How to specify a made to measure steel commission: the drawing must describe the interfaces
The mistake in most bespoke metalwork enquiries is not too little description. It is plenty of description about appearance and almost none about interfaces. An overall width and a reference photograph do not say what the steel must fit between, what it carries, which face must align, how it enters the site or who owns the structural decisions.
The short answer is to send a dimensioned sketch with one clear datum, photographs, intended use, surrounding materials, environment, access route and required finish. Mark which dimensions are critical and which may be adjusted. The workshop can help turn that into a fabrication drawing, but manufacture should begin only after one identified person approves the complete drawing.
Start with the problem, not the proposed section
Explain what the object must do and where it goes before prescribing 50 x 50 tube or 10 mm plate. Section choice follows span, stiffness, loading, connections, exposure and the visual result. S235JR is a common general-purpose structural steel grade. S355 provides higher strength where that helps, but it does not make a slender member less flexible in ordinary elastic use. A deeper section often controls deflection more effectively than a stronger grade.
Useful opening sentences are concrete: a table base for a solid oak top that must seat eight; a gate closing a measured sloping opening; a balustrade fixed to the side of an existing concrete stair; a pergola carrying a stated roof. Then identify the non-negotiable outcome. A workshop can propose steel around a real function. It cannot safely infer function from an inspiration image.
A drawing needs dimensions with hierarchy
| Dimension type | Example | Why it matters | How to mark it |
|---|---|---|---|
| Overall | Total width, height and depth | Controls footprint and transport | Give finished maximum envelope |
| Interface | Clear opening, bolt centres, top support | Controls whether parts meet | Mark critical and reference a datum |
| Functional clearance | Door swing, knee room, mattress allowance | Controls use rather than appearance | Show moving envelope or clear zone |
| Set-out | Column grid or baluster centres | Coordinates building work | Dimension from one origin, not a chain |
| Advisory | Decorative margin | Can absorb fabrication adjustment | Mark approximate or non-critical |
Do not dimension the same point from both ends unless one figure is explicitly a check. A chain of dimensions accumulates errors and can contradict the overall. Use millimetres, label the finished surfaces between which they were taken, and avoid scale-only drawings. A line that looks 400 mm long on a sketch is not a dimension.
Choose a datum and survey from it
A datum is the point, line or level from which other dimensions are taken. For a gate it might be the finished face of one post and the finished driveway level. For a table it can be the centreline and top surface. For a stair it may be the finished lower floor level. One datum stops several small site errors being added together.
Survey finished conditions where possible. If plaster, tiles, paving or timber cladding are not installed, state their build-up and who confirms the final level. Measure openings in several places and record deviation, not only an average. Photographs should include a tape or identifiable reference and show context from both directions.
Use the published practical companions rather than compressing them into the commission brief. The knowledge base covers measuring a driveway gate, a stairwell for a spiral staircase, selecting balustrade infill and receiving a heavy steel delivery. Link the relevant survey to the project and attach its completed measurements.
State loads and use without inventing a capacity
Describe what the steel supports, who uses it and whether loads move. A planter full of wet soil, a stone tabletop, a powered gate and a garden swing create different design questions. State attachments that may be added later, including roof coverings, glass, timber, automation and solar panels. A future addition is part of the present brief if the frame is expected to carry it.
For guarding, stairs, canopies and other safety-critical or structural work, identify the governing country, building type and use. Local rules determine actions, geometry and documentation. Agree who performs structural design, who checks the supporting building and what evidence is required. Do not assume that fabrication itself includes calculations, site approval or installation.
Standards and documentation belong in scope before the price
EN 1090 concerns execution and conformity routes for structural steel and aluminium work within its applicable scope. It is not a decorative quality label to paste onto every table, nor may a buyer assume any workshop status. Where a commission is structural, ask whether EN 1090 applies, which execution and documentation requirements govern, and who is responsible for design and declaration before ordering.
Nationally adopted Eurocodes and building rules may also apply. In an English-language EU brief, name the project country so DIN EN, PN-EN, NF EN, UNI EN or UNE-EN requirements are not silently mixed. For a decorative item, over-specifying structural documentation adds cost without resolving the actual interface. For a safety-critical item, omitting it is worse.
Finish is an environment specification
Do not write only black. Give RAL system and code, gloss or texture, indoor or outdoor exposure, and any coast, pool, irrigation, road salt or permanent damp. Decide whether visible welds should read as part of the fabrication or be dressed to a continuous appearance. State which faces are visible and which threaded or contact surfaces must remain uncoated.
Colour is not the corrosion system. The powder coating versus hot dip galvanising guide in the knowledge base is the companion for that decision. Drainage holes, sealed cavities and interfaces with timber or masonry should appear on the drawing before coating, because drilling after finishing breaks the protection.
Access and assembly can redesign the object
Give the narrowest doorway, stair turn, lift, gate and vehicle approach. Mark weight-handling constraints and the final assembly area. A single welded frame may be stiff and economical to fabricate but impossible to move into place. A demountable frame fits, while requiring stronger and more visible bolted nodes. That is a design tradeoff, not packaging detail.
For work delivered to another country, agree what arrives as welded assemblies, what the receiving party must assemble, and how interfaces are labelled. Do not state an installation assumption only in an email after approval. Put it on the drawing and packing list.
What actually drives relative cost
- Material quantity and section availability.
- Number of unique cuts, bends, holes and welded intersections.
- Tight tolerances and parts that must align with unknown site work.
- Grinding and cosmetic finishing of welds.
- Structural design, calculations and documentation where required.
- Prototype hardware or unusual moving mechanisms.
- Demountable joints and trial assembly.
- Glass, timber, stone or automation interfaces.
- Corrosion protection and masking complexity.
- Handling, protective packing and access constraints.
Size alone is a poor price predictor. A large rectangular frame with repeated parts can be simpler than a small object with thirty unique joints and an invisible fixing. If the budget is constrained, identify what may change: fewer divisions, standard section, visible weld character, simpler hardware or a different assembly split. Do not remove the interface that makes the object fit.
The approval pack
- Short function and site description.
- Dimensioned plan, elevations and necessary sections.
- Datum, critical dimensions and tolerances.
- Materials attached to the steel and their supplier details.
- Loads, movement and future additions.
- Site photographs and access dimensions.
- Finish and environment.
- Applicable country, regulations and responsibility for design.
- Assembly, fixing and installation responsibility.
- Name of the person authorised to approve revisions.
Use the appropriate catalogue area to make the brief concrete: garden and outdoor steel, interior steelwork, railings and staircases or structures and gates. The category identifies the family. The approved drawing identifies the commission.