Bi-fold Door Installation

Quick Answer: Bi-fold doors require a structural lintel or steel beam above the opening, FENSA or CERTASS certification if replacing existing glazing, and must meet Part L (U-value ≤1.4 W/m²K), Part Q (PAS 24:2016 security), and Part M (≤15mm threshold) of the Building Regulations. Installed aluminium bi-folds typically cost £800–£1,200 per m² of opening.

Summary

Bi-fold doors have become one of the most requested rear extension features in the UK over the past decade. The appeal is straightforward: a wide, unobstructed opening that connects an interior space to a garden or terrace, with panels that stack neatly to one side rather than swinging inward. But the installation is more technically demanding than it appears from a finished photograph.

The structural requirement is non-negotiable and frequently underestimated. Any opening wider than roughly 1.8m in a load-bearing wall requires a beam — usually a steel RSJ or SHS section — capable of carrying the load above. Getting this calculation wrong is a significant structural risk. Beyond the steel, Building Regulations apply to the glazing performance, the security of the units, the threshold height, and the waterproofing of the sill. Each of these areas has specific, measurable requirements.

This guide covers the structural, regulatory, and practical elements of bi-fold door installation for both the tradesperson quoting and specifying the job, and the homeowner commissioning it. The two most common failure modes are inadequate structural support and threshold waterproofing — both are addressed in detail below.

Key Facts

• Structural lintel always required — any opening wider than a standard door needs a properly sized steel or concrete beam; do not rely on the existing brickwork to span unsupported
• Part L compliance — glazed elements including doors must achieve a U-value ≤1.4 W/m²K in England (Approved Document L, 2021 edition)
• Part Q security — bi-fold doors must meet PAS 24:2016 for security performance; most major manufacturers supply Part Q-compliant units as standard
• Part M threshold — maximum 15mm threshold upstand where step-free access is required; flush (zero threshold) systems are available for level access
• FENSA or CERTASS — replacing any existing external glazing (doors or windows) requires notification to a competent person scheme; failure to certify means the installation cannot be verified at resale
• Folding direction — inward-folding panels require internal floor space when open; outward-folding require clear external space and are more exposed to weather damage
• Panel count — even-numbered panel configurations allow equal stacking to one side; odd numbers create an off-centre stack
• Minimum panel width — typically 600mm; maximum single panel width around 1,200mm (check manufacturer specification)
• Track systems — bottom-hung (rollers on lower track) are most common; top-hung systems offer a cleaner sill but are heavier and require stronger top beam fixing
• Low-profile threshold — most residential bi-folds have a raised threshold of 15–20mm; fully flush thresholds use a drainage channel recessed into the floor
• Drainage channel — flush thresholds require a linear drain (ACO or equivalent) set in the floor at the opening line; must be connected to drainage
• Aluminium frames — thermally broken aluminium is the industry standard; non-thermally broken aluminium cannot meet Part L
• CE marking — glazed units must carry CE (or UKCA) marking confirming compliance with BS EN 14351-1
• Opening tolerances — structural opening must be plumb, level, and square to ±3mm across the full span before frame installation
• Frame fixings — into masonry: M10 expanding anchors at 600mm centres; into timber stud: coach screws into noggins minimum 38mm × 89mm
• Head beam deflection — maximum L/300 deflection under full load; for a 3m span that is 10mm maximum

Quick Reference Table

Beam sizes are indicative for standard residential loading — always confirm with a structural engineer.

Detailed Guidance

Structural Lintel and Beam Sizing

The most important element of any bi-fold door installation is the beam above the opening. In a load-bearing wall, removing brickwork to create the opening transfers load to whatever is spanning above. The beam must be sized to carry:

• The dead load (weight of masonry, floors, or roof above)
• The imposed load (live loads on floors or roof above)
• Its own self-weight

For residential rear extensions and ground-floor openings where only a single storey sits above, a 203×133 Universal Beam (UB) at 25 kg/m is frequently adequate for spans up to 3.5m, but this must be confirmed by a structural engineer for every project. Variables include wall thickness, storey height, roof type, and whether the wall is load-bearing at all.

The beam must bear onto the masonry at each end for a minimum of 150mm (or as specified by the engineer). In cavity wall construction, padstones (engineering brick or concrete) are placed under each end of the beam to distribute point loads safely.

In new extension walls built to house a bi-fold, the structural frame (usually steel or timber portal) can be designed from the outset to carry the opening. The lintel question then becomes about the frame design, not retrofitting into existing masonry.

SHS sections (Square Hollow Sections) are sometimes used in place of UBs where a shallower depth is needed — for example, where head room is tight. An engineer should confirm the SHS specification as they behave differently to open sections under bending load.

Opening Preparation — Squareness and Level

The opening must be:

• Level — the sill line across the full width to within ±2mm
• Plumb — both jamb faces vertical to within ±3mm over the full height
• Square — diagonal measurements equal to within ±3mm

Most bi-fold frame systems have some adjustment built in (typically ±5mm), but starting with an out-of-square opening consumes that adjustment and leaves no margin for levelling the track. A spirit level and a 3m straight edge are the minimum tools for checking before ordering the frame.

Order the frame only after the structural opening is complete and measured. Frames are made to order — incorrect measurements result in re-ordering delays of 3–6 weeks.

Track, Threshold, and Weatherproofing

The lower track is the most vulnerable part of the installation from a weather and water management perspective. Key points:

Standard raised threshold (15–20mm): The most common and most reliable system. The threshold sits proud of the internal floor level, creating a physical barrier to wind-driven rain. A silicone mastic seal between the frame sill and the structural opening prevents water ingress around the frame perimeter.

Low-profile threshold: Achieves Part M compliance (≤15mm). The track is recessed into a sill channel, with a drainage groove that directs water away from the opening. Requires careful coordination with the floor finish level — the sill channel must be set in the structural floor, not the floor finish.

Flush threshold (zero upstand): Fully step-free access. Requires a linear drainage channel (ACO channel or similar) set in the floor immediately inside or outside the opening, connected to a surface water drain. The floor level inside and outside must be precisely matched — typically within 2mm. This is the most complex threshold to execute correctly and the most prone to flooding if the drainage is inadequate.

DPC under sill: The sill frame sits on a DPC slip (typically butyl rubber or lead-free DPC strip), isolating it from the masonry and preventing moisture transfer. The DPC extends behind the frame on each jamb.

Frame Types — Aluminium, uPVC, and Timber

Aluminium (thermally broken): The dominant choice for UK bi-folds. Aluminium is strong enough in thin section to allow narrow sightlines, and the thermal break (a polyamide strip separating inner and outer aluminium extrusions) achieves the required U-values. Powder-coated finish is standard; RAL colour choices are wide. Aluminium systems are low-maintenance and dimensionally stable.

uPVC: Lower cost. Achieves Part L and Part Q requirements. Wider sightlines than aluminium (profiles are bulkier) and limited to standard white/woodgrain foil finishes in most product ranges. Suitable for budget projects. Not generally used for spans above 4m due to rigidity limitations.

Timber: Premium appearance. Hardwood (oak, sapele) or engineered softwood. Requires ongoing maintenance (repainting or re-oiling every 3–5 years). Can achieve Part L with modern double-glazed units but engineering the track and hardware to match aluminium standards is harder — review manufacturer data carefully. Typically only used where aesthetic is the priority.

Building Regulations — Part L, Part Q, Part M

Part L (Energy efficiency): Approved Document L (2021) applies to replacement and new glazing. The threshold U-value for doors in England is ≤1.4 W/m²K as a whole-unit figure. For large glazed areas, the overall fabric energy efficiency of the building must not be worsened. For extensions, the total area of glazing should not exceed 25% of the floor area of the extension plus any existing windows or doors that are being replaced.

Part Q (Security): Applies to all new dwellinghouses and extensions. Bi-fold doors that provide access to the dwelling must meet PAS 24:2016 — the BSI publicly available specification for enhanced security performance for windows and doorsets. Most branded aluminium bi-fold systems (Schüco, Smart, AluK, Origin) supply Part Q-compliant units with the appropriate hardware. Verify compliance in writing before ordering.

Part M (Accessibility): Approved Document M Volume 1 (dwellings) requires that where step-free access is reasonably achievable, the threshold height should not exceed 15mm. For a ground-floor rear extension opening to a level garden, step-free access is usually achievable. A fully flush threshold is not mandated — the ≤15mm low-profile threshold satisfies Part M.

FENSA and CERTASS — Notification Requirements

Any replacement of an existing window or door with glazing in England and Wales requires either:

1. The work to be carried out by a company registered with a competent person scheme (FENSA or CERTASS), who self-certifies compliance, or
2. The work to be notified to the local authority building control and inspected

FENSA covers window and door installations specifically. CERTASS is a broader competent person scheme. Both issue a certificate of compliance after installation, which is required by conveyancers when the property is sold. For new openings (not replacement), the Building Regulations notification route through the local authority applies to the structural and Part L aspects. The FENSA/CERTASS requirement specifically covers the glazed units.

See FENSA registration and certification for how to register and what the certification covers.

Setting Out and Installation Sequence

1. Confirm structural opening is complete and beam is in place (minimum 48 hours after padstone mortar)
2. Measure opening — check level, plumb, square, and diagonal dimensions
3. Order frame to confirmed dimensions
4. Prepare sill — lay DPC slip, set any drainage channel
5. Offer up bottom track, level precisely, fix to structural sill
6. Fix head track — typically bolted to underside of steel beam or lintel soffit
7. Fix jamb frames — plumb, fixed to masonry or timber at 600mm centres
8. Hang panels starting from the slave end (fixed pivot panel)
9. Adjust rollers for level operation and even panel gaps
10. Install threshold bar and seals
11. Commission — check all panels fold smoothly, locking mechanism engages, no binding at extreme temperatures (aluminium expands approximately 23 µm per metre per °C)

Typical Costs

Installed costs for aluminium bi-fold doors in the UK (2024–2025 market):

• Standard 3-panel aluminium bi-fold (2.4m span): £3,000–£5,500 installed
• 4-panel aluminium bi-fold (3.2m span): £4,500–£7,500 installed
• 5-panel or wider (4m+): £6,000–£12,000 installed
• Per m² rate: £800–£1,200 per m² of opening (supply and install)

Flush threshold specification adds £500–£1,500 depending on drainage requirements. Timber bi-folds are typically 30–50% more expensive than equivalent aluminium.

Frequently Asked Questions

Do bi-fold doors need planning permission?

In most cases, no. Installing bi-fold doors in an existing external wall, or in a new extension built under permitted development, does not require a separate planning application for the doors themselves. If the extension itself requires planning permission (see permitted development rules), the doors are covered by that consent. In conservation areas, the choice of frame material and finish may be subject to planning conditions.

What is the maximum span for bi-fold doors without a structural engineer?

There is no safe maximum you can apply without a calculation. Any opening in a load-bearing wall requires a structural assessment regardless of span. Even a 1.8m opening needs a properly specified lintel. For openings above 2.5m, a steel beam is almost always required, and for spans above 4m, the beam specification and bearing details must be engineered.

Can bi-fold doors be fitted to an existing brick wall?

Yes, but it involves cutting out the existing masonry, temporarily propping the structure above, inserting a steel beam, and making good the reveals. This is more expensive and disruptive than installing in a new extension. The structural and building control requirements are identical.

How many panels can bi-fold doors have?

Most domestic installations use 3–7 panels. The practical limit is determined by the total opening width and the minimum panel width (typically 600mm). Very wide openings (above 6–7m) are usually better served by a combination of bi-fold sections or a sliding system, as the panel stack becomes very deep at one side.

What maintenance do bi-fold doors require?

Aluminium bi-fold doors require cleaning of tracks (quarterly — debris in the track causes roller wear), lubrication of rollers and pivots (annually with a dry or PTFE lubricant — not oil-based lubricants which attract dust), and inspection of seals and mastic (annually, resealing as required). The powder-coat finish is low-maintenance; mechanical damage should be touched up with matching powder-coat paint to prevent corrosion.

Regulations & Standards

• Building Regulations Approved Document L (2021) — energy efficiency requirements for glazed elements; U-value ≤1.4 W/m²K for doors

• Building Regulations Approved Document Q (2015, amended 2016) — security of dwellings; references PAS 24:2016

• Building Regulations Approved Document M Volume 1 (2015) — access to and use of dwellings; threshold height requirements

• PAS 24:2016 — Enhanced security performance requirements for doorsets and windows in the UK; mandatory reference standard for Part Q compliance

• BS EN 14351-1 — windows and doors; performance characteristics and classification; mandatory for CE/UKCA marking

• BS 8213-4 — installation of windows and doors; code of practice covering fixing, sealing, and DPC requirements

• Town and Country Planning (General Permitted Development) (England) Order 2015 — Schedule 2, Part 1; sets limits for domestic extensions

• The Competent Person Schemes (Amendment) Regulations — governs FENSA, CERTASS, and other self-certification schemes

• Planning Portal — Windows and doors

• FENSA — What needs FENSA certification

• GOV.UK — Approved Document L 2021

• GOV.UK — Approved Document Q

• FENSA registration and certification — how to register, what self-certification covers, and what to do if a certificate is missing at resale

• Permitted development rules for extensions — when a planning application is needed before the doors can go in

• Garage conversion guide — structural and building regulations considerations for converting adjacent spaces