Four metres is one of the most common clear spans people ask about. Kitchen knock-throughs, open-plan living rooms and garage conversions often land somewhere near that figure. The honest answer is there is no single correct RSJ for every 4 m opening. Load width, wall construction, roof load and finishes all change the answer. What you can do is narrow the field before you speak to a structural engineer.
Use this page for planning and budgeting. Final sizes need stamped calculations and Building Control approval. For a quick first pass, try the beam size calculator. For the wider domestic picture, keep the span table for domestic openings open in another tab.
What “4 m span” actually means
Clear span is the distance between the inside faces of the supports. The steel you order is longer than that. Most domestic beams need about 150 mm bearing on each end, so a 4.0 m clear opening often becomes a 4.3 m cut length. Miss that and the beam sits short on the padstones.
Measure twice. Check whether the supports are solid masonry, cavity wall or timber frame. The engineer will care, and so will the padstone detail. Use the bearing length calculator when you translate clear span into an order length.
Also write down:
- floor joist direction relative to the wall
- approximate room depths either side (load width)
- whether anything continues above the opening
- access route for a 4.3 m bar
That last point sounds dull until the delivery truck cannot get within 40 metres of the door.
Typical sizes for a domestic 4 m opening
These are planning ranges for simply supported beams carrying a domestic floor (roughly 0.75 kN/m² dead + 1.5 kN/m² live) with a moderate load width. They are not design certificates.
| Situation | Often discussed UB / RSJ | Notes |
|---|---|---|
| Light floor only, ~1.5–2.0 m load width | 203×133×25 | Common kitchen knock-through starting point |
| Heavier finishes / wider floor | 203×133×30 or 254×146×31 | Deflection often drives the jump |
| Floor + some wall / roof contribution | 254×146×31–37 | Needs engineer check |
| Long load width or awkward geometry | 305×165×40 | Do not guess. Get calcs |
A lot of UK quotes for a “standard” 4 m kitchen opening cluster around 203×133×25 or 254×146×31. That matches what people search for when they type “what size RSJ do I need for a 4m span”. Your house may not be standard.
If you are comparing serial sizes, the complete RSJ size chart shows depths, weights and section properties in one place.
Why deflection matters at 4 m
Strength and stiffness are different checks. A beam can pass bending and still feel soft underfoot if it sags too much. Domestic floors are often checked around span/360. At 4.0 m that limit is roughly 11 mm.
If plaster cracks or doors stick after installation, deflection was probably the weak link. Large format tiles, continuous plasterboard ceilings and glass screens hate movement. Tell the engineer about finishes early. A stiffer section can be cheaper than repairing cracked ceilings twice.
Run a rough check with the deflection calculator, then let the engineer refine it. Online calculators do not know your restraint conditions or exact load combinations.
Worked planning example: kitchen knock-through
Picture a two-storey semi. Owners want to remove the wall between kitchen and rear reception. Target clear opening 4.0 m. Timber first floor, joists spanning about 3.5 m into the wall from one side and 2.8 m from the other. No roof load on that wall line. Standard plasterboard finishes upstairs.
Early chat often mentions 203×133×25. After survey, trial holes and deflection checks, many drawings land on 254×146×31 with specified padstones and fire encasement. That jump is normal when both load widths and finishes are taken seriously.
Order length is typically about 4.3 m. Mass for 203×133×25 at that length is around 108 kg. A 254×146×31 at 4.3 m is around 133 kg. Plan mechanical help or a proper team lift. Logistics belong in the quote.
For a narrative walkthrough of a similar job, read the kitchen knock-through case study.
Rough cost for a 4 m beam (2026 UK)
Steel alone for a 4.3 m length of 203×133×25 often falls in the £250–£400 band before delivery, depending on region and finish. Add delivery, padstones, engineer fees, Building Control and labour and the installed project for a single opening commonly lands £2,500–£5,000+. Bigger making-good packages push higher.
Indicative stack:
| Item | Planning band |
|---|---|
| Structural engineer | £500–£1,200 |
| Building Control | £250–£600 |
| Steel + delivery | £300–£700 |
| Labour and temporary works | £1,000–£2,500 |
| Fire protection and making-good | £600–£3,000 |
See the cost calculator, installation cost guide and sizes and prices UK 2026.
Weight of that section at 4.3 m is about 108 kg for 203×133×25. Plan mechanical lifting. Use the weight calculator.
Padstones, bearings and temporary works
A 4 m beam still concentrates large end reactions on small flange areas. Soft blockwork without a padstone is a classic failure pattern. Follow the drawing. If you are still learning the vocabulary, the padstone size guide with worked examples and padstone calculator help.
Temporary works are not optional theatre. Props and needles (or an equivalent engineered scheme) go in before the wall comes out. Ask the installer to explain the sequence in plain English. If the answer is vague, get another quote. More on sequencing: how to support an RSJ during installation and how to install an RSJ in a load-bearing wall.
When 4 m is not a “standard kitchen beam”
Treat these as stop signs for internet sizing:
- wall or chimney breast continuing above
- roof loads or attic conversions feeding the wall
- point loads from trimmers, steel posts or stair landings
- openings close to party walls needing notices and shared bearings
- very wide rooms either side (deep load width)
- desire for minimal beam depth for architectural reasons (may force heavier or plated solutions)
In those cases the planning table is only a warm-up. The engineer may specify a deeper UB, a different grade, end plates, or a layout with posts.
Checklist before you order steel
- Clear span and proposed bearing length
- What sits above the beam (floor, wall, roof)
- Load width each side
- Padstone sizes from the engineer’s drawings
- Fire protection and finishes
- Temporary works plan (props, needles)
- Access route and lifting method
- Building Control application status
If any of those are fuzzy, stop. Ordering steel early to “save time” is how people buy the wrong section twice.
Related span guides
- What size RSJ for a 3 m span
- What size RSJ for a 5 m span
- What size RSJ for a 6 m span
- Domestic RSJ span table
- Safe load tables overview
FAQ
Is 203×133×25 always enough for 4 m?
No. It is a common planning starting point for light to moderate domestic floors. Wide load widths, heavy finishes or wall loads above often push the design to 203×133×30 or 254×146×31 and beyond.
Do I need Building Control for a kitchen knock-through?
If you are altering a load-bearing wall, yes, expect Building Regulations involvement. Internal aesthetic changes do not cancel structural duties. See Building Control approval process.
How does 4 m compare with 3 m and 5 m?
At 3 m, lighter sections appear more often. At 5 m, stiffness usually dominates and conversations move toward 254×146 and 305×165 families. Four metres sits in the busy middle where small brief changes swing the section.
Can I install it myself?
Most homeowners should not. You can clear rooms, organise skips and project-manage. Cutting a load-bearing wall without temporary works and approvals is how emergency propping invoices appear.
Briefing your engineer (4 m)
Bring: measured clear span, proposed bearing, photos of both supports, joist direction sketch, floor build-up upstairs, and whether any roof or wall sits on the member. Mention brittle finishes early.
Planning tools beforehand: span table, weight, cost. Real sequence flavour: kitchen case study.
Bottom line
For many 4 m domestic openings, conversations start with 203×133×25 or 254×146×31. That is a planning hint, not permission to cut a wall. Hire a chartered structural engineer, get Building Control on board, and only then order steel.
About James Hartley
Construction content editor focused on clear, practical guides for homeowners and builders planning RSJ projects across the UK.
