A loft conversion does not have one standard RSJ size. Different members may support floor joists, roof elements or other beams, and those members can need different sections and connections. The useful starting point is a beam-by-beam engineering brief.
A quoted room width or floor area leaves too much unknown for a structural selection. Our section comparison tool can demonstrate a uniform-load model, but it cannot determine the actual load arrangement in your roof or check the supporting building.
Give each member its own reference
Mark the proposed steel positions on a plan and give them references such as B1 and B2. Ask the engineer to confirm the arrangement. Keep the reference consistent across calculations, drawings, quotations and delivery documents.
For each member, record what bears on it and where. A floor load spread through joists is a different input from a concentrated reaction where another beam connects. Two members with the same length can therefore need different designs.
Use a brief like this to record known facts and questions. It is not a beam schedule for fabrication.
| Information | What to record |
|---|---|
| Member location | Plan reference and proposed support positions |
| Supported structure | Floor joists, roof elements, partitions or connected members |
| Load locations | Where reactions enter the member; do not guess unknown forces |
| Geometry | Measured distances, available headroom and ceiling constraints |
| Supports | Known wall or column construction and areas needing investigation |
| Access | Delivery route, opening sizes and obstructions |
| Interfaces | Stairs, services, roof alterations and fire-protection space |
Photographs can explain an access problem, but concealed construction may require further investigation. Keep uncertain dimensions marked as uncertain until the responsible person confirms them.
Headroom is more than the nominal beam depth
A designation contains nominal dimensions. The actual section depth, connection plates, joist arrangement, floor build-up and protection can all affect the finished level. Ask for a coordinated section drawing rather than assuming that the first number in a UB name equals the space the installation will occupy.
If a proposed floor or ceiling level conflicts with the design, raise it before ordering steel. Substituting a shallower section from an online table changes the structural question. It needs review by the designer, including any changes to supports and connections.
Keep the three length measurements separate
Record the clear distance you can measure, the span used in the analysis and the fabricated cutting length as distinct values. A support-centre span is not automatically the overall steel length, and a clear room width does not include every part of a bearing or connection.
The issued fabrication detail should resolve the order dimensions. Do not add a generic bearing allowance to a room measurement and turn that into an instruction to the supplier.
An access problem can change the design
Consider a hypothetical member that cannot pass through the available access route as one piece. That is a question for the engineer and contractor before fabrication. Cutting it into shorter pieces creates a connection to be designed, fabricated and installed; it is not equivalent to ordering the original beam in convenient lengths.
Request a proposed delivery and handling arrangement based on the actual fabricated item. Keep responsibility for temporary works explicit. HSE’s guidance describes the need for suitable design and management of the temporary condition.
What the issued package should clarify
Ask for the member designations and grades, supports, connection details, required restraint and protection. Check that the roof, stair and floor proposals use the same drawing revisions. Establish how site differences and later layout changes will be referred back to the designer.
For England, Approved Document A provides structural guidance. GOV.UK’s Building Regulations approval guidance is a starting point for the approval process; verify the requirements for the actual jurisdiction and building.
Use the UB size chart to read an already specified section. For planning the wider job, see the separate loft project guide. Structural work must be designed by a chartered structural engineer and receive the applicable Building Control approval. This article has not received an independent engineering review.