Approved Document A: Structural Requirements, Spans and When a Structural Engineer Is Needed

Summary

Approved Document A (ADA) is the Building Regulations guidance document for structural safety in England and Wales. It supports Requirement A of Schedule 1 to the Building Regulations 2010, which states: "The building shall be constructed so that the combined dead, imposed and wind loads are sustained and transmitted by it to the ground safely, and so that the movement of the ground caused by swelling, shrinkage or freezing of the subsoil will not impair the stability of the building."

The document operates on a "deemed-to-satisfy" principle: if the construction follows the dimensional and material limits in the ADA tables, the structural requirement is considered met without the need for structural calculations. This is the basis on which standard masonry houses, loft conversions with standard timber rafters, and straightforward extensions are built without involving structural engineers. However, the moment a project departs from the standard parameters — in span, loading, geometry, or ground conditions — calculations are needed.

Understanding what falls within the deemed-to-satisfy provisions and what requires engineering input is one of the most practically important bodies of knowledge for a builder or construction manager. Getting this wrong in either direction is costly: over-engineering adds unnecessary expense; under-engineering creates safety risk and Building Control rejection.

Key Facts

• Approved Document A 2004 (amended 2013) — the current version for England; Wales has its own edition; Scotland uses Technical Handbook Section 1
• Section 1A — deemed-to-satisfy provisions for houses up to three storeys; masonry and timber construction
• Section 2C — disproportionate collapse requirements; applies to buildings over four storeys and some other building types
• Section 1B — guidance on assessment of existing buildings (relevant for renovation and conversion projects)
• Cavity wall width — maximum 75mm for standard construction; up to 150mm permitted for EWI (external wall insulation) systems with appropriate ties
• Wall ties — stainless steel butterfly or double-triangle wire ties minimum 150mm long; 900mm horizontal × 450mm vertical centres
• Table 5 lintel spans — provides masonry arch and lintel sizing for openings in walls carrying floor and roof loads
• Floor joist span tables (Appendix A) — spans for C16 and C24 at 400mm and 600mm centres; for dead loads 0.5kN/m² and 1.25kN/m² imposed
• C24 timber — the structural grade for most domestic joists; C16 is the minimum; BS EN 338 grades
• Maximum safe span in Appendix A — approximately 4.5–5.0m for standard C24 joist sizes in Appendix A; spans beyond this require engineered timber or structural calculations
• When a structural engineer is required — spans over 6m, unusual loads, load-bearing wall removal, underpinning, retaining walls over 600mm, sloping sites, non-standard construction
• CEng MIStructE / CEng MICE — the minimum qualification levels for structural engineers carrying out residential structural work; CENG is the Chartered Engineer designation from the Engineering Council
• NHBC Chapter 4 — NHBC Buildmark requirements for foundations, paralleling Approved Document A
• BS EN 1997 (Eurocode 7) — geotechnical design standard; referenced for foundation design
• Disproportionate collapse — the requirement that a building does not collapse disproportionately if a structural element fails; relevant for tying and robustness design
• Structural calculations — must be submitted to Building Control before structural work commences; cannot be retrospectively submitted after concrete is poured or steelwork is installed

Quick Reference Table

Detailed Guidance

Structure of Approved Document A

Approved Document A is divided into:

Section 1: Resistance to collapse

• 1A: Dimensions and materials for standard construction (deemed-to-satisfy)
• 1B: Assessment of existing structures
• 1C: Alterations and changes of use

Section 2: Ground movement

• 2A: Strip and trench fill foundation tables
• 2B: Rafted foundations
• 2C: Foundations on shrinkable clay (NHBC/building form guidance)

Section 3: Disproportionate collapse

• Applies to buildings of 5 storeys or more, but can apply to unusual geometries in lower buildings

For standard residential construction — houses, bungalows, extensions up to three storeys — the Section 1A tables and Appendix A (floor joist tables) provide the primary reference.

Masonry Wall Construction: Section 1A Tables

Section 1A provides dimension tables for masonry walls based on:

• Height of the wall (storey height)
• Width of the wall
• Whether the wall is a loadbearing wall, a gable wall, or a compartment wall
• The number of storeys above

Minimum wall thicknesses (selected from Table 2, ADA):

• Ground storey, house up to three storeys: 190mm cavity wall minimum (two 90mm leaves with 10mm+ cavity)
• First storey: 190mm cavity wall
• Second storey: 190mm cavity wall minimum, 150mm if one-storey over

Cavity widths: Standard cavity is 50–75mm; extending the cavity to 100–150mm for full-fill cavity insulation is covered by ADA with appropriate wall ties. For EWI systems (external insulation on the outer leaf), the cavity may be filled and the outer leaf extended — this requires engineering confirmation that the wall tie design is appropriate for the modified cavity geometry.

Lintel Sizing: Table 5

Table 5 of ADA provides lintel spans for walls of known thickness carrying standard loads (floor and roof loads). The table entries are nominal spans — the clear span of the opening plus adequate bearing at each end.

Standard bearing lengths for lintels:

• Steel lintels on masonry: 150mm minimum bearing each end
• Concrete lintels: 100mm minimum bearing each end
• For spans over 1.8m or unusual loads: structural calculation required

Table 5 covers spans up to approximately 3.0m with standard domestic loads. For larger openings (bi-fold doors, sliding glass walls, garage door openings), structural calculations are required regardless of whether the lintel is steel, concrete, or engineered timber.

Padstones: concentrated loads from lintels, beams, or joists bearing on masonry require padstones to spread the load over an adequate area of masonry. Standard padstone specification: engineering brick (Class A or B, ≥50N/mm² crushing strength) or concrete padstone (C25 minimum), minimum 100mm × 200mm × one course height. The structural engineer will specify the padstone size based on calculated bearing stress.

Floor Joist Span Tables: Appendix A

Appendix A of ADA provides span tables for floor joists in C16 and C24 grade sawn timber at 400mm and 600mm centres, for standard domestic loading (dead load 0.5kN/m² and live load 1.5kN/m² for rooms used for general purposes).

Typical spans from Appendix A (C24 at 400mm centres, 50×200mm joist):

• 4.5–4.8m clear span (approximately)

For longer spans, the options are:

1. Increase joist depth (use deeper sections from the table)
2. Reduce joist centres (from 600mm to 400mm)
3. Add a mid-span beam with column support
4. Switch to engineered timber (I-joists, LVL) with manufacturer span tables

Appendix A tables do not cover:

• Point loads from partitions or concentrated loads
• Spans over 4.8m (check current table limits)
• Water tanks or concentrated equipment loads
• Loft conversions with different load patterns

When a Structural Engineer Is Required

1. Spans over 6m (any material) Any horizontal structural element spanning over 6m — floor beam, roof beam, lintel — requires structural calculations. This threshold is a practical guide, not a regulatory rule; technically, structural calculations are needed whenever the deemed-to-satisfy tables are insufficient, which can occur at spans well below 6m with unusual loads.

2. Load-bearing wall removal Any wall that carries floor, roof, or other wall loads must have a structural engineer design the replacement beam, its connections, padstones, and any temporary works. The Building Control inspector will require the engineer's calculations before sign-off.

3. Underpinning Underpinning (deepening existing foundations) requires a site investigation (borehole or trial pit to confirm subsoil), structural calculations for the underpinning method, and a permanent works design. Never underpin without engineering input — incorrect underpinning can cause more settlement than it prevents.

4. Retaining walls over 600mm retained height A retaining wall with more than 600mm of retained soil requires structural design. The design must consider both the vertical stability (overturning) and the bearing capacity of the foundation soils. For retaining walls adjacent to properties, highways, or public spaces, an engineer's design and sign-off is essential.

5. Sloping sites Building on a slope introduces ground pressure, cut and fill requirements, and potential for slope instability. Any new build on a slope steeper than approximately 1:5, or any building with significant fill under the ground floor, requires geotechnical assessment.

6. Non-standard construction Timber frame, SIPs, steel frame, structural glass, CLT (cross-laminated timber), and other non-masonry systems are not covered by the ADA deemed-to-satisfy tables and require structural calculations or a manufacturer's certified design.

7. Removing intermediate floors or staircases in large open-plan conversions These can change the diaphragm action of a floor structure, affecting the overall stability of the building. Engineering assessment is needed.

Structural Calculations Submission to Building Control

Structural calculations must be submitted to the Local Authority Building Control (LABC) or an Approved Inspector before construction of the structural element begins. Key points:

• Calculations are prepared by the structural engineer and submitted alongside or before the Building Notice or Full Plans application
• The BCO (Building Control Officer) reviews the calculations to confirm compliance with Part A
• If BCO requests amendments, the engineer revises the calculations before work proceeds
• For Building Notice applications (where no prior approval is given), calculations may still be requested by the BCO on site inspection
• Always ask the engineer for a copy of the calculations to retain in the client's building records

Calculations format: calculations should include a load takedown (calculating loads at each level), member sizing (beams, columns, lintels), connection design (bolts, welds, anchors), and foundation bearing pressure checks. Drawings should show the structural layout clearly.

Frequently Asked Questions

Do I need a structural engineer to remove a chimney breast?

Yes, in most cases. Chimney breasts are load-bearing in the sense that the chimney stack above relies on the breast below for support. Removing the breast at any level while the stack remains above requires a steel beam, padstones, and careful temporary support during works. Engineering calculations are needed.

Can I use BRE guidance instead of structural calculations?

BRE (Building Research Establishment) publications provide guidance on some structural topics, but they are not a substitute for site-specific structural calculations. Approved Document A's deemed-to-satisfy tables are the appropriate shortcut for standard construction; for non-standard work, calculations are required.

How do I find a structural engineer?

The Institution of Structural Engineers (IStructE) has a member search at istructe.org. Look for Chartered Members (AMIStructE does not hold CEng status; MIStructE does). For residential projects, ask for recent residential experience — a large commercial engineer may charge more and communicate less effectively than a smaller practice specialising in residential and extensions work.

What is the difference between Full Plans and Building Notice for structural work?

A Full Plans application requires you to submit all plans and structural calculations before work starts. The BCO reviews and approves in advance — if the plans are approved, you have certainty that the work is compliant before you start. A Building Notice does not require prior plan submission; the BCO inspects during construction. For structural work with complex engineering, Full Plans is recommended to avoid costly revisions after construction begins.

How long are structural calculations valid?

Structural calculations do not expire, but they are site-specific and load-specific. If the design loads change (adding a floor above, changing the roof structure, converting a loft) the original calculations should be reviewed by the engineer to confirm they are still valid.

Regulations & Standards

• Approved Document A: Structure (2004, amended 2013) — Building Regulations structural guidance for England

• Building Act 1984 — primary legislation

• Building Regulations 2010 (SI 2010/2214, Schedule 1, Part A) — the structural requirement

• BS EN 1995-1-1:2004 (Eurocode 5) — design of timber structures

• BS EN 1992-1-1:2004 (Eurocode 2) — design of concrete structures

• BS EN 1993-1-1:2005 (Eurocode 3) — design of steel structures

• BS EN 1997-1:2004 (Eurocode 7) — geotechnical design; foundations

• BS 8103-1:2011 — structural recommendations for loadbearing walls in masonry construction

• Approved Document A Full Text — official current version

• IStructE: Find a Structural Engineer — Institution of Structural Engineers member directory

• LABC: When Do You Need Building Regs? — Building Control guidance for residential projects

• NHBC Standards Chapter 4 — foundations and structural requirements for NHBC warranty

• BRE: Structural Guidance — Building Research Establishment publications

• nhbc warranty — NHBC structural requirements and warranty period

• roof trusses — truss design and modification rules

• engineered timber — LVL, glulam and I-joist specifications

• planning vs building regs — when Building Regs apply vs planning permission