Flat Roof Structural Specification: Loads, Falls, Insulation

Flat roofs are deceptively simple. The geometry is straightforward; the structural, thermal, and waterproofing detailing is unforgiving. Most flat roof failures trace back to specification mistakes, not workmanship.

Structural loading

• Self-weight: 0.5–1.5 kN/m² depending on covering
• Snow: 0.6 kN/m² minimum (much higher in northern South Africa)
• Imposed (maintenance access): 0.6 kN/m²
• Imposed (terrace use): 1.5–4.0 kN/m² depending on occupancy

Joist sizing rules of thumb

For typical 0.6 kN/m² roofs at 400mm centres:

• 200×47 C24: spans up to 3.5m
• 225×47 C24: up to 4.0m
• I-joists: up to 6.0m for deeper sections

Always verify with calculations for your specific load case.

Falls and drainage

Building Regulations require minimum 1:80 fall to drainage. Practical advice: design for 1:40 to allow for construction tolerance and long-term deflection.

Achieving falls

• Tapered insulation: Most common modern method. Insulation board cut to falls.
• Firring strips: Tapered timber strips on top of joists.
• Sloping joists: Joist depth varies — structurally inefficient.

Warm roof vs cold roof

Warm roof (recommended)

Insulation above the deck. Joists stay at internal temperature — no condensation risk in joist void.

• Thicker overall build-up
• No ventilation required
• Better thermal performance

Cold roof (legacy)

Insulation between joists. Cross-ventilation required to prevent condensation.

• Thinner build-up
• Ventilation gaps interrupt insulation continuity
• Higher condensation risk
• Largely superseded by warm roof in new build

Coverings

• EPDM rubber: 25–40 year life, single membrane, no seams in field
• GRP fibreglass: 25–30 year life, seamless, can be slippery
• Hot-melt asphalt: 30+ year life, suits inverted roofs
• Built-up felt: 10–20 year life, lowest cost

Common failures

• Inadequate falls — ponding water shortens membrane life
• Cold roof condensation in unventilated voids
• Thermal bridges at perimeter
• Detailing failure at upstands and penetrations

MCFAR specifies flat roof structure for extensions and commercial projects.

Frequently Asked Questions