Steel Beams in Domestic Extensions: RSJ Sizing, Padstones & Building Control

Quick Answer: Any steel beam installation in a domestic extension requires a structural engineer's calculation and Building Regulations Part A notification. The engineer will specify the steel section size (e.g., 203 × 133 UB for a 3m span), minimum bearing length (typically 150mm), and padstone specification. Building control must inspect before the beam is enclosed. Never size a beam without a structural engineer's calculation — undersized steels are a life-safety risk.

Summary

Steel beams — most commonly Universal Beams (UBs, informally called RSJs) — are used in domestic extensions to span openings that masonry lintels cannot bridge and to transfer floor or roof loads where walls are removed. Whether it's a rear kitchen extension requiring a wide span to create an open-plan living space, a flat above requiring steel on a ground floor, or an internal wall removal to merge rooms, steel beams are one of the most regulated elements of domestic construction.

The term "RSJ" (Rolled Steel Joist) is technically obsolete — modern steels are Universal Beams (UBs) with parallel flanges, which are stronger than the old tapered-flange RSJs. The terminology persists on site, but when ordering steel or discussing with a structural engineer, use the correct designation.

The regulatory process for steel beam installation is non-negotiable: structural engineer's calculations must be produced before work starts, building control must be notified and must inspect the installation before it is enclosed in plasterboard, and padstones must be correctly specified and installed. Every year, building control teams deal with cases where beams have been installed without notification and must be exposed for retrospective inspection — an expensive and disruptive problem that is entirely avoidable.

Key Facts

• Building Regulations Part A — structural stability; any new or modified structural element (including steels) is notifiable
• Structural engineer required — beam sizing MUST be calculated by a structural engineer; no exceptions for safe practice
• Building control inspection — steel must be inspected before it is enclosed; do not plasterboard over before inspection
• Universal Beam (UB) — standard beam section; designation e.g., "203 × 133 × 30 UB" = 203mm depth × 133mm flange width × 30 kg/metre
• Padstones — required at each bearing point to distribute the concentrated load into the masonry; typically engineering brick or precast concrete
• Minimum bearing length — typically 150mm minimum on each end; engineer to specify
• Fire protection — steel exposed internally in a fire compartment requires intumescent paint or board encasement (Part B); typically 30 or 60 minutes depending on location
• Corrosion protection — external or partially exposed steels require hot-dip galvanising or priming/painting with appropriate primer
• Temporary propping — the structure above must be supported before any existing lintel or load path is removed; Acrow props are standard
• Weight — a 5m × 305 × 165 UB weighs approximately 250kg; always plan the lifting operation before ordering
• Delivery — steels are usually 7–10m lengths; check access before ordering; on-site cutting with angle grinder/disc
• Flitch beam — alternative to solid steel: timber with steel plate sandwich; lighter but less efficient for large spans
• Compound beam — two or more steels bolted or welded together for very large spans or loads; structural engineer design required

Quick Reference Table

Critical: These are indicative only. A structural engineer must calculate the actual section required for the specific loading, span, and bearing conditions.

Detailed Guidance

How a Structural Engineer Sizes a Beam

Understanding the process helps you brief the engineer correctly and manage client expectations:

Load assessment: The engineer establishes what loads act on the beam. These include:

• Dead load: the permanent weight of the structure above (roof weight, floor weight, wall weight, finishes)
• Imposed (live) load: people, furniture, snow
• The load tributary area: the width of floor or roof that the beam supports

Span and bearing: The clear span (the distance between supports) plus the bearing length at each end determine the effective span. A beam spanning 4m clear with 150mm bearing each end has an effective span of 4.3m for calculation purposes.

Section selection: Using BS 5950 or Eurocode 3 (EN 1993), the engineer selects a section that:

• Has adequate bending strength (moment capacity > applied bending moment)
• Has adequate shear strength
• Has adequate deflection (typically span/360 under imposed load)
• Does not fail in lateral torsional buckling (critical for deep sections with no restraint to the top flange)

The result: A specific section designation (e.g., 203 × 133 × 25 UB, Grade S275), bearing length, padstone specification, and propping requirements.

Padstone Design and Installation

A padstone distributes the concentrated point load at the beam end into the masonry below. Without a padstone, the local bearing stress in the masonry exceeds its capacity, causing crushing at the support point.

Sizing a padstone: The padstone area required = beam end reaction force ÷ masonry bearing capacity.

For a typical house extension:

• Masonry bearing capacity: 0.4–2.0 N/mm² depending on brick/block type and mortar
• A common rule of thumb: a padstone for domestic work should extend at least as wide as the beam flange and be at least 100mm deep

Installation:

1. Prepare the bearing zone: clear mortar crumble from the course to receive the padstone; ensure the surface is level and solid
2. Bed the padstone in a full mortar bed (1:3 cement mortar for masonry; structural grout for heavier loads)
3. Allow the mortar to achieve initial set before loading the padstone (minimum several hours; ideally overnight)
4. Check the padstone is level in both directions — a tilted padstone causes eccentric loading and can crack
5. Do not use ordinary house-building brick as a padstone — the compressive strength is too low and variable

Temporary Propping

Before any existing structure is disturbed, temporary propping must be in place. This is both a safety requirement and a Building Regulations compliance issue — the structure must be maintained in a safe condition throughout the works.

Propping arrangement for a load-bearing wall removal:

1. Identify what the wall supports: joists, rafters, floor above, roof load
2. Identify the load path from the existing wall to the new steel
3. Place temporary props 500–1,000mm back from the proposed opening on both sides of the wall
4. Props must bear on spreader boards (150mm × 50mm minimum) top and bottom
5. Top spreader board: must be continuous across the full span; does not need to be against the ceiling if the load is distributed through the floor structure above
6. Bottom spreader board: must spread the load across multiple floor joists; size depends on prop load
7. Never remove an existing lintel or load-bearing wall element until the new steel is in place and bearing fully

Typical propping arrangement for a 3m wide opening:

• 4 × Acrow props in pairs either side of the opening
• 150mm × 50mm timber spreader top and bottom on each pair
• Props extend to both above and below the floor structure if the first floor is also supported

Fire Protection for Steel

Steel loses strength rapidly at elevated temperatures. At 550°C, structural steel retains only about 60% of its room temperature strength. Building Regulations Part B requires structural elements to maintain their load-bearing capacity for a specified fire resistance period.

Domestic requirements (Part B):

• Ground floor steels in ground-floor-only extensions where no floor is above: minimum 30 minutes resistance (REI30)
• First floor steels supporting upper floors or used as floor beams: minimum 60 minutes (REI60)
• Steels in compartment walls or floors: as required by the compartmentation strategy

Providing fire protection:

1. Intumescent paint — water or solvent-based paint applied in multiple coats to achieve required DFT (dry film thickness); typically 1,000–3,000 microns for 30–60 minutes; must be applied to clean, primed steel; manufacturer-specific systems
2. Board encasement — fire-resistant board (Supalux, Knauf Fireboard) fixed in a box profile around the steel; 30–60 minutes depending on board thickness; common method for concealed beams
3. Spray protection — cement-based or mineral fibre spray applied on site; more common for commercial work

For domestic work: Board encasement (boxing in with plasterboard or fire board) is the most common approach. The box around the steel can be plastered or decorated as part of the ceiling/wall finish.

Ordering and Handling Steel

Ordering:

• Contact a local steel stockholder or builders' merchant
• Specify: grade (S275 is standard for most domestic work; S355 for higher strength where depth is limited), section designation (e.g., 254 × 146 × 31 UB), and length
• Standard cut-to-length service available from most stockholders; cutting on site with angle grinder is also possible
• Lead time: usually 1–3 days for standard sections

Weight and handling:

• A 5m × 305 × 165 × 46 UB weighs approximately 230kg — not manageable by hand
• Most domestic steel installations are achieved by 2–4 people using scaffold boards as ramps and ratchet straps
• For longer or heavier sections, a crane or telehandler is required
• Plan the lift carefully: where will the steel be stored before installation? What is the access route? Can the beam be maneuvered through doorways?

Connections:

• Most domestic end-bearing steels have no formal connection to the padstone — they simply bear under gravity
• For cantilever beams, side-bearing steels, or any situation where uplift is possible (e.g., in a high wind uplift zone), bolted connection or anchor bolts are required as specified by the engineer

Frequently Asked Questions

Can I look up a steel size from a table rather than getting an engineer?

No. Span tables for steel beams exist in some guidance documents, but they are based on specific loading assumptions that may not apply to your project. The structural engineer's calculation accounts for the actual loads from the specific structure above, including the contribution of any walls, floors, and roof loads that the building owner's load path analysis has identified. An undersized steel beam in a domestic building can fail catastrophically. The cost of a structural engineer's calculation (typically £300–£800 for a domestic beam calculation) is trivial compared to the cost of a structural failure.

My building control inspector says I need to expose the steel before they can sign it off. The plasterer has already boarded it over. What do I do?

The steel must be exposed for the building control inspection. This means removing the plasterboard or boarding to allow the inspector to verify the section size, bearing length, padstone condition, and fire protection. This is expensive and disruptive — it is entirely avoidable by not boarding over until after the inspection. Contact the inspector before plastering and wait for their visit.

How do I know if a wall is load-bearing?

The indicators that a wall is likely load-bearing:

• It runs perpendicular to the floor joists (can be checked by looking at the joist direction in the loft or floor void)
• It is on the ground floor below a wall on the first floor
• It is an external wall or a central spine wall
• Floor joists are notched or have joist hangers into the wall

Indicators that a wall is likely non-load-bearing:

• It runs parallel to the floor joists
• It is a recent addition (thinner construction, different brick or block)
• It sits above an I-joist or engineered timber floor that could span freely without it

When in doubt, consult a structural engineer. Visual inspection alone can mislead — some walls appear non-structural but carry significant loads through complex load paths.

Does the engineer's calculation cover the connection to the existing wall at each end?

The calculation will specify the bearing length and padstone requirements. The connection to the existing masonry is typically through bearing only (gravity). The structural engineer should review the condition of the masonry at each bearing point and may specify strengthening (additional courses, repointing) if it is inadequate. If the existing wall is in poor condition, the bearing capacity may be less than assumed, and the padstone or bearing zone may need to be engineered more carefully.

Regulations & Standards

• Building Regulations Approved Document A (2004, 2013 amendment) — structural stability; all structural work including steels is notifiable

• BS EN 1993-1-1 (Eurocode 3) — design of steel structures; the current UK structural design standard for steel

• BS 5950 — structural use of steelwork in buildings; older standard still referenced

• Building Regulations Approved Document B — fire safety; fire resistance periods for structural elements

• BS EN 10025 — hot rolled products of structural steels; material specifications (Grade S275, S355)

• Approved Document A (2013 amendment) — structural requirements

• Steel Construction Institute (SCI) — free technical guidance on steel design including simple span beam design

• IStructE: Guide to Domestic Extensions — structural engineering guidance for residential work

• British Steel: Section Properties — UB and UC section tables with weights and properties

• foundations — foundation types supporting the beam bearing points

• party wall — party wall implications of steel beam installation near boundaries

• planning permission — planning requirements alongside structural notifications

• thermal bridging — thermal bridging through steel beam penetrations