How Do You Size a Steel Beam for an Extension?

What Actually Governs the Size

Every beam size traces back to the same handful of inputs. Change any one of them and the answer changes, which is exactly why a number from a different job, or from a neighbour’s extension, cannot be trusted for yours.

• The clear span. The opening the beam has to cross. Longer spans need a deeper, heavier section. Simple rule-of-thumb sizing tables, including our own, are generally scoped to domestic openings up to around 5m: beyond that, or for anything unusual, the job moves straight to a full engineer-designed calculation.
• What is bearing on the beam. This is the single biggest variable on an extension. A beam carrying only a lightweight roof above a single-storey rear extension is a very different job from a beam carrying an upstairs floor, its finishes and the walls and roof above that, on a two-storey side extension of the same span.
• The loads themselves, in kN.Under Eurocode 1 (BS EN 1991-1-1), an ordinary domestic floor is Category A: imposed load 1.5 kN/m², with the dead load (the self-weight of the joists, boarding, ceiling and finishes) typically in the range 1.0–2.0 kN/m² depending on construction. A roof with access only for maintenance is Category H, with a much lower imposed load of 0.6 kN/m² that reduces further as the roof pitch steepens, plus any snow load from BS EN 1991-1-3 where it governs. These characteristic loads are then increased by partial safety factors, 1.35 on dead load and 1.5 on imposed load, to get the factored design load used in the strength check.
• The deflection limit. A beam can be strong enough not to fail and still sag too much for the finishes it carries. Beams supporting a brittle finish such as a plasterboard ceiling are usually held to a tighter deflection limit than beams with nothing brittle underneath.
• Bearing at each end. Every beam needs to sit on enough sound wall to spread its reaction without crushing the masonry. Planning Portal guidance gives 150mm as the normal minimum bearing at each end, often achieved with a padstone, a pad of dense concrete or engineering brickwork built into the wall under the beam.

How the Calculation Actually Works

This is the sequence a structural engineer works through. It is also, not coincidentally, why nobody can shortcut it with a table:

• 1. Establish the structure. Confirm the wall is load-bearing and record exactly what sits above the opening, floor by floor, up to and including the roof.
• 2. Measure the span and the supported width.The clear opening, plus the width of floor or roof that drains its load onto this particular beam (its “tributary width”), usually half the distance to the next support on each side.
• 3. Calculate the loads. Dead and imposed loads per square metre are multiplied by the supported width to get a load per metre run of beam, then factored (1.35 dead, 1.5 imposed) to get the design load.
• 4. Find the bending moment.For the common case of a simply supported beam carrying a uniformly distributed load, the maximum bending moment is M = wL² ÷ 8, where w is the factored load per metre and L is the span.
• 5. Select and check a section. The engineer picks a candidate steel section whose moment capacity exceeds M, then checks it against the deflection limit, and against shear, buckling and connection checks that go beyond what a simple table can show. Whichever check the candidate section fails first is the one that actually sizes the beam.
• 6. Specify the bearing. The end reactions are checked against the bearing area and, where necessary, a padstone is specified to spread the load into the wall.

A Worked Example of the Arithmetic (Not a Design)

To show the method rather than hand you a number, here is one illustrative case. None of the span or load figures below are a recommendation: they are chosen only to demonstrate the arithmetic in step 3 and step 4 above.

• Clear span, L: 3.6m
• Supported width (tributary width): 4m of upstairs bedroom floor, for a beam in the rear wall of a single-storey extension carrying the floor above
• Imposed load (Category A domestic floor): 1.5 kN/m²
• Dead load: 1.5 kN/m² (an illustrative mid-point of the typical 1.0–2.0 kN/m² range, not a design value for any real floor)

Factored load per square metre: (1.35 × 1.5) + (1.5 × 1.5) = 2.025 + 2.25 = 4.275 kN/m²

Load per metre run of beam, w: 4.275 × 4 = 17.1 kN/m

Bending moment, M = wL² ÷ 8: (17.1 × 3.6²) ÷ 8 = (17.1 × 12.96) ÷ 8 = 221.616 ÷ 8 = approximately 27.7 kNm

That factored bending moment, alongside a separate deflection check, is what the engineer then compares against steel section moment-capacity tables to find the lightest section that passes both. This page stops there deliberately: which section actually satisfies 27.7 kNm depends on the steel grade, the lateral restraint conditions and the deflection result, and confirming those is the calculation a structural engineer has to carry out and put their name to.

The Three Checks Every Beam Has to Pass

Why an Extension Beam Cannot Be Sized From a Rule of Thumb

Two extensions with the same 3.6m opening can need very different beams. A single-storey rear extension where the beam only picks up a lightweight roof is carrying a small fraction of the load of a two-storey side extension where the same-width beam supports a full upstairs floor, its finishes, and the walls and roof structure above that. Copying a beam size from a similar-looking job, or from a builders’ merchant guess based on the opening width alone, ignores the one variable that changes the answer the most: what is actually bearing down on the beam.

The consequences of getting it wrong are structural and legal. An undersized beam can deflect, crack finishes, and in the worst case fail to carry the load safely. Work carried out without Building Control approval has no completion certificate, which is routinely picked up when a property is sold or remortgaged, and the local authority can require unapproved structural work to be opened up, redone or removed at the owner’s expense.

Temporary Support During the Work

Before any masonry comes out, the wall above the new opening has to be temporarily supported, typically with needles carried on Acrow props. On the calculator built into our steel beam tool, that temporary support is modelled with needles at up to 900mm centres, extending at least 600mm beyond each side of the opening, a starting point your contractor and engineer will confirm for your actual wall and loads.

Who Must Do This

• A structural engineer confirms the wall is load-bearing, calculates the loads, works through the bending, deflection and bearing checks, and specifies the beam and its bearing. Planning Portal guidance requires their structural calculations to be submitted as part of getting Building Control approval.
• Building Control, either the local authority or an approved inspector, checks the engineer’s design against the Building Regulations, principally Approved Document A: Structure, and inspects the work, including the temporary support, the padstones and the finished beam, before it is covered up.
• Your buildercarries out the temporary support, removes the masonry, installs the beam and padstones to the engineer’s specification, and applies any required fire protection, commonly two layers of fire-rated plasterboard, so the exposed steel achieves the fire resistance the design calls for.

Once the beam is specified, our structural steelwork cost guide covers what supply, installation, engineer and Building Control fees typically add up to.