Fluid Steel Window (External): Slender Steel Windows from 20mm

At 20mm on the window vent and 25mm on fixed glazing, this is the narrowest face width of any system we supply. On a construction depth of 60mm, the slimline steel frame helps achieve a larger glass area and more natural light because steel has much higher tensile strength than aluminium or uPVC, giving steel windows the structural benefits of slender sections with minimal visual obstruction and a stronger sense of light, space, form, and beauty to suit your design vision and style. It produces the intricate, glass-dominant look that period steel windows are recognised by, which is why a heritage replacement is specified in steel rather than approximated in aluminium, since aluminium windows need thicker sections that compromise the traditional look and create more visual obstruction.

The system is tailored to reproduce the proportion of the window being replaced rather than impose a new one, which is why the profile range is wide and the face widths are so narrow; that versatility helps create glazing that can suit each project, from traditional replacement work to modern architectural extensions in residential settings.

It is equally specified in new construction where a period language is wanted, and for non-load-bearing partitions and replacement walls, including heritage conversions where slimline steel windows maintain architectural integrity in commercial and other commercial buildings.

For the matching door product, see our Fluid Door (External)

Windows run to 1000mm wide by 2400mm high at weights to 150kg — a sash proportion, matched to the openings found in Victorian, Edwardian and industrial buildings. Wider openings are composed from multiple lights with a 60mm meeting stile, which is how the original would have been built. For a genuinely large span in a slim steel section, a different system is the honest answer and we will say so.

Side-hung in single and double vent, inward and outward opening; inward-opening bottom-hung; top-hung and projected top-hung outward; horizontal pivot; vertical pivot; fixed glazing; and windows. The range also includes configurations used for internal screens where the opening pattern calls for it. That range is what lets a replacement scheme reproduce the opening pattern of the original rather than rationalise it — usually the difference between a sympathetic replacement and an obvious one.

Galvanised steel, powder coated or enamelled, is the standard specification. This can be tailored to suit the project and finished in any RAL or BS colour, whether you're after a traditional look or a more contemporary style. Stainless steel 1.4401 is specified in coastal and pool environments where chloride exposure would attack a galvanised finish. Corten is chosen where a weathered, oxidised exterior is wanted — as at Rusty House, where a Corten facade sets the material language for the whole extension. Together, these options offer real versatility, from traditional black to stunning colour choices and brass-toned hardware where that helps align the design intent, balancing beauty with timeless elegance in carefully crafted schemes.

Wind resistance is Class C5 at 2000 Pa and water tightness Class 9A at 600 Pa — the highest rating in either scale, which is what makes the system specifiable on exposed and coastal elevations. Sound reduction reaches 45dB. Durability is Class 4 at 50,000 cycles, with Class 4 impact resistance and mechanical strength, and prefabricated glazing meets DIN 18008-4 for safety barrier loading.

Tested to EN 1627 up to RC2 and CE marked under EN 14351-1, with multi-point locking as standard and surface-mounted and electric drive options among the key features. One constraint worth knowing at design stage: a non-RC-rated unit can use a handle from the Classic or Luxury collections, while a window specified for security takes standard fittings.

The published thermal transmittance is Uf from 1.9 W/m²K. Uf is the frame alone; the whole-window figure (Uw) depends on the glazing specification, including high-performance double or triple glazing, the spacer and the ratio of frame to glass in your window, and we calculate it per configuration rather than quoting a headline. Steel conducts more readily than aluminium, so a heritage steel window will not match a modern thermally broken aluminium casement. Thermally broken steel systems such as Crittall’s W20 TE can achieve U-values of 1.4 W/m²K, reducing heat transfer and energy costs, and these figures are often used to demonstrate compliance with building regulations. Where it is specified, it is because the profile is required. If energy efficiency is the priority and the profile is not mandated, the Fluid Timeless Window reaches Uw from 1.3 W/m²K at roughly a third of the cost, with whole-window values giving the more useful measure of overall fenestration performance and features.

Steel replacement on a period building is survey-led — openings are rarely square and rarely consistent between bays. We survey, supply, install and have each project manufactured in the UK by the same company team, so the measurements that set the sizes and the team that fits them are the same. See the work at Chevening, or get in touch to discuss a project, request a free quote or visit the Watford showroom.

Architects — for listed buildings and conservation work where the face width and the opening pattern will be examined. Request section details, BIM models and test certification.

Homeowners — for replacing original steel windows where the character of the house depends on them, without the draughts, rust and repainting.

Developers — for heritage conversions and period schemes where one certified system has to cover every opening.