When you open up a wall in a house extension — to create an open-plan kitchen or to integrate the extension with the existing house — a structural steel or timber beam must be installed to carry the loads that were previously carried by the wall. Understanding how these beams are sized, what factors the structural engineer considers, and what the installation involves helps you plan your project effectively and ask the right questions of your design team.

Why Are Structural Beams Needed?

Walls in houses are not just partitions — many are structural, carrying the loads of floors, roofs and walls above and transferring them to the foundations below. When a structural wall is removed or partially removed to create an opening, those loads must be redirected to a beam spanning the opening, which then transfers the load down to the remaining structure at each end (typically onto padstones, columns or existing walls).

Common situations where a structural beam is needed in an extension project:

• Removing the rear wall between the existing house and the new extension to create an open-plan space
• Creating a wide opening (doors or glazing) in a load-bearing wall
• Installing a loft conversion dormer or hip-to-gable where the new roof structure spans between new supports
• Opening up the ground floor to span the void below a first-floor extension
• Bridging the opening above a large set of bifold doors in a rear extension

Types of Structural Beam Used in UK Extensions

Universal Beam (UB) — “I-Beam” or “RSJ”

The most common structural steel section used in domestic extensions. A Universal Beam has a distinctive I-shaped cross-section with wide flanges top and bottom. The UB is specified by its nominal depth and weight per metre — for example, a 178x102x19 UB is approximately 178mm deep, 102mm wide and weighs 19 kg/m.

UBs are efficient in bending — the wide flanges at top and bottom are where the stress concentrates during bending, and most material is placed there rather than in the web (the vertical plate between the flanges). This efficiency makes them the standard choice for spanning openings in domestic extensions.

Universal Column (UC)

A UC has a more square cross-section than a UB — it is designed for compression loading (columns) but is sometimes used as a beam where depth is restricted and a squatter profile is acceptable.

Timber Beams (LVL, Glulam, Solid Timber)

For shorter spans or where exposed timber is desired aesthetically, engineered timber beams (Laminated Veneer Lumber or Glulam) can be used. They are lighter than steel, can be left visible, and are sustainable. They are typically not suitable for spans over 5–6m without very large sections due to the lower stiffness of timber compared to steel.

Flitch Beams

A flitch beam is a sandwich of a steel plate between two timber planks, bolted together. Used historically and occasionally today where timber is aesthetically preferred but additional stiffness is needed. Generally less efficient than a standard UB and requires careful detailing.

How Does a Structural Engineer Size a Beam?

Beam sizing is an engineering calculation based on the loads the beam must carry and the span it must bridge. The key variables are:

• Span: The distance between supports. A 4m opening requires a beam spanning at least 4m (plus the bearing at each end, typically 150mm per side).
• Load: What the beam is carrying — this includes the dead load (weight of the structure above, including walls, floors, roofs) and the imposed load (live loads from occupants and furniture). Loads must be calculated from the area of structure being supported.
• Deflection limit: Beams must not deflect excessively under load — excessive deflection causes plaster cracking and structural damage. The typical limit is span/360 or less under imposed load.
• End support: The beam must bear on adequate supports — typically a padstone (a steel bearing plate or concrete block) at each end, designed to distribute the concentrated beam reaction into the wall or column below.

Typical beam sizes for domestic openings (as a guide only — always get a qualified structural engineer to calculate for your specific situation):

• 2m opening carrying roof only: 152x89x16 UB or similar
• 3m opening, two storeys above: 203x133x25 UB or similar
• 4m opening, two storeys above: 254x146x31 UB or similar
• 5m opening (large bifolds), two storeys: 305x165x40 UB or similar
• 6m full-width opening, two storeys: 356x171x45 UB or 406x178x54 UB or similar

Note: These are illustrative only. Always commission a structural engineer to calculate the correct beam size for your specific loading and span.

The Installation Process

Installing a structural beam in an existing wall involves:

1. Temporary propping: The structure above the opening is propped using Acrow props or engineered propping to transfer the load safely during the beam installation. This must be done before any of the wall is removed.
2. Wall removal: The masonry or timber stud wall in the opening is carefully removed. For masonry, this is typically done by hand to avoid excessive vibration.
3. Padstone installation: Concrete or steel padstones are bedded on each end of the opening to distribute the beam bearing reaction into the wall below.
4. Beam installation: The steel beam is lifted into position (this often requires a scaffold lift or temporary crane for heavier sections) and placed on the padstones.
5. Intumescent fire protection: Steel beams in domestic construction must be protected against fire — typically with intumescent paint or by encasing in fire-rated plasterboard. This is checked during Building Control inspection.
6. Propping removal: Once the beam is in place and the bedding mortar has cured, the temporary propping is removed and the structure is transferred to the new beam.

Building Control and Structural Calculations

Building Regulations require a structural engineer’s calculations to be submitted to Building Control before work starts (for a Full Plans application) or before the relevant structural stage is reached (for a Building Notice). Building Control will inspect the open wall before the beam is installed and again after installation to confirm compliance with the structural design.

Crown Architecture & Structural Engineering provides structural calculations and engineering drawings for all beam installations as part of the in-house engineering service.

Frequently Asked Questions

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

A structural engineer must make this assessment based on inspection of the construction. General indicators that a wall is load-bearing include: it runs perpendicular to the floor joists; it is a gable or external wall; it aligns with walls on other floors. However, these are only indicators — only a structural inspection confirms whether a wall is load-bearing.

Can I install a beam myself?

Removing a load-bearing wall and installing a structural beam is a high-risk activity that requires proper temporary propping and engineering supervision. It must be notified to Building Control. While a competent person can do some of the physical work, a structural engineer must design the beam and padstones, and a Building Control inspector must approve the work. Crown Architecture strongly advises against DIY structural work.

How much does a structural beam cost to supply and install?

A typical domestic structural beam installation costs £2,000–£6,000 including temporary propping, steel supply, installation, padstones and fire protection. The structural engineering calculation and drawings add £500–£1,500. Larger spans or more complex loading situations cost more.

Can Crown Architecture provide structural calculations and beam design?

Yes. Crown Architecture & Structural Engineering provides structural calculations, beam design and Building Regulations drawings for all structural opening works as part of the in-house engineering service. Call 07443 804841 to discuss your project.

Get Expert Structural Engineering for Your Extension

Every structural beam in your extension needs to be designed by a qualified engineer. Crown Architecture & Structural Engineering provides in-house structural engineering alongside architectural design, ensuring your project is structurally sound and Building Control compliant.

Call 07443 804841 or use the form above to get started.