When Do You Need a Structural Engineer? Beam Calcs, Underpinning & Load-Bearing Walls

Summary

The question "do I need a structural engineer?" comes up on almost every domestic extension or alteration project that involves removing walls, installing beams, or digging new foundations. The honest answer is that many smaller structural elements — like standard timber lintels over windows or a simple single-storey extension on firm ground — can be specified by an experienced builder using standard span tables and published guidance without a formal engineer's input. But anything outside the standard tables, any steel beam carrying significant load, and anything involving ground movement or existing foundations requires a structural engineer.

Building control bodies routinely request structural calculations as a condition of plan approval. If an engineer's calculations are required, they should be produced by a Chartered Structural Engineer (MIStructE or CEng with the Institution of Structural Engineers) or a Chartered Civil Engineer with structural experience. The engineer takes professional responsibility for the design, providing a signed and stamped calculation pack and typically a drawing showing the structural scheme.

Structural engineers are also involved in forensic investigations — assessing the cause of cracking, diagnosing subsidence, recommending underpinning, and advising on whether cracks in walls are structural or cosmetic. For any property showing signs of serious movement (cracking over 15mm wide, doors and windows sticking, visible deflection in floors or roofs), a structural engineer's inspection is strongly advised before any building work begins.

Key Facts

• When building control requires calcs — for any structural element that cannot be specified from standard tables; conditions are common on Full Plans approvals
• MIStructE / CEng — qualified structural engineer designations; look for these when appointing
• RSJ/universal beam — steel beams used to support loads when walls are removed; size determined by span, load, and support conditions; must be calculated by or checked by an engineer
• Padstones — concrete or engineering brick bearing pads under beam ends; size specified in structural calculations; typically 215x215mm minimum for domestic beams [verify for specific loads]
• Propping — temporary propping of the structure above any opening must be designed and supervised; inadequate propping is a common cause of structural collapse during domestic projects
• Underpinning — deepening or replacing existing shallow foundations to prevent or arrest settlement; requires a structural engineer; traditionally mass concrete underpins in 1.0m bays; now often mini-piled or resin injection
• Load-bearing wall identification — a wall running at right angles to floor joists, a wall with beams sitting on it, a wall with thickened footings, or any wall on the structural engineer's drawings is likely load-bearing
• Timber span tables — Approved Document A (Structure) includes span tables for floors, rafters, and joists; these are acceptable without calculations for most routine domestic timber work
• Eurocode 5 (BS EN 1995) — design of timber structures; used by engineers for non-standard timber design
• Eurocode 3 (BS EN 1993) — design of steel structures; used by engineers for steel beam design
• Eurocode 7 (BS EN 1997) — geotechnical design; applies to foundations, underpinning, and ground investigation
• Ground investigation — for underpinning and new foundations on suspect ground, a soil investigation (trial pit or borehole) is required before the structural engineer can design the foundations
• Party wall — if structural work involves a party wall, the Party Wall Act 1996 applies separately to building control; a party wall surveyor may be needed as well as a structural engineer
• Cost of structural calculations — typically £300–£1,500 for simple domestic beam calcs; more for underpinning or complex projects

Quick Reference Table

Detailed Guidance

Identifying Load-Bearing Walls

Before removing or altering any wall, determine whether it is load-bearing. Warning signs that a wall IS load-bearing:

• Runs perpendicular to floor joists (floor joists bear on it)
• Has a beam or joist immediately above it in the floor or roof structure
• Continues through multiple floors with walls directly above it
• Has thicker or deeper foundations than partition walls
• Is marked as structural on any existing drawings (if available)
• Is an external wall (almost always structural)

A partition wall that runs parallel to floor joists, sits on a floating floor without direct foundation support, and carries no joists is typically non-load-bearing — but always verify before removing.

When in doubt: commission a structural engineer's opinion visit (typically £200–£400 for a site visit and written opinion) before proceeding.

Steel Beam Design — What the Engineer Calculates

For a typical domestic steel beam (RSJ/Universal Beam) over a removed load-bearing wall, the structural engineer will:

1. Establish the load — calculate the weight of structure above the beam (floor loads, roof loads, wall loads) using standard loads from BS EN 1991 (Eurocode 1)

2. Determine span and support conditions — the clear span of the opening plus the required bearing length at each end (typically minimum 100mm on masonry, ideally 150mm)

3. Select beam size — calculate the required section modulus and second moment of area; select from the standard universal beam (UB) catalogue

4. Design the padstones — size concrete or engineering brick padstones at each end to spread the concentrated load into the masonry below

5. Check deflection — ensure the beam deflects no more than span/360 under full load

6. Issue calculations — signed and stamped calculation pack submitted to building control as part of the Full Plans application or as a condition discharge

Underpinning — When and How

Underpinning is the deepening or replacement of existing foundations. It is required when:

• The existing foundations are too shallow and soil is moving (clay shrinkage/swelling, tree root desiccation)
• Additional load is being applied to existing foundations (e.g. adding a storey)
• Adjacent excavation has undermined existing foundations

Traditional mass concrete underpinning is done in alternating 1.0m bays to avoid destabilising the whole wall at once. Bays are dug by hand to the new founding level, inspected by building control, and poured with concrete before moving to the next bay.

Mini-piled underpinning is less disruptive — small-diameter piles are drilled or driven adjacent to the existing foundations and connected by a ground beam.

Resin injection is a newer technique for minor ground consolidation and foundation strengthening; less suitable for significant settlement.

All underpinning requires a structural engineer's design, ground investigation (trial pits or boreholes), and building control involvement.

Ground Investigation for Extensions and Underpinning

For any extension on suspect ground (made ground, known filled land, properties in areas of mining, properties with large trees nearby, or properties showing signs of movement), a ground investigation is required before foundation design.

Trial pits — excavated by JCB to 2–4m depth; the soil profile is described and recorded; disturbed soil samples taken for lab testing. Most straightforward.

Boreholes — drilled investigation to greater depths; used when piles or deep foundations are being considered.

Tree proximity — trees can cause clay soils to shrink significantly in summer. The National House Building Council (NHBC) publishes guidance on foundation depth relative to tree species and distance [verify current NHBC standards chapter relevant to this].

Chimney Breast Removal

Removing a chimney breast is a common project that absolutely requires structural engineer input. The chimney stack above continues to sit on chimney breasts in the floors above. If a ground-floor chimney breast is removed:

• The chimney breast in the room above is left unsupported
• It must be corbelled (brick projection supported by a steel angle or beam) from the party wall or supported on a steel frame
• The structural engineer specifies the corbel design and the required support

Building control approval and structural calculations are mandatory for chimney breast removal.

Frequently Asked Questions

My builder says he doesn't need structural calculations for a simple beam — is he right?

For very small spans using standard timber lintels (up to about 1.2m), he may be correct if the design follows the standard tables in Approved Document A or the lintel manufacturer's tables. For any steel beam, or for timber spans beyond the standard tables, calculations are required by building control. A builder who says "I've done this a hundred times, I know what size beam to use" may be experienced, but without signed calculations, neither he nor you have a professional document that indemnifies the design.

How do I find a structural engineer for domestic work?

The Institution of Structural Engineers has a directory at istructe.org/find-an-engineer. Also search for local practices via the RICS directory or ask your architect or building control officer for recommendations. For simple domestic work, a local practice will be more cost-effective than a large commercial firm.

My wall has cracks — do I need a structural engineer?

It depends on the cracks. Fine hairline cracks in plaster are almost always cosmetic and caused by thermal movement or normal building settlement. Cracks over 5mm wide, diagonal cracks at window or door corners, cracks that are wider at the top than the bottom, or any crack that appears to be growing are warning signs. A structural engineer's inspection is recommended for cracks over 15mm wide or cracks showing differential movement. The Building Research Establishment (BRE) Digest 251 classifies crack severity for guidance [verify current edition].

Regulations & Standards

• Approved Document A (Structure) — Building Regulations guidance on structural requirements; includes span tables for domestic timber elements

• BS EN 1990 Eurocode — Basis of structural design: adopted as the UK structural design standard framework

• BS EN 1993 Eurocode 3 — Design of steel structures: used for beam calculations

• BS EN 1997 Eurocode 7 — Geotechnical design: applies to foundation design and underpinning

• Building Act 1984 — authorises structural requirements within Building Regulations

• Institution of Structural Engineers — Find a structural engineer directory

• RICS — Structural Surveys — Guidance on structural inspections and reports

• Approved Document A (Structure) — Free to download from Planning Portal

• NHBC Standards — Foundations — Foundation depth guidance including tree proximity tables

• building control process — Full Plans and Building Notice application process

• structural steel — RSJ installation, padstones, and temporary propping

• foundations — Foundation types and building control requirements

• building control — What work requires building control