Does a Steel Beam Need Fire Protection? Plasterboard vs Intumescent Paint

Put a new steel beam into a house and building control will almost always ask how it's protected from fire. Here's why, how much protection is normally needed, and the usual ways to do it: fire-rated plasterboard, intumescent paint, or the ceiling around it.

Why a steel beam needs fire protection

Steel doesn't burn, but it does lose strength as it heats up. In a house fire, an unprotected beam can heat quickly, lose a large share of its strength and sag, and whatever it carries (a wall, a floor, a chimney breast) comes down with it.

Fire protection slows down how fast the steel heats. That buys time for people to get out and for the fire service to work, which is exactly what the Building Regulations are after.

What the regulations ask for

In England, fire safety is covered by Approved Document B. A beam that holds up a floor or a load-bearing wall is part of the structure, and the structure has to resist fire for a minimum period.

• Most houses: typically 30 minutes of fire resistance for the beam and the structure around it.
• Taller or larger buildings, flats and commercial work: often 60 minutes or more, set by the designer or fire engineer.
• A beam that only supports a roof may not need fire resistance at all, unless the roof is used as a floor or holds up something that does need protecting.

Your building control officer (council or approved inspector) confirms what applies to your project. It's usually stated in the plan approval or raised at inspection, so ask early rather than finding out when the ceiling's already boarded.

If you're not yet sure whether your beam needs approval at all, start with do I need building regulations for a steel beam.

The options

Method	How it works	Best for
Fire-rated plasterboard	The beam is boxed in with fire-rated board, taped and skimmed	Beams that will be hidden, the usual domestic choice
Intumescent paint	A coating that swells into an insulating char when heated	Beams left on show, or where boxing in won't fit
Fire-resisting ceiling	The beam sits within the floor, protected by the fire-rated ceiling below	Beams fully inside the floor depth
Boards or encasement	Specialist fire boards, or concrete or blockwork around the steel	Commercial work and longer fire periods

Fire-rated plasterboard

The most common answer in houses. The beam is boxed in with fire-rated board (not standard wallboard), fixed to battens or a frame around the steel, then the joints are taped and the box is skimmed.

For 30 minutes this is commonly a single or double layer of 12.5mm fire-rated board, but the exact build-up should follow the board manufacturer's tested detail or what your building control officer accepts. The box has to go all the way round the exposed faces of the beam, with no gaps where it meets the wall or ceiling.

Intumescent paint

Intumescent paint looks like ordinary paint until it gets hot. Then it swells into a thick, insulating char that slows down how fast the steel heats.

It's the usual choice when you want an exposed steel beam as a feature, or where there isn't room to box it in. The things to get right:

• The thickness. The required coating thickness depends on the size and shape of the beam and the fire period. A light, slim section heats faster than a heavy one, so it needs more coating. The product's data sheet gives the figure.
• The primer. Intumescent paint has to go over a compatible primer. If you plan to use it, tell your fabricator when you order the steel.
• The record. Building control may want the product details and confirmation of the thickness applied, so keep the paperwork from whoever applies it.

Ordinary paint, even fire-retardant decorative paint, isn't a substitute. Only an intumescent product applied to the specified thickness counts.

A fire-resisting ceiling

Where the beam sits fully within the depth of the floor, the fire-rated ceiling underneath can provide the protection. This depends on the ceiling being built as a fire-resisting construction and the beam being fully enclosed, so check with building control before relying on it.

Boards and encasement

On commercial jobs and where longer fire periods are needed, steel is often protected with specialist fire boards or sprayed systems, designed by the project's fire engineer. Older buildings sometimes have beams cased in concrete or blockwork, which still works but is heavy and slow to build.

Common situations

• Knocking through a load-bearing wall. The beam is usually boxed in with fire-rated board as part of the new ceiling line.
• Removing a chimney breast. The steel supporting the remaining breast above needs protecting like any other beam carrying load.
• Loft conversions. New steels in the floor of a loft conversion are part of the conversion's fire strategy, and building control will check them along with the protected escape route.
• Feature beams in kitchens and open-plan spaces. Intumescent paint, then a decorative topcoat in the colour you want.

What building control will check

The inspector will usually want to see the beam, its bearings and its fire protection before they're covered up. For boarded beams that means seeing the board going on; for intumescent paint, the product details and thickness. Don't plaster over anything until it's been signed off.

Getting the steel right from the start

The fire protection is added on site, but it's easiest when the steel is ordered with it in mind. When you order a beam from us, tell us:

• whether it will be boxed in or left exposed
• if it will be painted with intumescent, so the primer is compatible
• any finish the engineer or building control has specified

We fabricate beams and goalpost frames to the engineer's drawing and supply them ready to install. See steel beams for extensions or get in touch with your drawings.

Frequently asked questions

Does every steel beam need fire protection? Most beams that hold up a floor or a load-bearing wall in a house do. A beam that only supports a roof may not. Building control confirms.

How much fire protection does a steel beam need? For most houses, typically 30 minutes. Flats, taller buildings and commercial work often need 60 minutes or more.

Is ordinary plasterboard enough? Use fire-rated board. Standard wallboard isn't a fire-rated product, so don't rely on it unless building control has agreed it for your detail.

Can I leave a steel beam exposed? Yes, if it's protected with intumescent paint to the thickness the product specifies for that beam and fire period, then decorated as you like.

Can you supply a beam ready for intumescent paint? Yes. Tell us when you order and we'll make sure the primer suits the coating system you're using.

Who applies the fire protection? Your builder or plasterer usually boxes beams in. Intumescent paint is often applied by a specialist, who can provide the record building control asks for.