Planning Belgian Steel-Look Window Openings in Existing Walls

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Planning a steel-look window replacement starts with accurate measurements of the existing opening, checked at several points, along with a review of the wall structure, lintel and sill condition. The measurements should then be turned into a written specification and drawing approved by the client before production begins.

Why Accurate Opening Measurements Matter

A Belgian steel-look profile window is made to measure, so the quality of the replacement depends on the quality of the first measurements. Existing openings are rarely perfect rectangles. Plaster, old tiles, uneven masonry and previous repairs can all change the dimensions from one side of the opening to the other.

Measure before you order anything, and expect to find differences. The goal is to capture the true size of the opening, not the size shown on old plans or remembered from the original installation.

Tools and Preparation Before Measuring

Prepare a rigid measuring tape, a spirit level, a long straight edge, a pencil and a notebook or printed sheet. A laser distance meter helps on higher floors. Clear the area around the opening so you can reach every corner comfortably.

Remove loose debris and old sealant where it is safe to do so.
Note whether the wall is plastered, rendered, stone or concrete.
Take photos of each opening from inside and outside.

How to Measure Width and Height Correctly

Measure the width at the top, middle and bottom of the opening, and the height at the left, centre and right. Record the smallest and largest value for each direction. Differences of more than a few millimetres should be noted, because they affect frame fitting and sealing.

Also check the diagonals. If the two diagonal measurements differ noticeably, the opening is out of square, and this must be shown on the drawing so the frame can be made to suit the actual geometry.

Checking Depth, Reveals and Wall Structure

Depth matters as much as width and height. Measure the wall thickness at the opening, the depth of any existing reveal, and the position of the frame line relative to the outer face of the wall. These figures determine how the new frame sits and how it is anchored.

Inspect the lintel above and the sill below. Look for cracks, moisture stains or crumbling material. Any structural concern should be assessed by a qualified engineer before a new window is fixed into the opening.

Turning Measurements into a Written Specification

Once measurements are taken, prepare a drawing with clear dimensions, a note of the wall material, the orientation and the opening type, such as fixed, sliding or casement. Include photos and any site remarks, such as obstacles, access limits or existing shutters.

Opening width and height, with minimum and maximum values.
Diagonal check results.
Wall depth and reveal details.
Intended window or door type and opening direction.

A clear written specification avoids misunderstandings between the homeowner, the contractor and the fabricator.

Approval, Production Timing and Installation Coordination

Ask the client to review and approve the drawing and specification in writing before production starts. Any later change to the opening, such as widening it or altering the sill, should be recorded as a revision so the factory works from the correct data.

Coordinate with the installer and the site contractor. Confirm when the opening will be ready, whether the wall has been plastered or finished, and who will handle sealing and final cleaning. Good coordination keeps the installation smooth and protects the new frames.

Frequently asked questions

Can I measure the opening myself?

Yes, you can take basic measurements yourself using a rigid tape and a level. For a precise fit, the fabricator usually prefers to verify the dimensions on site before production.

Why should I measure in several places?

Walls are rarely perfectly straight or square, so the width and height often vary across the opening. Several readings reveal the true size and help prevent fitting problems.

Should I measure before or after plastering?

Measure after the wall finishes are in their final state, or clearly account for the thickness of plaster or tiles. Measurements taken before finishing may not match the final opening.

Do I need an engineer to check the opening?

If you notice cracks, moisture damage or a weak lintel, have a qualified engineer check the structure. Exact structural and railing standards, such as SI 1142 for railings, are confirmed with the engineer on your project.

Common mistakes to avoid

The most common mistake is relying on a single measurement or old plans instead of checking the real opening. Ignoring out-of-square openings and uneven wall depth also causes fitting problems later.

Our recommendation

Take at least three width and height readings, check both diagonals, and photograph the opening before ordering. Send these details with a sketch to the fabricator so the first production run fits correctly.

Summary

Accurate, multi-point measurements, a clear written specification and written client approval are the foundation of a successful steel-look window replacement.

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