Flat Roof Drainage: How to Prevent Water Pooling

Every flat roof must have effective drainage. Despite the name, flat roofs aren't truly flat — they're built with a slight slope (called a fall) to direct rainwater towards outlets. When drainage fails, problems follow quickly.

What is a Roof Fall?

A roof fall is the slope built into a flat roof to encourage water to run off. Building Regulations recommend a minimum fall of 1:80 (about 12mm per metre). In practice, most contractors aim for 1:40 to allow for settlement. Insufficient fall is one of the most common causes of flat roof problems.

Drainage Options

• Internal roof drains — outlets within the roof surface connected to internal downpipes
• Parapet gutters — gutters behind a parapet wall around the roof perimeter
• Scuppers — openings through the parapet wall to drain water to external downpipes
• Siphonic drainage — uses negative pressure for faster drainage of large commercial roofs

Keeping Drains Clear

Roof drains and outlets should be checked and cleared at least twice a year. Leaf guards can help reduce blockage, but aren't a substitute for regular inspection. A blocked drain can quickly cause ponding that stresses the waterproof membrane.

Improving Existing Drainage

If your flat roof consistently ponds water, the drainage may need to be improved. Options include tapered insulation systems (which create a slope above the existing deck), additional outlets, or raising existing outlets. Contact us for a drainage survey.