Steel Beam (RSJ) Installation Guide: UK Building Regs

Quick Answer: Installing a steel beam in a domestic property requires (1) a structural engineer's calculation specifying the beam size, padstones and connections, (2) Building Regulations approval under Approved Document A, (3) temporary propping of the load above with acrows or strong-boys before any masonry is cut, (4) padstones of correct grade concrete to spread the load onto the masonry below, and (5) 30–60 minutes fire protection under Approved Document B (typically intumescent paint or plasterboard boxing). The beam must be S275 or S355 grade steel to BS EN 10025 and installed to the engineer's specification exactly — no field substitution.

Summary

Installing a steel beam — opening up a kitchen-diner, supporting an extension head, replacing a load-bearing wall — is a regulated, high-risk task that needs careful sequencing. Mistakes during temporary propping have killed builders and homeowners. The work is also one of the most common reasons Building Control rejects a project — incorrect padstones, wrong fire protection or unspecified bearing length come up regularly at final inspection.

The competent tradesperson treats the engineer's calc as gospel. The beam, its grade, its length, the padstone size and concrete grade, the bearing length, the fire protection — all are specified for a reason. The site role is to source the specified beam and execute the install in the sequence that keeps the building above safe through the cut, lift, set and reinstate.

This article covers domestic single-beam installs (typically UB 152, UB 203, UB 254 size). Heavier industrial beams, composite floor structures, and multi-span systems are outside the typical kitchen-fitter / builder scope and need specialist fabricator + erector teams.

Key Facts

• Building Regulations Approved Document A — Structure (statutory requirement)
• Building Regulations Approved Document B — Fire safety (fire protection of steel)
• BS EN 10025 — Hot rolled products of structural steels (S275, S355 grades)
• BS EN 1090-2 — Execution of steel structures
• BS 5950 (legacy) / BS EN 1993 (current Eurocode 3) — Steel design
• Typical residential beam grade — S275 or S355; S355 is now default
• Beam designation — Universal Beam (UB) e.g. UB 152×89×16; depth × width × kg/m
• Padstone concrete grade — minimum C30/37 (often C32/40 specified)
• Padstone reinforcement — typically 4 × T8 mild steel bars
• Bearing length onto padstone — typically 100–150mm (calc specifies)
• Temporary propping — Acrow / Adjustaprop type 0–4 sized for span and load
• Acrow safe working load — depends on size and extension (10–20 kN typical)
• Strong-boy / dead shore — alternative wall support for masonry above
• Beam fire protection — 30 minutes minimum domestic (Part B)
• Fire protection methods — intumescent paint (sprayed, 1.5mm DFT typical), 15mm gypsum plasterboard double-layered, fire-line board
• Steel weight — 1.0 kg/m steel ≈ 1.0 kg per m length; e.g. UB 203×133×30 over 4m = 120 kg
• Lifting equipment — beam slings (BS EN 1492-1), beam clamps, gantry hoist, scaffold tower lift

Quick Reference Table

Detailed Guidance

Pre-install checks

Pre-Install Readiness Checklist
□ Structural engineer's calc on site with current revision
□ Beam delivered — check grade marking and length matches drawing
□ Padstone materials on site — reinforcement, concrete, formwork
□ Acrows and props of correct rating
□ Strong-boys (if propping masonry directly)
□ Spreader beams / timber lintels for top of props
□ Lifting equipment booked / hired
□ Building Control inspection booked
□ Existing services located and isolated where in working zone
□ Dust sheets, clean-up plan
□ Risk assessment / method statement (RAMS)

Temporary propping

The most dangerous moment in any beam install is propping the masonry before the cut. The wall above the new opening must be supported before the lintel/wall below is removed.

Two main methods:

• Acrow props with timber lintels — props placed directly under the masonry on a 100×75mm timber bearer; props extended to take load; second row at the back of the wall if wall is more than 1 brick thick (i.e. a party wall or cavity wall in solid construction)
• Strong-boys — a steel paddle that clips to an acrow and supports masonry from a needle hole punched through the wall above; needles spaced 600–900mm; props each side carry load down

Whichever method, the props must:

• Bear on a strong floor (concrete slab acceptable; timber floor needs spreader plates to distribute load)
• Be plumb (out-of-plumb props can buckle under load)
• Be tightened to support the load (the prop "starts to take weight" feeling)
• Have safety pins inserted in the prop holes once at length

Once propped, a small section of masonry can be removed at one end to test the support. If the props are loaded, no settlement should occur. Cut the full opening only after this test.

Cutting the opening

With the wall supported:

1. Mark the opening on both sides of the wall (always check both sides — old buildings have rooms at different floor levels)
2. Cut top of opening 100mm above final beam soffit position (allow for mortar bed)
3. Cut sides plumb
4. Cut bottom (if opening reaches floor) clean and level
5. Remove masonry in courses from the top, working down
6. Clear all rubble before lifting the beam

Use a stitch-drill / disc cutter for clean cuts. A diamond chainsaw makes faster work but creates more dust. A masonry breaker may be needed on harder bricks or stonework.

Building padstones

Padstones must be cast or set in place before the beam is lifted in.

• Pre-cast padstones — bought ready-made; bedded onto the wall on M6 mortar; quick install
• Cast in-situ padstones — formwork built into the wall, reinforcement placed, concrete poured; takes 24h+ to cure before loading

Specification follows the engineer's calc:

• Concrete grade C30/37 or C32/40
• 4 × T8 bars (typical)
• 50mm cover to outside face
• Size — typically 215 × 215 × 215mm or 440 × 215 × 100mm
• Position — under the beam's full bearing length (typically 100–150mm)
• Bedded on M6 mortar (1:1:6 cement:lime:sand)

The wall under the padstone must be sound. Loose masonry, perished mortar joints, or voids must be made good or the wall localised under the padstone may fail in compression.

Lifting the beam

For beams up to ~30 kg/m and 5m long, two-person manual handling with a sack truck or genie hoist is typical. For larger beams or restricted access:

• Strops — woven slings rated BS EN 1492-1
• Beam clamps — pin or screw-type clamp around the flange
• Gantry hoist — chain block in a portable A-frame
• Scaffold tower lifting platform — sometimes used for beams installed above ground floor

CDM 2015 manual handling rules — two-person lift for >25 kg per person; mechanical lifting for awkward or heavy beams. RAMS (Risk Assessment and Method Statement) documents the lift method.

Installing the beam

1. Position the beam on temporary props at the correct height (typically 5mm below final position to allow for mortar bed)
2. Apply mortar bed to padstones (10mm M6 mortar)
3. Lower the beam onto the mortar bed
4. Adjust position — should be central, level, parallel to wall
5. Check level along the length
6. Tighten any clamps or fixings
7. Remove temporary props one at a time, monitoring for movement

If the beam shows movement when props are removed — STOP. Recheck propping, recheck padstone strength, check beam grade and length match calc.

Reinstating masonry above

The masonry above the beam is the original wall structure. After beam install:

1. Allow padstones and bed mortar to cure (24h)
2. Build back any missing courses above the beam in matching brick/block
3. Use M6 mortar joints
4. Tie new masonry to old with stainless steel ties (BS EN 845-1)
5. Provide expansion gap (5mm) between top of beam and bottom of brickwork above
6. Pack expansion gap with compressible material (e.g. fibre packing); seal with sealant

Fire protection

Building Regulations Part B requires structural steel to retain its strength for the minimum fire resistance period — typically 30 minutes in domestic up to two storeys, 60 minutes for three-storey domestic.

Methods:

• Intumescent paint — sprayed coating that swells at 200°C+, insulating the steel. Dry film thickness specified by paint manufacturer (typically 1.5–3mm for 30 min). Final colour coat over the intumescent.
• Plasterboard boxing — two layers of 15mm Fyreline or fire-line board; corner beads at edges; fire-rated screws; finished with skim
• Fire-resistant timber boxing — typically thicker timber (45mm) with FR rating — less common
• Concrete or masonry encasement — older buildings; not standard for new install

The fire protection method should be in the engineer's calc or the architect's specification. Don't omit.

Building Control inspection

Building Control will typically want to see:

• Beam in place with grade marking visible (before fire protection)
• Padstones cured and bedded
• Bearing length conforming to calc
• Engineer's letter or certificate of conformity
• Photographs of work in progress (if not inspected in person)

The Building Control officer signs off the structural element. Without their sign-off, the building cannot be certified compliant.

Frequently Asked Questions

Do I need to lift the beam in one piece?

Almost always — the beam is a single section by design. Splicing two shorter beams into one structural element requires a designed splice connection from the engineer (typically bolted plate splice) and is rarely done on small jobs because of cost and time. For long spans where access is impossible, the engineer may design a spliced or articulated solution.

Can I use a wooden beam instead of steel?

Sometimes, with engineer's design. Glulam (glue-laminated timber) and LVL (laminated veneer lumber) beams are increasingly used in domestic work for similar spans to RSJs. Cost is comparable, install easier (lighter), and aesthetic better (timber can be left exposed). Specified by the engineer; not interchangeable with steel without redesign.

Why does the calc specify a 215×215×215mm padstone?

Padstone size comes from the bearing pressure calculation: beam reaction (kN) divided by allowable bearing pressure of the wall below (kN/m²). Smaller padstones concentrate the load — risk of crushing the brickwork under the bearing. The engineer's calc shows the calculation explicitly; don't shrink the padstone "because it fits better".

What if the beam is delivered with the wrong grade?

Reject the delivery and order the correct grade. S275 cannot be substituted for S355 — capacity is roughly 30% lower for S275 compared to S355. The beam mark on the web should be legible (acid-etched or stamped). If unmarked, ask the supplier for a certificate of conformity to BS EN 10025; no certificate = no install.

How quickly can I remove props after install?

After the beam is bedded on mortar and the padstones have cured (24h minimum), props can be removed. Common practice: leave props in place 48–72h after bedding to allow padstones to reach near-design strength. Remove one prop at a time, monitor for movement.

Regulations & Standards

• Building Regulations Approved Document A — Structure

• Building Regulations Approved Document B — Fire safety

• BS EN 1993 parts 1-1 to 1-12 — Eurocode 3: Steel design

• BS EN 1090-2 — Execution of steel structures

• BS EN 10025 parts 1–6 — Hot rolled products of structural steels

• BS EN 10210 / 10219 — Hollow sections (SHS, RHS, CHS)

• BS EN 1492-1 — Lifting slings (textile, flat-woven)

• BS EN 845-1 — Wall ties

• CDM Regulations 2015 — Construction work obligations

• LOLER 1998 — Lifting Operations and Lifting Equipment Regulations

• PUWER 1998 — Provision and Use of Work Equipment Regulations

• HSG150 — Health and safety in construction

• Manual Handling Operations Regulations 1992 — Lifting limits

• Steel for Life — Steel Construction Institute — Steel design and install

• British Constructional Steelwork Association — UK steel industry portal

• GOV.UK — Approved Document A — Structural Building Regs

• GOV.UK — Approved Document B — Fire safety

• Institution of Structural Engineers — Manual for the design of plain masonry — Padstone design

• HSE — Manual handling — Lifting safety

• structural calculations guide — Calc pack content

• party wall agreement guide — Beam work in party walls

• garage conversion guide — Common beam install context

• concrete mix ratios guide — Padstone concrete grade

• building control process — Inspection sign-off