Roof Drainage Design

Roof drainage design is the planning of slopes and outlets so that rainwater leaves a roof quickly and safely. Good drainage protects the roof covering, limits leaks, prevents overloading and directs water away from walls and foundations. It becomes more critical as heavy downpours grow more intense in many regions.

How much water a roof collects

Every millimetre of rain on one square metre of roof produces about one litre of water. In imperial units, about 0.62 US gallons fall on each square foot of roof per inch of rain. A 200 m² (about 2,150 sq ft) roof receiving 50 mm (2 in) of rain therefore sheds roughly 10,000 litres (about 2,650 US gallons).

Steep-slope roofs

Pitched roofs shed water by gravity into gutters along the eaves and then into downspouts (downpipes). Design considers:

• gutter size and fall toward outlets;
• number and size of downspouts relative to the roof area served;
• valleys, which concentrate flow and need wider flashings;
• overflow paths so that blocked gutters spill outward, not into walls or soffits.

Low-slope roofs

Flat roofs are never truly flat. Commercial membranes usually sit at slopes of about 1 to 3 degrees, created by structural slope or tapered insulation. Water is removed by:

• Internal roof drains connected to piping inside the building;
• Scuppers through parapets at the roof edge;
• Gutters at open edges;
• Crickets and saddles that divert water around curbs and toward outlets.

Plumbing codes such as Chapter 11 of the International Plumbing Code in the United States size drains and leaders using design rainfall intensity maps and require secondary (emergency overflow) drains or scuppers where parapets could trap water if primary drains block. Other countries use their own national drainage standards and rainfall data.

Why redundancy matters

Leaves, gravel and debris can clog outlets. Water that cannot escape builds up, adding dead load the structure may not be designed to carry and driving water into small defects. Secondary drainage set slightly above the primary level gives water an escape route.

Maintenance

Inspecting and cleaning gutters, drains and strainers, especially after autumn leaf fall and before storm seasons, is one of the most effective roof maintenance tasks.