Six metres is a long hole in a house. Plenty of loft, extension and open-plan schemes ask for it. This is not a “buy a catalogue beam and hope” length. Lateral restraint, temporary works and deflection usually matter as much as raw bending strength.
Use this for budgeting talk. Design belongs with a chartered structural engineer. Planning tools: beam size calculator and domestic span table.
Order length and access
Clear span 6.0 m + bearings ≈ 6.3 m of steel in many details. Delivery vehicles and site access become real constraints at this length. Measure the route from kerb to final position. Check stair turns, door heads and garden side access before you fall in love with a single clean opening.
Convert opening to cut length with the bearing length calculator. Confirm the bearing length on the drawings rather than assuming 150 mm forever.
Planning size bands
These are conversation starters for simply supported domestic floor beams with controlled assumptions. They are not certificates.
| Situation | Sections often discussed | Notes |
|---|---|---|
| Domestic floor, controlled load width | 305×165×40–46 | Common planning territory |
| Wider rooms / heavier finishes | 356×171×45–51 | Stiffness-led |
| Floor + significant roof/wall loads | 406×178×54 or engineered solutions | May need plated/built-up options |
| Very demanding geometry | Engineer-specified | Do not extrapolate from 4 m guides |
If someone online says “just use a 254 for 6 m”, treat that as noise. At this length, stiffness and restraint details dominate. Compare properties in the complete size chart.
Deflection and finishes
L/360 at 6 m ≈ 17 mm. That sounds small until you watch a ceiling crack or feel a lively floor. Long spans often push deeper sections or alternate layouts (two shorter beams, posts, portal frames).
Brittle finishes upstairs or large glazed screens near the opening make serviceability non-negotiable. Tell the engineer what you plan to install, not only what the structure must carry in kN.
Check serviceability with the deflection calculator, then stop guessing.
Worked planning example: long open-plan wall
A 1960s detached house wants a 6.0 m rear opening to link kitchen, dining and sitting areas. Load width is generous because both rooms are deep. First-floor joists land from both sides. Owners want large porcelain tiles on the new ground floor and a continuous plasterboard ceiling above.
Planning talk that starts at “maybe a 305” can quickly become 305×165×46 or 356×171×45/51 after deflection and load width are taken seriously. Order length around 6.3 m.
Mass example: 305×165×46 × 6.3 m ≈ 290 kg. That is crane territory in most homes. Price the lift, road closures if needed, and temporary works footprint as part of the same decision that chose the opening width. Use the weight calculator.
Weight, cranage and neighbours
A 6.3 m heavy UB is not a hallway carry for three mates and a cushion. Get masses early. Ask installers:
- mobile crane vs telehandler vs specialist beam trolley
- outrigger positions and ground conditions
- overhead cables and tree canopies
- noise and timing constraints next to neighbours
Party wall and neighbour politics often matter as much as steel grade on long openings near shared walls.
Cost (2026 UK planning bands)
Expect the installed total to move well beyond a short kitchen knock-through. Steel, lifting, temporary works, making-good and fees stack up.
| Item | Planning band |
|---|---|
| Engineer / drawings | £700–£1,800+ |
| Building Control | £300–£700+ |
| Steel + fabrication details | £700–£2,000+ |
| Cranage / delivery | £300–£1,500+ |
| Labour and temporary works | £1,500–£4,500+ |
| Fire protection and making-good | £1,000–£5,000+ |
Structure-focused project totals of £5,000–£12,000+ for a single long opening are not unusual before decorative fit-out. Complex roofs and multi-beam schemes climb further. See install costs, sizes and prices and the cost calculator.
Padstones, restraint and connections
Long beams still sit on padstones. Reactions are larger, so skimping on bearing details is foolish. Read the padstone guide and use the padstone calculator to follow the arithmetic on the drawing.
Ask the engineer about lateral restraint to the compression flange, connection details at bearings, and whether end plates or cleats are part of the package. Catalogue “beam only” quotes sometimes omit fabrication that the drawing assumes.
Temporary works at 6 m
Removing six metres of load-bearing wall is a serious temporary condition. Props, needles or framed temporary support need a clear design and inspection points. If a quote cannot explain the temporary works, it is incomplete. Background: supporting an RSJ during installation and common installation mistakes.
Alternative layouts worth discussing
Before you commit to one heroic span, ask the engineer whether a post (or two) would:
- shrink beam depth for headroom
- cut steel weight and cranage
- simplify deflection
- reduce making-good risk on long ceilings
Architecture sometimes forbids posts. Fair enough. Make that a stated brief item, not a surprise after the first drawing.
Loft and roof schemes with long steels have their own patterns. See the loft conversion steels case study and what size RSJ for loft conversion.
Related spans
3 m · 4 m · 5 m · span table · safe load tables
FAQ
Can a 305×165×40 span 6 m in a house?
Sometimes under controlled load width and finishes. Often the design needs a heavier serial size or a different arrangement. Do not order from a forum answer.
Is 6 m too long for an RSJ in a domestic setting?
No. It is common in open-plan and loft work. It is too long for casual sizing without an engineer.
Will Building Control accept a span table printout?
No. They want calculations for your building. Our tables are planning aids only.
How long does installation take?
Steel day may still be one intense day once temporary works are in. Design, notices, fabrication and making-good stretch to weeks or months. See how long installation takes.
Site realities at 6 m
Long steels need a delivery plan, not just a price. Check street width, parking suspensions and whether a crane outrigger fits. Mass climbs fast � weight calculator. Order length includes bearings � bearing length calculator.
Deflection conversations dominate. If the first quote looks cheap because it uses a shallow section �like the 4 m job�, challenge it. Read 4 m and 5 m for how advice scales.
FAQ � 6 m spans
Is DIY temporary works okay? No. Long openings without designed props are how partial collapses happen.
Will Building Control accept an online size? No. Submit engineer calculations.
How much extra for making-good? Often thousands � see install cost guide.
Party wall? If bearings sit in a shared wall, start notices early.
Bottom line
At 6 m, plan for 305+ depth conversations, real cranage and proper engineering. The calculator helps you ask better questions. It does not replace calculations, and a screenshot of this page is not a design.
About James Hartley
Construction content editor focused on clear, practical guides for homeowners and builders planning RSJ projects across the UK.
