What Size Rainwater Downpipe Do I Need?

Downpipe Capacity by Size

• 50mm round: Up to ~25m² effective roof area
• 68mm round: Up to ~50m² effective roof area
• 75mm round: Up to ~65m² effective roof area
• 100mm round: Up to ~130m² effective roof area
• 110mm round: Up to ~160m² effective roof area
• 65×50mm rectangular: Up to ~30m² effective roof area

Effective Roof Area

The effective roof area is the plan area of the roof section draining to each downpipe. For pitched roofs, use the plan area (not the actual sloped area). For walls adjacent to roofs, add 50% of the wall area to the roof area.

Number of Downpipes

A typical 3-bed semi has a roof plan area of about 40–50m² per side. With 68mm downpipes, you need one per 50m². Most houses have 2–4 downpipes depending on roof configuration.

Gutter Sizing

• 75mm half-round: Up to ~25m² roof area
• 100mm half-round: Up to ~50m² roof area
• 112mm half-round: Up to ~65m² roof area
• 125mm half-round: Up to ~85m² roof area
• 150mm half-round: Up to ~130m² roof area

Tips

Match the downpipe size to the gutter outlet size. Downpipe offsets and bends reduce capacity — each bend reduces flow by about 10–15%. Connect downpipes to the underground drainage system via a back-inlet gully or rainwater shoe.