Why Roof Drainage Matters

Large industrial roofs collect significant amounts of rainwater.

If the gutter or downpipe system is undersized, blocked or incorrectly detailed, water can overflow and increase the risk of roof leakage.

Roof drainage should therefore be considered during the early engineering stage rather than treated as a simple accessory.

Common Gutter Types

External Eave Gutters

External gutters are installed outside the roof edge.

Advantages include:

• Easier inspection

• Easier maintenance

• Blockages can be identified quickly

They are commonly used in industrial buildings.

Internal Gutters

Internal gutters are located within the building roof profile.

They can provide a cleaner exterior appearance but require careful waterproofing and maintenance.

Intermediate Gutters

Intermediate gutters are often used between roof spans in multi-span steel buildings.

Because they may collect water from multiple roof areas, their capacity requires careful calculation.

Valley Gutters

Valley gutters are installed where two roof slopes meet.

These areas can receive significant water flow and require special attention.

Gutter Capacity

The required drainage capacity depends mainly on:

• Horizontal roof catchment area

• Local rainfall intensity

• Gutter geometry

• Number of downpipes

• Downpipe diameter

The design should use local rainfall data for the actual project location.

Projects in tropical or high-rainfall regions may require larger gutters and additional downpipes.

Downpipes

Downpipes transfer rainwater from gutters to ground drainage.

Their size and spacing should be calculated according to roof catchment area and rainfall intensity.

Common industrial projects may use downpipes within approximately the DN100–DN150 range, but final sizing should always follow engineering calculations and local drainage requirements.

Roof Daylighting Systems

Natural daylight can reduce artificial lighting demand inside workshops and warehouses.

Common daylighting solutions include:

• FRP rooflight panels

• Polycarbonate panels

• Continuous skylights

• Point skylights

• Opening skylights

• Combined daylighting and ventilation units

FRP Rooflight Panels

FRP rooflights are commonly manufactured to match the profile of metal roofing sheets.

They are relatively economical and widely used in industrial buildings.

Typical advantages include:

• Low weight

• Easy integration with profiled metal roofing

• Competitive cost

• Suitable natural light transmission

Polycarbonate Rooflights

Polycarbonate panels can provide higher light transmission and improved weathering performance depending on product design.

Multi-layer polycarbonate products can also improve thermal performance.

They may be used where higher daylighting quality or longer-term performance is required.

Installation Details

Rooflights are often more vulnerable to leakage than conventional metal roofing.

Important considerations include:

• Adequate overlap

• Compatible sealing materials

• Butyl sealing tape

• Waterproof closure strips

• Correct fixing positions

• Allowance for thermal expansion

Rooflight details should be coordinated with the surrounding roofing profile.

Skylights and Ventilation

Some skylight systems can combine daylighting with ventilation.

For example:

• Electrically operated skylights

• Continuous ridge skylights

• Combined rooflight and ventilator systems

This can be useful in large workshops requiring both daylight and heat removal.

Conclusion

A successful roof envelope should manage both water and light.

The drainage system must safely remove rainwater, while the daylighting system should introduce natural light without creating leakage or excessive heat gain.

For overseas steel building projects, gutter sizing, downpipe layout and rooflight detailing should be adapted to local climate and project requirements.