Field note: Opening a load-bearing wall
It’s not always a steel beam
Lintel, RSJ, goalpost or box frame, and which one your opening needs. Scroll to follow one wall, from a window to a fully open span.
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In short
What a load-bearing wall is actually doing
Almost everyone starts with the same picture: it’s a wall, so it needs a beam to hold up what’s above and a wider opening just needs a bigger beam.
But a load-bearing wall rarely does one job. It carries the floor and the brickwork above it; it braces the house sideways against the wind; and it hands its load down to a foundation.
So the real question isn’t which beam replaces the wall, but what the wall was doing and where each of those jobs goes now.
You don’t replace the wall with a beam. You replace everything the wall was doing.
How an opening grows in a load-bearing wall, stage by stage
Can I put an opening in a load-bearing wall?
Take the rear wall of a brick house.
StructuralIt’s doing several jobs at once: it’s the weather skin, it braces your building, it carries its own weight, the floor and roof above, and it passes all of that down to the foundation.
Start with a simple window.
StructuralA small opening needs only an off-the-shelf lintel. The brickwork arches over it, so the lintel takes just a 45° triangle of masonry directly above, plus the floor load falling within a wider 60° zone of influence.
Can we make it bigger, for more light?
StructuralAbove 1.2 m a heavier stock lintel still copes; however, from there it needs more bearing length each end. The load keeps the same triangular shape that grows with the opening.
What about wide doors onto the garden?
StructuralGo wider still, windows above prevent the masonry from forming an effective arch. A tall rectangle of wall and floor load now bears straight onto the opening rather than a neat triangle, so a steel beam takes over. The size of beam may surprise you: set bifold or sliding doors beneath it, and their slim frames tolerate so little movement that it’s governed by how little it may deflect rather than by strength, often far heavier than the span alone suggests.
So why won’t a beam just do?
Now where the load goes becomes the problem.
StructuralIt has to go somewhere, and two things can give. A narrow masonry pier, often soft in Victorian houses, can be easily overstressed. Or the load reaching the foundation climbs past what the ground already carries; more than about 10% over the existing condition, and you risk settling the building. It’s a common cause of the cracking that appears after building work.
The goalpost frame.
StructuralA goalpost lifts the load off the fragile masonry entirely. Two posts carry the beam straight down to their own pad foundations, bypassing the old piers and footings.
There’s a basement under there.
The box frame.
StructuralSometimes a pad isn’t the way, a boundary too close, a basement below, or an existing footing generous enough to reuse. Closing the goalpost with a ground beam makes a box frame: the load spreads back along the original foundation line, over ground already settled under the old wall, and you save the excavation and concrete of new pads.
And if the extension needs a clear opening right through?
StructuralA bigger opening means bigger loads, so the beam gets deeper. And because the load still concentrates where the frame lands, the old footing rarely has the depth or width to take it, a long strip footing is easier to dig, and avoids the underpinning required by a pair of big pads.
We want it really wide — most of the back wall gone.
Now it stops being about weight.
StructuralWith most of the wall removed, too little masonry is left to resist wind. The frame takes that on instead, acting as a stiff box that keeps the house square.
But I’m mid-terrace — the wind hits my neighbours, not me.
Building Regulations, Part A: “regard shall be had to the … wind loads … in the ordinary course of its use.”
StructuralLittle direct wind reaches the party walls, true, but the extension’s own flank walls still catch it. And removing your rear wall strips the buttressing it gave the party wall you share, weakening a structure whose stability you hold jointly with your neighbours.
So we design each house to retain a proper share and to have a suitable level of independent stability.
StructuralWe proportion the wind load through the terrace if adjoining properties retain effective stability systems: 100% to the end house, 50% to the second, 33% to the third and 25% to subsequent mid-terrace houses.
This ensures each property makes a defined contribution to a coherent stability system, so the terrace is not critically dependent on the end houses or on unverified restraint from neighbouring properties, while still recognising the beneficial load sharing inherent in the existing terrace construction.
Let’s go full width.
The open-plan rear.
StructuralThe wall comes out entirely and the steel frame takes over every job it did, carrying the floors and roof, and bracing the whole house. What was the external wall is now an internal opening into the extension.
Why the answer keeps changing
The width sets the problem — the limit it meets first sets the answer
As the opening widened it met one limit after another: the masonry’s ability to arch, then to bear; the length of wall left to land on, and to brace against the wind. The first limit it reached sets the solution, which limit that is depends on the house, not the tape measure. So two openings the same width can still need very different structures.
Lintel or beam
While there’s generous wall either side, the masonry returns carry the bearing and brace the house. A lintel is used where masonry can arch, then a beam when loads are significantly higher.
Goalpost
Run short of wall either side; roughly 550 mm of return is the rule of thumb, and the beam ends have insufficient masonry to sit on. A goalpost on its own pad footings bypasses weak masonry elements.
Box frame
Where the foundation can’t take concentrated pad loads, over a cellar, or weak ground, a closed box frame ties the column bases with a ground beam and spreads the load along the footing.
Moment frame
Take enough of the wall and the job stops being about weight. With too little left to brace against the wind, the frame’s corners are made rigid so it can do the lateral work the wall used to.
Questions people ask
Opening a load-bearing wall: common questions
Is this wall actually load-bearing?
Direction of the floor joists above, the wall’s thickness, and what sits over it all matter, and the only reliable way to know is to have it checked.
What size RSJ or beam do I need?
There is no lookup table that survives contact with a real house. The span direction of the floor, the masonry and any chimney above, and the roof all change the load so two terraces that can still need different beams. And even where the structure genuinely is identical, the glazing can decide it. A beam is often sized by how far it is allowed to deflect, and different window or door systems tolerate anywhere from about 2 mm to 10 mm.
Is an ‘RSJ’ the same as a UB or steel beam?
In everyday use, yes, ‘RSJ’ is just what most people call the steel beam over an opening, and any engineer will know what you mean. Strictly, a Rolled Steel Joist is an older rolled section with tapered flanges; the beams specified today are almost always Universal Beams (UB) or Universal Columns (UC), whose flat, parallel flanges are far easier to bolt and connect.
Can I just put a beam in, or do I need columns?
It depends on how much wall is left either side to take the bearing and to keep bracing the house. Run short of return, or overload the foundation beneath, and the beam has to grow legs: a goalpost or a box frame down to the ground.
How wide can I go — can I take the whole wall out?
Often yes, but the further you go the more the solution changes shape. With little return left, you’re into a frame rather than a beam, and the cost steps with it.
Do I need a structural engineer and Building Control?
Yes to both for anything load-bearing. An engineer designs and calculates the opening; the calculations and drawing form part of the Building Control submission your local authority signs off.
Will it cause cracks, sagging or a bouncy floor?
Done properly, no, deflection is designed to a sensible minimum, not just strength. Cracks often occur due to poor temporary propping during construction, if say the contractor has used strong boys instead of needles and pins.
Do the foundations need underpinning?
Sometimes. New columns put large loads into small areas of ground, and if the existing foundation can’t take that, you need new pads, a ground beam, or local strengthening. It is one of the most overlooked parts of the job.
Do I need a temporary works design?
Usually not, the builder normally takes responsibility for the temporary support. Traditionally that means needles through the wall with props on both sides while the beam goes in; increasingly it’s a proprietary needle system such as a Prop Pal, which supports both leaves of the wall and lifts the props clear of the work. Be wary of anyone propping from one side only with Strongboys; rated at around 340 kg each and prone to bending the prop sideways, they are easily overloaded under real floor and roof loads. For larger or more complex openings, a proper temporary works design is worth having.
Do I need a party wall agreement?
If you are working on or near a shared wall, common in terraces, then very likely. It’s a separate process from Building Control and worth starting early.