How Much Intumescent Paint Do I Need?

How We Calculated This

Intumescent paint consumption depends on the dry film thickness (DFT) required, which in turn depends on the steel section factor (Hp/A) and the fire resistance period needed:

• 30-minute protection: 0.8–1.5 kg/m² (DFT ~0.5–1.0mm)
• 60-minute protection: 1.5–3.0 kg/m² (DFT ~1.0–2.0mm)
• 90-minute protection: 2.5–5.0 kg/m² (DFT ~2.0–3.5mm)

What Is Section Factor?

The section factor (Hp/A) is the ratio of the heated perimeter to the cross-sectional area of the steel member. A higher section factor means thinner steel relative to its perimeter, requiring more coating. Typical domestic beams: 100–250 m¹.

Application

• Surface prep: Sa 2½ blast clean or St 2 wire brush
• Primer: Compatible anti-corrosion primer required first
• Coats: Multiple coats typically needed, each 0.25–0.5mm DFT
• Top coat: Decorative sealer/top coat recommended for visible steel

Cost Guide

Intumescent paint costs £30–60 per litre for thin-film products. For a typical domestic steel beam (203×133 UB, 3m long), expect to use 2–4 litres for 30-minute protection, costing £60–240 in materials.

Important

Intumescent coatings must be specified by a fire engineer and applied by a certified contractor. The coating system must be third-party certified (e.g., Warrington, BRE). Always refer to the manufacturer’s assessment report for exact DFT requirements.