When you need an RSJ
An RSJ is required wherever a load-bearing wall is opened up or a new opening is cut in a structural wall. The standard London use cases: a knockthrough between front and rear reception rooms on a Victorian terrace; opening up the kitchen wall to a new rear extension; supporting an existing chimney breast above a removed lower section; flank beams over wide bifold or sliding-door openings in extensions; new opening cut for a wider doorway in a corridor wall. The structural engineer calculates the load (dead load of structure above + imposed load of floors and occupancy + roof loads if applicable) and sizes the beam for the span. Standard residential spans up to 4m use a 152x89 or 203x102 RSJ at 23–30 kg/m; longer spans demand deeper sections or twin-RSJ.
Beam types: RSJ, UC, flitch explained
RSJ (Rolled Steel Joist): the traditional I-section with tapered flanges. Standard sizes 127x76, 152x89, 178x102, 203x102, 254x102. Used for most domestic openings. Cheapest beam type per metre. UC (Universal Column): squarer cross-section with parallel flanges, used where bending stiffness matters or ceiling depth is limited. Common sizes 152x152, 203x203, 254x254. About 10–20% more expensive per metre but allows shallower depths. UB (Universal Beam): similar to RSJ but with parallel flanges, slightly stiffer for deeper sections; common where engineer specifies. Flitch beam: timber beam with steel plate fixed to one or both faces, used where appearance matters or where space is too constrained for a steel section alone. More labour-intensive. The engineer specifies which type — we don't substitute.
Install method and padstones
RSJ installs follow a strict sequence. First, temporary propping: Acrow props with cross-needles take the load above the planned opening. Engineer inspects the propping before any wall is cut. Second, padstone preparation: each end of the new beam bears onto a load-distributing padstone — typically engineered concrete (Class C25/30) or precast at 215mm wide x 215mm deep x 200mm long, calculated by the engineer based on bearing stress. Third, controlled wall removal below the props: cut out the masonry in sections, set padstones in position with bedding mortar. Fourth, beam hoist: standard residential RSJs (under 80kg) hoisted manually by 4 trades; longer or heavier beams craned in via window or door opening. Fifth, beam bedded onto padstones with non-shrink grout, ensuring full contact. Sixth, engineer signs off install before propping is removed. Seventh, infill above beam, fire-protect, plaster, decorate.
Fire protection requirements
Building Regulations Part B requires 30 minutes of fire resistance on all new structural steel in domestic properties (60 minutes if the steel is in a flat or above a flat). The two compliance routes: (1) Plasterboard encasement — two layers of 12.5mm pink fire-rated plasterboard, all joints staggered and taped with fire-rated tape and skim plaster. Cheapest route; adds 25mm thickness to each side and underneath. (2) Intumescent paint — applied as a 2-4mm coat directly to the steel; expands when heated to insulate the beam. More expensive but maintains the slim profile of the steel. Used where the beam is left visually exposed. (3) Concrete encasement — only for new-build situations, rarely used in domestic retrofits. Builderr defaults to plasterboard encasement on ceiling-line beams (best price and reliability) and intumescent paint where the beam is feature-exposed.
