The short answer
It depends on whether you are altering the opening. If you fit bifold doors into an existing opening of the same size, the existing lintel or beam above it usually already carries the load, and you may not need a new one — though it should be checked. But if you are widening the opening, forming a new opening, or removing part of a load-bearing wall to fit wider bifolds, then a properly sized steel beam (RSJ) or lintel is almost always required to support the structure above. The beam must be sized by a structural engineer based on the loads it carries, and the work is notifiable under Building Regulations, with building control inspecting the structural work before it is covered up. Never widen an opening without engineered support.
Whether your bifold project needs a steel beam comes down to one question: are you changing the size of the opening? Here is how to tell, and what the structural process involves.
Structural support
- Same-size openingExisting support often enough
- Widened openingSteel beam usually needed
- New openingBeam / lintel required
- Who sizes itStructural engineer
- Building controlInspects before covering
When you don't need a new beam
The most common bifold project — replacing an existing set of patio or French doors with bifolds in the same opening — often does not need a new structural beam. The wall above the opening is already supported by an existing lintel or beam that was sized when the opening was originally formed. If the opening width is unchanged and the existing support is sound and adequate, it can usually carry the new doors just as it carried the old ones.
That said, "usually" is not "always". The existing lintel should be inspected and confirmed adequate, particularly if its condition is uncertain, if the original doors were lighter, or if there is any sign of cracking or movement above the opening. A building professional will check that the existing support is in good order and properly bearing onto the wall either side. Where it is, you avoid the cost and disruption of structural work entirely, and the project becomes a relatively quick replacement. The key point is that reusing an opening at the same width is the scenario where you are most likely to keep the existing support — but it is verified, not assumed.
When a steel beam is essential
A new steel beam or engineered lintel becomes necessary whenever you change the structure above the opening:
- Widening an existing opening: bifolds are often chosen to create a wide span of glass. Extending the opening beyond the existing lintel means a new, longer beam must span the wider gap and carry the load above.
- Forming a brand-new opening: knocking a new aperture through a wall requires a beam to support the masonry, floor or roof loads that were previously carried by the wall.
- Removing a load-bearing wall: the most involved case, transferring substantial loads onto a beam and its supports.
In all these cases the wall is temporarily propped, the beam installed, and the load transferred onto it before the opening is finished and the doors fitted. The type and size of beam — a rolled steel joist (RSJ), other steel section, or a heavy-duty lintel — depends on the span and the loads involved.
The structural engineer and building control process
Where structural support is needed, there is a defined process to follow, and it is regulated:
- Structural engineer's calculations: an engineer assesses the loads above the opening and the span, then specifies the correct beam size and the bearing required at each end. These calculations are the basis of a safe design.
- Building Regulations approval: structural alterations are notifiable. You apply to building control (or use an approved inspector), submitting the engineer's calculations.
- Inspection at the right stage: building control inspects the structural work — the beam, its bearings and padstones — before it is plastered over or hidden, so they can confirm it is built as designed.
- Completion certificate: once satisfied, building control issues a certificate confirming compliance.
This process is not optional for load-bearing work. It protects the structural safety of the home and gives you documented evidence the work was done correctly — which buyers' solicitors will ask for. Engaging the engineer and building control early keeps the project on schedule and avoids work having to be opened up again for inspection.
Padstones, bearings and what holds the beam up
A steel beam is only as good as what it sits on. When an engineer specifies a beam to span a widened bifold opening, the design also covers how the load is carried down into the structure at each end — and this detail matters as much as the beam itself. The ends of the beam bear onto padstones or spreader plates, which distribute the concentrated load over enough masonry to avoid crushing the brick or block below. The wall beneath each bearing must be sound and capable of taking the load down to the foundations.
This is why a bifold opening is more than just "put a steel in". The engineer considers the bearing length at each end, the padstone size, and whether the existing masonry and foundations below can carry the redistributed load. Where the bifolds create a very wide opening, the loads at the ends can be substantial, and occasionally additional support or strengthening below is needed. During the work, the wall is temporarily propped while the beam goes in, then the props are removed once the load has transferred. Getting these details right is what stops the classic problems — cracking above a window nearby, a door frame that racks out of square months later, or movement in the structure. It also explains why this is engineer-and-building-control work rather than a job to improvise: the visible beam is only part of a load path that has to be sound from the opening all the way down to the ground.
A practical sequence helps keep a beam job on track: engage the structural engineer early to confirm the beam size, submit the calculations to building control before work starts, arrange temporary propping for the day the wall is opened, and book the inspection for the stage before the beam is plastered over. Lining these up in advance avoids the common hold-up of structural work waiting on a calculation or an inspection visit, and keeps the project moving towards the day the doors finally go in.
Frequently asked questions
Do I need a steel beam to fit bifold doors in an existing opening?
Usually not, if the opening stays the same size. The existing lintel or beam often already carries the load and can be reused, though it should be checked for adequacy and condition. A new steel beam is generally only needed when you widen or form a new opening.
Who decides what size steel beam bifold doors need?
A structural engineer. They calculate the loads above the opening and the span, then specify the correct beam size and the bearing needed at each end. This is notifiable work, so building control reviews the calculations and inspects the installed beam before it is covered up.
Does installing a steel beam for bifolds need building regulations?
Yes. Forming or widening an opening and installing a supporting beam is structural work that is notifiable under Building Regulations. Building control inspects the beam and its supports before they are hidden, then issues a completion certificate confirming the work complies.
Sources & further reading
- Planning Portal — Structural alterations and building control
- HomeOwners Alliance — Removing a load-bearing wall
- GOV.UK — Building regulations approval
Figures on this page are typical UK ranges drawn from published sources and depend on your specific opening and material. They are guidance, not a quotation.