What is a trimmer beam, and why does the steel cost so much?
A trimmer (or lintel-and-trimmer) is the beam that carries what a removed wall used to carry. The cost follows the span — and the bearings at each end often cost more than the beam itself.
Short answer: A trimmer (also called a lintel-and-trimmer, or simply a structural beam) is the beam — typically a steel section or a glulam timber beam — placed over a new opening to carry the load the removed wall used to carry. It rests at both ends on bearings: points strong enough to carry the load onward down to the foundation. The bill has four parts: the engineer's calculation, the beam itself, the bearings, and the temporary propping needed while the work is done. Budget for a bill that isn't small, and for it to climb a good deal further when the span is large — prices vary widely by country and region, so get a local quote rather than assume a figure. What moves the number is the span and what sits above it, not the beam's price per metre.
What a trimmer actually is
When a load-bearing wall is removed, the support that the floor above or the roof was resting on disappears with it. A trimmer replaces it: a beam is placed horizontally across the opening, and everything the wall used to carry now rests on the beam instead.
The beam doesn't carry the load on its own. It gathers the load and channels it out to its two ends, where it has to be handed off to something. Those two points are called bearings, and they matter just as much as the beam: a beam resting on masonry that's too weak presses down into it and creates exactly the kind of settlement damage the whole exercise was meant to avoid.
So a trimmer is always three things at once: the beam, the bearings, and the path onward down through the house to the foundation. All three have to be calculated by a structural engineer.
Why the span costs so much
This is the most surprising part of the bill: a 5 m opening doesn't cost 25% more than a 4 m one, even though it's only 25% longer. It costs a good deal more, and the physics explains why.
For a beam under a uniformly distributed load, the bending moment — the stress the beam has to withstand — grows with the square of the span, while the deflection grows with the fourth power. Go from 4.00 m to 5.00 m with the same load per metre and the same profile, and the moment rises by just over 50% (1.25² = 1.56), while the deflection becomes almost 2.5 times as large (1.25⁴ = 2.44).
That's why you often have to jump from one steel section to a bigger one, and the heavier beam is at once more expensive per metre, harder to get into the house, and needs stronger bearings. Three costs rise at once.
The practical move: always ask the engineer what it costs to keep a short stub of wall or add a column in the middle. Two openings of 1.80 m with a 40 cm pier between them — adding up to exactly the same 4.00 m — can be markedly cheaper than one continuous opening. And the wall stub often ends up being exactly where the fridge, the tall cabinet or the end of the sofa can go. It's a decision that both saves money and gives a more usable room, and it needs to be made on the drawing, not on site.
Steel or glulam?
- Steel (a universal beam or similar rolled section) is slimmer for the same load capacity. That means the beam can be hidden within the depth of the floor structure, or only project down a little. On the other hand, steel is heavy — a 5 m beam in an ordinary section weighs several hundred kilograms — and it has to be physically got into the house. Steel also normally needs fire protection cladding.
- Glulam (glued laminated timber) is easier to handle, easier to fix things into, and can in some cases be left exposed as an architectural feature. On the other hand, a glulam beam has to be deeper than a steel one to carry the same load, so it projects further down into the room.
The choice is made based on the load, the span, how much headroom is available — and often on whether the beam can get through the door at all. Access is a real line item on the bill: if a 6 m steel beam can't be carried in, you either need to hire a crane or have the beam split and joined on site, and both cost money.
Deflection: the requirement people don't know about
A beam mustn't just be able to carry the load. It also mustn't bend too much. A beam that visibly sags in the middle causes cracks in the plasterboard ceiling above, doors that no longer close, and a joint line at the ceiling that opens up.
That's why trimmers are often sized by deflection rather than by strength alone: the beam ends up stronger than it strictly needs to be to avoid breaking, because it has to be stiff enough that the structure above doesn't get damaged. That's one reason an apparently modest opening can call for a surprisingly large beam.
It's also why you shouldn't paint and finish the joints the day after the temporary propping comes out. The structure settles a little once the load is transferred onto it. Ask the contractor to hold off on the final finish — it's standard practice, but it isn't always said out loud.
The bearings: the hidden half of the cost
The beam is the visible part of a trimmer. The bearings — the two points the beam rests on — are the part that surprises people on the bill.
All the load the beam gathers is concentrated into two points. Where the wall used to spread the load evenly over 4-5 m, two points now have to take all of it. That places demands on:
- The masonry or column the beam rests on — sometimes new piers have to be built, or steel columns installed.
- The floors below. The load has to continue straight down. If there's a room underneath, there needs to be something under the bearing point there too — it's why the engineer will want to go down to the basement.
- The foundation. In some cases the foundation needs underpinning beneath the bearing points. That's an expensive line item, but a rare one.
It's also why the question "which walls line up in the same place across floors?" matters so early in the planning. With a measured floor plan of each storey — scanned with an iPhone's LiDAR or drawn on graph paper in HouseSense — the floors can be laid on top of each other so the vertical alignment becomes visible. It's a clue, not an answer, but it's exactly the clue the engineer starts with.
What an app can and can't do here
A measuring app can measure the wall: length, thickness, room height and position, and produce a measured floor plan as a PDF that can be sent to the engineer instead of a sketch. The app can also show what the room would look like with a 3.60 m opening instead of a 4.80 m one — and how much continuous wall space is left afterwards.
The app cannot size a trimmer, and it cannot decide whether the wall is load-bearing in the first place. That's a structural assessment that has to be made on site by a building surveyor or a structural engineer. Breaking through a load-bearing wall also usually needs approval before work starts — what's required depends on where the home is. An app can measure the wall. It can't tell you what the wall is holding up.
Five questions you should ask the engineer
- What does it cost to make the opening 1 m narrower? The answer is often surprisingly large.
- Can the beam be hidden within the floor depth, or will it project down — and by how much?
- What needs to happen at the bearings, and does anything need to go down through the floor below?
- Can the beam physically get into the house, or does it need to be split or craned in?
- What does the local authority need, and who's submitting it?
These five questions take ten minutes to ask, and can move the final bill by a great deal — precisely because they're asked while the drawing can still be changed.