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Steel Beams for Loft Conversions
A loft conversion turns a space that was never designed to be lived in into a room that has to carry people, furniture and a staircase. Steel is usually how that happens.
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What the steel is actually doing
The ceiling joists in an unconverted loft are not floor joists. They were sized to hold up a ceiling and stop the walls spreading, not to carry a bed, a bathroom and people walking about. A conversion has to introduce a new floor structure that carries residential loading, and in most houses that means steel beams spanning between the party walls or the flank walls, with new timber joists hung off them.
There is often a second job going on at the same time. Cutting a dormer or a rooflight into a roof removes rafters that were doing structural work, so the roof load has to be picked up and taken somewhere — usually a ridge beam or a pair of beams at the dormer opening. It is common for a conversion to need three or four separate steel members, not one.
What drives the size
Span is the first thing: how far the beam has to reach between the walls it bears on. In a typical terrace that is the width of the house, often somewhere between 3 and 5 metres. Then the load — floor only, or floor plus part of the roof, or floor plus a dormer above.
Depth is the constraint that catches people out. A deeper beam is a more efficient beam, but the deeper it is the more headroom it eats. Loft conversions are frequently tight on height, and the Building Regulations set out what headroom the new staircase needs, so the engineer is often working to a maximum depth as well as a minimum capacity. That is why loft beams sometimes end up heavier than the span alone would suggest: the design is buying capacity through weight because it cannot buy it through depth.
Getting the steel into the loft
This is worth thinking about before the order, not after. A beam that has to go up two flights of stairs and through a hatch is a different proposition from one that can be craned over the roof or fed through a rear window. Weight per metre matters here as much as total weight — two people can manage a light section over a short distance, but very little else is going up a domestic staircase by hand.
Tell us at quote stage how the steel will reach the loft. It affects how the beam is cut, whether it is better supplied in sections with a spliced connection, and how the delivery is arranged.
Typical sizes for this job
Sizes commonly specified for loft floor beams in domestic work. The right one for your loft depends on the span, what the beam is carrying and how much depth you can give it.
| Size | Weight | Indicative Span | Typically Used For |
|---|---|---|---|
| 152 x 89 x 16 | 16 kg/m | 2.5–3.2m | Short spans in a narrow terrace, floor load only |
| 178 x 102 x 19 | 19 kg/m | 3–3.5m | Typical terrace width, floor load only |
| 203 x 133 x 25 | 25 kg/m | 3.8–4.5m | Mid-size conversions, floor plus a dormer above |
| 254 x 102 x 22 | 22 kg/m | 4–4.5m | Where depth is available but the wall will not take a wide flange |
| 254 x 146 x 31 | 31 kg/m | 4.5–5.2m | Wider houses, or floor plus roof load |
What else the steel will need
Most beams need more than cutting to length before they reach site. All of it is arranged on the same order:
- Cut to size
- Welded cleats, end plates and connections
- Painting, priming and intumescent coating
- All processing and fabrication services
Related jobs
- Steel Beams for Removing a Load-Bearing Wall
- Steel Beams for Rear and Side Extensions
- All project types
Common questions
How many steels does a loft conversion need?
Frequently more than one. A typical conversion has floor beams spanning between the party walls, and often additional steel at a dormer opening or a ridge beam where rafters have been removed. Your engineer’s drawings will show the full schedule.
Can the beams be got up an ordinary staircase?
Sometimes, for lighter sections over short lengths. Heavier or longer members usually go in through a window, through the roof, or in spliced sections. Tell us the access route when you ask for a price so the steel is supplied to suit.
Does loft steel need fire protection?
It can. A new habitable storey brings fire resistance requirements, and where the steel is left exposed rather than encased it may need protecting. Your building control officer will confirm what is required.
Why is my loft beam heavier than the span suggests?
Usually headroom. If the design cannot afford the depth, capacity has to come from a heavier section instead. That is normal in a conversion tight on height.
Ready for a price?
Send your drawings or a cutting list and we’ll come back with quantities and a price. Large and small orders both welcome.
We supply structural steel across London, the South and the Midlands, delivered direct to site. Information on this page is general guidance; the specification for your project must come from your structural engineer.
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