or both. Quote what the code permits.
• Where does it discharge, and would anyone notice? A piped secondary system discharging into the same stack as the primary hides the failure. A scupper discharging onto a public sidewalk is visible and is a different problem. This is a design decision with a maintenance consequence, and it is usually made by default.
• What the scupper does to the parapet and the elevation. Point at design-parapet.
• What the discharge lands on: paving, planting, an entrance canopy, a neighbour.

Step 5. Name the blocked case and hand it over

If the primary drains block, water stands on the roof to the height of the secondary inlet, and that water has weight.

State plainly:

• How deep the water stands before the secondary system takes it, as a question for the engineer rather than a figure.
• That the structural engineer has to check the roof for that load, and whether anyone has asked.
• Whether the roof deflects under that load in a way that deepens the pond, which is the mechanism behind roof collapses and is not obvious.
• What maintains the drains, how they are reached, and whether the strainers can be cleared without special access.

Step 6. Report

The fall strategy and what it costs, the drain table with conflicts, the secondary system with its discharge named, the blocked case. Then:

• Every value that has to be established, with who supplies it: rainfall intensity, drain sizing, ponding load.
• Every conflict between a drain location and the structure or the space below.
• What the thinnest point of the assembly is and what governs there.
• What has to reach the plumbing engineer, the structural engineer and the civil engineer.

Then ask whether to write the result to a file and where.

Rules

• Never supply a rainfall intensity, a drain size, a slope, or a load.
• Say where the falls come from before placing a drain.
• Check every drain against the roof plan, the structure and the space below.
• Name where the secondary system discharges and whether anyone would notice it running.
• Raise the blocked-drain load as a question for the structural engineer, every run.
• Name the thinnest point of a tapered assembly.

Anti-patterns

• Placing drains on the roof plan and leaving the drop to the plumbing engineer.
• Tapering insulation without checking the parapet height at the low corner.
• Reporting the assembly's average performance rather than its performance at the thinnest point.
• Running the secondary system into the same stack as the primary.
• A drain over an electrical room.
• Recalling a minimum slope or a rainfall figure.
• Treating the ponding case as somebody else's without naming who.
• Designing scuppers without looking at the elevation.

Resources

None. This skill is one file. Output is written directly at the path you choose.