How to Box In a Steel Beam: UK Plasterboard Guide
To box in a steel beam you build a light frame around it — timber battens or metal furring — and clad it in plasterboard to give a clean, square, paintable surface. It is a common finishing job after an RSJ goes in over a knocked-through opening. But there is one thing to settle before you reach for the plasterboard: fire protection. In most UK homes a structural steel beam needs a fire-resisting encasement, and plasterboard is how that is usually achieved. This guide covers both the fire requirement and the practical method.
Why Box In a Steel Beam?
Boxing in does three jobs at once. It hides the bare steel for a tidy, plastered finish that matches the surrounding walls and ceiling. It gives you a flat surface to decorate, fix coving to, or run services past. And — most importantly — the plasterboard casing provides the fire protection that exposed structural steel almost always needs in a domestic building. Leaving an RSJ exposed is sometimes done for an industrial look, but it must still meet the fire requirement, usually with intumescent coating instead.
Fire Protection Comes First
Unprotected steel loses strength rapidly in a fire, so Approved Document B (fire safety) of the Building Regulations requires structural steel to achieve a minimum period of fire resistance — typically 30 minutes in a standard two-storey house, rising to 60 minutes where the beam protects an escape route, separates a loft conversion, or divides higher-risk areas. Plasterboard delivers this: a single layer of 12.5mm fire-rated plasterboard (the pink fire-resistant board, not ordinary wallboard) gives around 30 minutes' fire resistance, and two layers of fire-rated board are used for 60 minutes. Confirm the exact requirement with your building control body before you start — your structural engineer's drawings or building control will state the period needed.
How to Box In a Steel Beam
There are two reliable methods. Both create a three-sided frame (two sides and a bottom) around the beam, clad in plasterboard.
Method 1 — Timber Batten Frame
Fix timber battens (typically 50×50mm CLS or similar) to the underside and both sides of the structure around the beam, creating a ladder-like frame that follows the beam's profile. Fix the battens into the masonry or joists above and below the beam — not into the beam itself unless you are using beam clips, as drilling structural steel is best avoided. Check everything is plumb and level, then clad the two vertical faces and the soffit with plasterboard. This is the most common DIY method and needs only basic tools.
Method 2 — Metal Furring (MF) Frame
For a straighter, more stable casing — especially on longer spans — use a metal furring system. Galvanised steel channels and brackets form the frame, giving a dead-straight line that resists twisting or shrinkage over time. Metal frame systems are also the basis of proprietary fire-protection encasements rated to specific periods, which is worth using where a 60-minute rating is required.
How to Plasterboard a Steel Beam
Cut the plasterboard to the width of each face plus the board thickness so the sheets meet cleanly at the corners. Fix the boards to the frame (not the steel) with drywall screws at roughly 150mm centres at board edges and 230mm in the field, keeping screws back from the board edge to avoid breakout. Board the two sides first, then the soffit, so the bottom board supports the side edges. Where a 30-minute rating applies, a single layer of 12.5mm fire-rated board is the standard; for 60 minutes use two layers of fire-rated board, staggering the joints between layers. Back and fill the joints, as unsealed gaps undermine the rating. Finish by taping and jointing the corners, or skim the whole casing for a seamless plaster finish ready to paint.
Common Mistakes to Avoid
Do not screw or drill into the structural beam without checking with your engineer — fixings go into the frame and surrounding structure. Do not skip the fire-rating check; a casing that looks fine but doesn't meet the required period will fail building control. Allow the beam and any padstones to be fully built in and the structure signed off before boxing in, so the work isn't hidden before inspection. And leave a small gap for any movement rather than wedging boards tight against masonry.
Steel Beams at Rackerman
Boxing in is the last step after the beam is installed. If you are still at the planning stage, Rackerman supplies RSJ steel beams cut to size in mill or red oxide finish — and our guides explain what an RSJ is, how much a steel beam costs, and whether an RSJ needs painting before it is boxed in. Use our RSJ size guide to understand the section sizes you'll be casing.
Browse RSJ steel beams at Rackerman →
Frequently Asked Questions
How do you box in a steel beam?
Build a light frame of timber battens or metal furring around the beam, fixed into the surrounding masonry and joists rather than the steel itself, then clad the two sides and soffit in plasterboard. Tape and joint or skim the corners for a clean, paintable finish that also provides fire protection.
How do you plasterboard a steel beam?
Fix plasterboard to the frame around the beam — not to the steel — using drywall screws at 150–230mm centres. Board the sides first, then the soffit. Use 12.5mm fire-rated board for a 30-minute fire rating, or two layers of fire-rated board for 60 minutes, then tape and joint or skim the casing.
Does a steel beam need fire protection?
Yes, in almost all domestic situations. Approved Document B requires structural steel to achieve a minimum period of fire resistance, usually 30 minutes in a standard house and 60 minutes where it protects an escape route or separates areas. Plasterboard casing is the normal way to provide it.
How many layers of plasterboard for a steel beam?
One layer of 12.5mm fire-rated plasterboard (the pink board) provides about 30 minutes' fire resistance, which suits most standard domestic beams. For a 60-minute requirement, use two layers of fire-rated board with staggered joints. Confirm the required period with building control.
Can you screw into a steel beam to box it in?
It is best avoided. Drilling a structural beam can affect its performance and should only be done with your structural engineer's approval. Instead, fix the boxing frame into the surrounding masonry and joists, or use proprietary beam clips designed to grip the flange without drilling.
Should you leave a steel beam exposed or box it in?
Either is possible, but exposed steel still needs to meet the fire requirement — usually with an intumescent coating that expands in a fire. Boxing in with plasterboard is simpler, cheaper and provides both fire protection and a decorative finish, which is why most domestic beams are cased.
What size battens do you use to box in an RSJ?
50×50mm CLS timber is a common choice for the framing battens, fixed around the beam to form the casing. Larger sections give a more robust frame on longer spans. A metal furring system is an alternative that stays dead straight and avoids timber movement.
Do you need building control to box in a steel beam?
The beam installation itself requires building control approval, and the fire protection of that beam is part of it. Boxing in is how the fire requirement is usually met, so it forms part of the approved work. Check the required fire period with your building control body before cladding.