An RSJ is a rolled steel joist, a steel beam with an I-shaped or H-shaped cross-section used to support heavy loads in buildings. You’ll most commonly hear the term when someone is knocking through a load-bearing wall and needs a beam to carry the weight above the opening. While “RSJ” technically refers to an older style of steel beam with tapered edges, the name has stuck in everyday language as shorthand for any structural steel beam used in residential and commercial construction.
How an RSJ Works
If you look at an RSJ from the end, it resembles the letter “I” or “H.” The top and bottom horizontal plates are called flanges, and the vertical section connecting them is the web. This shape isn’t decorative. It’s engineered so that each part handles a different type of force: the flanges resist bending, while the web handles shear (the sliding force that acts when weight pushes down on the beam). This division of labor means the beam can support enormous loads without needing to be a solid block of steel, saving both weight and material.
RSJ vs. Universal Beam
Strictly speaking, the RSJ you’ll find in older buildings is a different product from the Universal Beam (UB) used in modern construction, though people use the terms interchangeably. The key differences are in the flange design and standardization.
Traditional RSJs have tapered flanges, with the inner faces sloping at roughly 8 to 9 degrees. Their dimensions also varied between manufacturers, which made them less predictable for engineers to work with. Universal Beams, by contrast, have perfectly parallel flanges and strictly standardized dimensions across the industry. That parallel design distributes loads more evenly and gives a better strength-to-weight ratio, meaning a UB can often do the same job as an RSJ while being lighter.
The naming conventions also differ. An RSJ was specified by its depth and weight per meter (for example, “8 × 4 RSJ at 17.5 kg/m”), while a UB uses a more precise format listing depth, width, and weight in millimeters (such as “203 × 133 × 25 UB”). If you’re renovating an older property and a structural engineer recommends replacing an existing RSJ, they’ll typically specify a modern UB that matches or exceeds the original load capacity.
What RSJs Are Made Of
RSJs are made from structural steel, an alloy of iron with a small amount of carbon (typically under 0.25%). The most common grades used in European construction are S275 and S355, where the number indicates the minimum yield strength in megapascals. In practical terms, S275 steel can withstand about 36,000 psi of force before it starts to permanently deform, while S355 handles around 50,000 psi. Your structural engineer will specify the grade based on the loads the beam needs to carry. Residential wall removals often use S275, while heavier commercial applications may call for S355.
The Most Common Use: Removing a Wall
The reason most people encounter the term RSJ is home renovation. When you want to create an open-plan living space by removing a load-bearing wall, something needs to take over the job of holding up the structure above. That something is usually a steel beam.
The process starts with a structural engineer, who calculates the loads the wall currently carries and specifies the correct beam size and steel grade. This isn’t optional. A beam that’s too small risks cracking walls, sagging floors, or worse. The engineer produces calculations and drawings, which you’ll typically need for building regulations approval.
During installation, temporary supports (called Acrow props) hold the structure while the wall is removed. The beam is then lifted into position, spanning the full width of the opening. At each end, the beam sits on a padstone, a dense block of concrete or engineering brick that spreads the beam’s load across the supporting wall. Without padstones, the concentrated weight of the steel would crush standard bricks or blocks over time. The gaps between the beam and surrounding masonry are then dry-packed with mortar. In many cases, two beams are installed side by side and hidden within the ceiling void so the finished room shows no visible steelwork.
Fire Protection
Steel is strong but performs poorly in fire. It doesn’t burn, but it loses structural strength rapidly as temperatures climb, which is why building codes require fire protection for exposed steel beams in certain situations. The two most common methods are intumescent coatings and board encasement. Intumescent paint looks like a normal finish at room temperature but swells into a thick insulating char when heated, buying time before the steel weakens. Gypsum board encasement (essentially boxing the beam in fire-rated plasterboard) can achieve one- or two-hour fire ratings. For residential wall removals where the beam is buried in the ceiling, the surrounding plasterboard and plaster typically provide adequate protection, but your building inspector will confirm what’s needed.
Other Places You’ll Find RSJs
Wall removals get the most attention, but steel beams with I-shaped profiles appear throughout construction. They form the structural frames of multi-story buildings, support upper floors in loft conversions, span garage openings, and serve as lintels above large windows and doors. In bridge construction, they’re used in composite deck systems where steel beams work together with concrete slabs. Anywhere a structure needs to span a gap while carrying significant weight, a steel beam of this type is a likely solution.