Bi-fold Door Installation: A Tradesperson's Guide

Summary

Bi-fold doors have become one of the most requested items on extension and renovation jobs, and they are a profitable install for joiners, builders and window fitters who get the preparation right. The doors themselves are precision-engineered units — aluminium, timber or uPVC — running on a track system, and they will only perform as well as the opening they sit in. A bi-fold that drags, fails to fold flat, or lets in draughts is almost always a symptom of an opening that was not level, square and structurally stable before the unit went in.

The work is not just carpentry. There is a structural element — bi-folds usually span a wide opening that needs a correctly sized steel beam or lintel above it, and on a new opening that means a structural calculation and Building Control involvement. There is a weatherproofing element — the threshold detail and cavity tray determine whether the customer gets a dry floor or a puddle. And there is a compliance element — replacement glazing is notifiable work under Part L, and on an extension the doors form part of the overall energy and structural sign-off.

The misconception to correct early: customers think the door is the job. The door is the easy part. The opening, the beam, the threshold and the cill are the job. Quote accordingly, and never price a bi-fold install without seeing — or specifying — what is happening above and below the opening.

Key Facts

• Part L compliance — replacement external doors with more than 50% glazed area must meet the energy standard; the practical target is a whole-door U-value of 1.4 W/m²K or better (verify the current Approved Document L figure at point of install).
• Notification — installing replacement doors is notifiable work; either self-certify through FENSA or CERTASS, or submit a Building Notice to Building Control.
• Structural opening — a new or widened opening needs a steel beam or lintel designed by a structural engineer and approved by Building Control under Part A.
• Opening tolerance — bi-fold tracks demand a level and square opening; most manufacturers specify the head and cill must be level within roughly 2–3mm over the full span.
• Threshold options — a rebated/weathered threshold (better weatherproofing, small upstand) or a flush/low threshold (level access, higher water ingress risk needing careful detailing and drainage).
• Glazing — toughened safety glass to BS 6206 / BS EN 12600 is required in doors and side panels (critical locations under Part K/N); double or triple glazed sealed units.
• Configuration — described as number of panels and fold direction, e.g. "3-pane, 3+0" or "5-pane, 3+2"; a traffic/access door allows use without folding the whole set back.
• Cavity tray and DPC — a cavity tray with weep vents must sit above the opening and the DPC must be continuous around the reveals and threshold.
• Lead times — made-to-measure bi-folds typically run 4–10 weeks; never start demolition until the unit is confirmed and measured.
• Survey, don't assume — always survey the structural opening yourself or have it surveyed; manufacturer sizes are finished sizes and need a fitting tolerance.

Quick Reference Table

Detailed Guidance

Getting the opening right

Bi-folds are unforgiving. A standard hinged door has one pivot point and tolerates a slightly out-of-true frame; a bi-fold has a multi-panel track that must run true along its whole length or the leaves bind, sag or fail to align when folded. Before the unit arrives:

• The structural opening must be square — measure both diagonals, they should match.
• The head must be level — a sagging or uneven beam underside will telegraph straight into the track.
• The cill must be level and flat — pack and bed the threshold dead level; this is where most callbacks come from.
• The opening must be the correct size — finished door size plus the manufacturer's fitting tolerance (commonly 10–15mm per side and head). Too tight and you cannot adjust; too loose and you have a large gap to make good and seal.

The structural element

Most bi-folds span an opening wider than a standard lintel. On a new opening or a widened one, this means a steel beam (RSJ/UB) sized by a structural engineer, with padstones or spreader plates at the bearings, and Building Control approval under Part A. You cannot eyeball this. See structural steel for beam selection and bearing detail and structural calculations for when an engineer's calculations are required. Temporary propping must be in place before any masonry is removed — get this wrong and you have a structural failure, not a callback.

Threshold and weatherproofing

The threshold detail decides whether the customer's floor stays dry. Key points:

• A cavity tray with weep vents must sit above the opening to throw water out of the cavity.
• The DPC must be continuous around the reveals and link to the cavity tray and the threshold.
• A drainage channel in front of a low or flush threshold is strongly recommended, especially on exposed elevations — it gives wind-driven water somewhere to go.
• Bed the threshold on a continuous bead of suitable sealant/mastic, not spot-fixed, so there is no path for water under the cill.
• Where the internal floor finish meets a flush threshold, detail the junction so the door's drainage cannot back up into the room.

Glazing and safety

Doors and any side screens are "critical locations" under the glazing safety requirements: the panes must be toughened safety glass marked to BS 6206 or BS EN 12600. Specify the sealed units to meet the current Part L whole-door U-value target — double glazing with a warm-edge spacer and low-E coating usually achieves it; triple glazing where the customer wants better performance. Confirm the manufacturer's U-value certification covers the whole door, not just the centre-pane glass figure.

Notification and sign-off

Replacing or installing external doors with significant glazing is notifiable. The two routes:

1. Competent Person Scheme — if you or the supplier-installer is registered with FENSA or CERTASS, the work is self-certified and the customer gets a certificate. This is the normal route for a straight replacement.
2. Building Control — submit a Building Notice. This is the route when there is no registered installer, or when the bi-fold is part of a wider project (an extension) that already has a Building Control application running. The structural opening is signed off under the same application.

If the bi-fold is part of an extension, it folds into the extension's Building Control sign-off — but the glazing performance still has to be demonstrated.

Frequently Asked Questions

Do bi-fold doors need planning permission?

Replacing existing doors or installing them into an extension that already has consent does not need separate planning permission. Forming a large new opening in a rear wall is normally permitted development, but planning permission may be needed in a conservation area, on a listed building, where an Article 4 Direction applies, or if the opening faces a highway on a front or side elevation. Building Regulations approval is always required regardless of the planning position.

What size beam do bi-fold doors need?

There is no standard answer — the beam is sized by a structural engineer based on the span, the load above (roof, floors, masonry), and the bearing conditions. A 3m opening in a single-storey rear wall needs a very different beam to a 5m opening carrying a floor above. Never reuse a beam spec from a previous job. Get a calculation, get Building Control approval, and prop properly before cutting.

Should the customer choose a flush threshold?

A flush (level-access) threshold looks the best and is the right choice for accessibility, but it carries the highest water ingress risk and only works reliably with a proper drainage channel and careful DPC detailing — and ideally a sheltered elevation. On an exposed, weather-facing wall, a low-line weathered threshold is the safer specification. Talk the customer through the trade-off; do not let them pick flush on an exposed gable without understanding the drainage requirement.

How long do bi-fold doors take to install?

The fitting of a made-to-measure unit into a prepared opening is typically one to two days for a competent two-person team. The structural work — forming the opening, the beam, the cavity tray, making good — is the bulk of the time and can run a week or more. Lead time on the door itself is usually 4–10 weeks, and you should never strip out the old opening until the new unit is confirmed and on order.

Regulations & Standards

• Building Regulations 2010, Part L1B — energy efficiency of replacement glazing and doors in existing dwellings.

• Building Regulations 2010, Part A (Structure) — the beam or lintel over the opening and its bearings.

• Building Regulations 2010, Part K — protection from falling, collision and impact, including glazing in critical locations.

• Building Regulations 2010, Part C — resistance to moisture; cavity tray, DPC and threshold detailing.

• BS 6206 / BS EN 12600 — impact performance of safety glass.

• FENSA and CERTASS Competent Person Schemes — self-certification routes for replacement glazing installers.

• Planning Portal — Doors and windows — planning and Building Regulations position

• Approved Document L (Volume 1: Dwellings) — replacement glazing energy standards

• FENSA — Competent Person Scheme for replacement glazing

• Approved Document K — glazing in critical locations

• structural steel — RSJ beam selection, padstones and temporary propping for wide openings

• structural calculations — when a structural engineer's calculations are required

• house extension process — how bi-fold installation fits into a wider extension project

• building control process — Building Notice vs Full Plans and the inspection stages