If you ask ten specifiers how they’re protecting their steelwork this month, you’ll get twelve answers.
The problem is that most of the time, the method doesn’t feel especially consequential. Steel is, for all intents and purposes, hidden. Plasterboard is cheap. Paint is familiar. The design moves on. And the structure, despite holding up the entire building, quietly disappears into the void.
Fire doesn’t care about cost or coverage, though. It just burns.
Part of the reason that steel is a solid favourite is that its behaviour under heat is relatively well known, but the protection is still being chosen based on habit. Plasterboard encasement remains a default for mid-rise and low-complexity buildings, often because it’s there, and it’s familiar.
It’s one of those things in construction that’s a bit of an uncomfortable topic. But when steel structures (or any, for that matter) fail, it doesn’t happen quietly, and it doesn’t happen politely. It can happen quite quickly, and in ways that make the timelines of evacuation and rescue rather challenging indeed.
Intumescent coatings dominate many commercial builds, and plasterboard encasements remain a go-to on the residential and mid-rise side (particularly in cost-sensitive or constrained programmes). But are a catch-all.
Paint, board, repeat
Intumescent coatings often win on access. They can go where boards can't, which is a big win for exposed or awkward geometries. But they do bring their own problems - site conditions, film thickness variation, durability, and touch-ups all create challenges that can impact their effectiveness. Many avoid them for internal applications for this reason - because they’re too dependent on perfect execution.
Plasterboard, by contrast, seems like quite a simple solution. It’s cheap. It’s fast. It’s familiar. We’re big fans, on the whole.
A boxed-out column wrapped in board looks pretty tidy, inspectable even, and fairly benign. And in low-rise applications, where tolerances may be a little more generous, it probably does the job. That’s how it ended up in thousands of buildings, effectively by default.
But as buildings grow taller, building loads get heavier, design interfaces grow tighter, and regulatory oversight grows sharper, that default is being stress-tested. This might be the one occasion that we would not tell you to use plasterboard.
Protection is not an afterthought
Approved Document B is very clear: structural elements must maintain loadbearing capacity for the full designated fire resistance period of the building (this does not just apply for structural protection – in any fire compartment, the resistance period is defined by the lowest denominator). BS EN 1993-1-2 and related fire design standards provide calculation methodologies to help determine the necessary protection methods, but performance can be proven only through full-system testing.
Unprotected steel can reach failure temperatures within 15–20 minutes. This may not always be visible.
As temperature rises, and a structural member hots up, steel can undergo creep, buckling, and loss of stiffness well before yielding. Research through testing has repeatedly proven this. Collapse cascades not from one glaring omission, but from small missteps. Structural protection buys the necessary time, but only if it works as specified.
Which brings us to plasterboard encasements.
Discontinuity at connections
The challenge: Board systems may be detailed for clean spans, failing to take into account the complexity of steel junctions
Why this matters: Connections are critical under fire, but BRE testing has found them to have localised stress and high heat transfer. Without the same level of protection (with the same build up) as the main member, they can become points of early failure. Unless that protection is part of a tested solution, it may not deliver the necessary fire resistance. Plus, where intumescent paint is used, connections are the most likely area to have a junction with the board, introducing an extra layer of complexity – and risk.
Movement and deformation risk
The challenge: Primary structures move. Under load, under fire, and under thermal expansions. Plasterboard encasements may not be specified to fully account for this.
Why this matters: EN 1363-1 does allow for specimen deformation under fire conditions, but board systems need to account for it. Structural steel can deflect significantly over its span during fire. If the encasement system wasn’t tested for this, it can crack or detach, exposing the steel.
Invisibility of protection solutions
The challenge: Many structural protection methods are buried behind finishes, or even the façade, making inspection post-installation near impossible.
Why this matters: The Building Safety Act 2022 places legal accountability on duty holders for evidencing compliance. But if protection systems are closed up before verification, or covered over, installation errors can go unnoticed. It might be incorrect fixing centres, it might be uneven application, or it might be a failure in the bonding of the paint – but they all affect fire resistance. Without photographic records or pre-close sign-off, it’s more or less guesswork.
Responsibility falls down the cracks
The challenge: The junction between fire-rated partitions and structural encasements is one big issue in itself. Trade packages may not coordinate the edges, and where the responsibility sits may not be defined in the paperwork.
Why this matters: The interface between structural protection and partitions is often a weak link. Even if both systems have valid test reports in their own right, that’s no guarantee the interface does. Without integrated testing or a verified method for these interfaces, fire can breach the line of protection. It’s a critical failure that might not show up in the design, but it will show up in the final risk profile. If left to installer discretion or solved ad hoc, the protection strategy can be invalidated.
Building a better habit
The challenge is not necessarily plasterboard.
The way forward isn’t to flat-out reject boarded encasement solutions, but to stop relying on habit. Plasterboard systems can be viable (when specified and installed exactly as tested). But for complex, high-load, or multi-interface designs, systems designed to maintain structural integrity under fire - like frameless, specialist protection board systems - may now offer a more robust answer.
Specialist protection board systems like the ones offered by Promat. Take a look: we hope you like them as much as we do.