How to measure an opening for steel doors and partitions

The costly error is often measuring the wrong surfaces. A frame made to bare blockwork may not fit after plaster, tile or lining reduces the opening, and a door measured from the slab may drag after flooring. For steel doors and partitions, survey the finished opening or record every unfinished layer. Do not ask the workshop to guess.

Define the assembly before measuring

Write down what the opening will contain: a fixed glazed screen, hinged door, pair of doors, pivot door, surface slider or pocket slider. State whether it is internal or external, and whether the project has fire, smoke, acoustic, thermal, security or accessibility requirements. These are design inputs, not options to add after the frame has been fabricated.

Decide which face the frame aligns with. It may sit flush with one room, centre on the wall or line through with adjacent joinery. Mark that face on the sketch. For a connected run of screens, use one continuous datum rather than measuring every panel from the last one. Products across interior steelwork become visibly crooked when small chain-measuring errors accumulate.

Use a fixed reference and millimetres

Use a checked tape or laser, level, straightedge, square and a helper for long dimensions. Record millimetres without rounding. Keep one viewpoint, such as Room A towards Room B, and label it on plans and photographs.

Establish the finished floor level, abbreviated FFL, on both sides. If the floor is incomplete, mark a durable datum on the wall and record the vertical distance from that mark to the future FFL. List each remaining layer and its confirmed thickness. A verbal note such as “tiles still to come” is not a usable dimension.

Measure the finished width and height at several points

1. Measure width between finished jambs at the top, middle and bottom. Add more readings where a tall reveal bows.
2. Measure height from finished floor to finished head at the left, centre and right.
3. Record every reading, not only the smallest, and mark the exact position of each.
4. Repeat the critical readings with the second person holding the tape or target.

The smallest reading shows the tightest part of the opening, but it is not automatically the manufacturing size. The workshop must apply the fitting clearance and frame detail for the selected system. Do not deduct an allowance yourself unless the approved survey sheet explicitly asks for it.

Check square, plumb, level and bow

Measure both diagonals between the same four finished corners. Different diagonals show that the opening is out of square, but do not reveal which jamb is wrong. Check each jamb for plumb on its room face and reveal face, check the head for level and lay a straightedge along jambs and head to find bow or local humps.

Draw the opening as surveyed, not as it ought to be. Note the direction and amount of every deviation. A steel frame cannot follow a corkscrew reveal, and forcing it into one can twist the door so hinges, latch and glazing no longer align. The project team must decide whether to correct the opening, use a wider perimeter joint or alter the frame detail before fabrication.

Record wall depth and all returns

Measure finished wall thickness at the top, middle and bottom on both jambs. Walls often change where old plaster, tile or panelling stops. Record:

• skirting height, thickness and projection on both sides;
• cornice, ceiling drop, beams and bulkheads near the head;
• tile, wall panelling, architrave or acoustic lining that meets the frame;
• reveal depth and any rebate, step or sloping return;
• adjacent sockets, switches, thermostats and ventilation grilles.

State door handing as a sentence

“Left hand” is ambiguous without a viewpoint. Use the named reference: “Viewed from Room A, hinges on the left, door pulls towards Room A.” Mark the handle, latch and any inactive leaf. For double doors, identify the leaf that opens first and where the floor or head bolt lands.

Trace the entire swing on the floor. Check skirting, radiator, furniture, light switch, ceiling slope and anything carried through the doorway. Record the required opening angle and any stop position. A door that fits the aperture but hits a cupboard at 70 degrees has not been specified successfully.

For sliding doors, survey the travel as well as the opening

A surface slider needs clear wall for the complete leaf, plus hardware and stop zones. Measure from the opening along the wall at floor, handle and head height. Note skirting, sockets, pictures, radiators and changes in wall plane. Check that the floor-guide position is structurally suitable and free of underfloor heating or services.

A pocket door requires the finished pocket width, depth, plumb and internal clearance, not merely the visible doorway. Confirm who supplies the pocket structure and the interface with the steel door.

Survey the floor and threshold

Record finished levels on both sides and across the full swing or sliding travel. Identify a threshold, floor transition, rug, drainage channel or slope. With a laser, give all readings from one datum rather than describing one room as “slightly higher”. If an accessible route is required, the designer must approve threshold geometry under the local building rules.

Mark underfloor heating, waterproofing and other no-drill zones from reliable drawings or detection. Do not assume a floor guide or stop can be screwed anywhere in a finished floor.

Identify the fixing faces

Remove the guesswork from each jamb and the head: state whether the structural material is reinforced concrete, solid or perforated brick, aerated block, timber stud, steel or a non-structural lining. Record thickness, edges, cavities, lintel position and known services. Plasterboard, tile and timber trim are not anchoring substrates by themselves.

Where a fixing carries the door or restrains a tall screen, the structural engineer defines the load path and the anchor manufacturer's current technical data governs drilling, embedment, spacing and edge distance. Do not choose a concrete anchor for perforated brick or aerated block. If a concealed support frame is needed, coordinate it as part of steel structures before closing the wall.

Set glazing and compliance requirements early

The project designer determines where safety glass is required and its make-up. EN 12600 classifies flat glass by pendulum impact performance, but the standard does not decide on its own which classification or glass construction the building needs. Fire-resisting, smoke-control and acoustic doors must be specified and evidenced as complete assemblies. Changing glass, seals, hinges, closer or gaps can invalidate the required performance.

Send a survey pack, not a list of numbers

When the opening changes during building work, survey it again. Check every resulting dimension, frame alignment, handing and fixing face on the fabrication drawing before approval. Confirm the order and any measurement-service question through the FAQ and written documents.