Intumescent Coatings vs Fire Boards vs Cementitious Spray — Which is Best?

When it comes to protecting structural steel from fire, there are three main options: intumescent coatings, fire board encasement, and cementitious spray. Each has its strengths and weaknesses. The right choice depends on your building type, budget, aesthetics, and programme. Here's a straightforward comparison.

The Three Methods at a Glance

1. Intumescent Coatings

Intumescent coatings are specialist paints applied directly to the steel surface. They look and feel like a normal paint finish at room temperature. When exposed to fire, the coating expands up to 50 times its original thickness, forming a carbonaceous char that insulates the steel.

Best for:

• Exposed structural steel where aesthetics matter (offices, restaurants, retail, residential conversions)
• Projects where the architect wants to see the steel
• Off-site application — steel can be coated in the fabrication shop before delivery
• External steelwork (with appropriate topcoat system)
• Complex steel geometries — connections, castellated beams, curved members

Limitations:

• Higher cost per m² than board or cementitious for high fire ratings (90–120 min)
• Multiple coats required for higher DFTs — needs drying time between coats
• Requires clean, properly prepared steel surface

2. Fire Board Encasement

Fire boards (such as Promat SUPALUX, Knauf Fireboard, or Vermiculux) are cut and fixed around the steel to form a protective encasement. The boards are typically 15–30mm thick depending on the fire rating required.

Best for:

• Steel that will be hidden behind ceilings, walls, or cladding
• High fire ratings (90–120 min) where cost is a priority
• Simple, regular steel sections (UC columns, UB beams)
• Projects where a clean, boxed finish is acceptable

Limitations:

• Hides the steel completely — not suitable for exposed steel designs
• Slow to install — each piece must be measured, cut, and fixed
• Difficult around complex connections and irregular steel shapes
• Adds bulk and weight to the steel
• Cannot be applied off-site

3. Cementitious Spray

Cementitious spray (such as Cafco or Mandolite) is a cement-based material sprayed onto the steel to build up a thick, fire-resistant layer. It's the cheapest option per square metre but produces a rough, textured finish.

Best for:

• Large industrial or warehouse structures where aesthetics don't matter
• Steel hidden above suspended ceilings
• Budget-driven projects with high fire rating requirements
• Rapid coverage of large areas

Limitations:

• Rough, unattractive finish — must be hidden
• Heavy — adds significant weight to the structure
• Messy application — requires extensive masking and protection
• Not suitable for external use
• Can be damaged easily before other trades have finished

Which Should You Choose?

The decision usually comes down to three questions:

• Will the steel be visible? If yes, intumescent coatings are the only practical option. Board and cementitious spray both hide the steel.
• What's the fire rating? For 30–60 minutes, intumescent coatings are competitive on price. For 90–120 minutes, board or cementitious may be cheaper per m² — but intumescent is faster to apply.
• What's the programme? If speed matters, intumescent coatings (especially applied off-site) or cementitious spray are faster than board encasement.

Can You Mix Methods?

Yes — and it's common. Many projects use intumescent coatings on exposed steel (reception areas, atriums, feature steelwork) and fire board or cementitious spray on hidden steel (above ceilings, in risers, plant rooms). This gives you the best of both worlds: aesthetics where it matters, cost savings where it doesn't.