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Aluminium Vertical Sliding Sash Windows

Aluminium Sash Windows

Two systems, one decision. The Smart VS600 replicates a timber sash — square-cut joints and a deep bottom rail. The new Smart VS400 is the modern one — mitred corners, PAS 24:2022 as standard and U-values to 1.0 W/m²K. We fabricate both in Slough and fit nationwide.

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Two systems. Which one is yours?

We fabricate both of Smart's vertical sliders, so the choice is about your building rather than our stock. The short version is below; the full specification for each is on its own page.
Anthracite grey aluminium vertical sliding sash windows in a contemporary brick extension

Smart VS400

The modern one. Mitred and crimped sash corners, PAS 24:2022 as standard, and U-values down to 1.0 W/m²K with glazing up to 36mm.

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White aluminium vertical sliding sash windows with Georgian glazing bars on a Victorian London terrace

Smart VS600

The timber replicate. Square-cut sash joints and a deep bottom rail — the details a conservation officer is trained to look for.

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The sash window look, without the decorating cycle

Sash is a style, not a material. The vertical sliding sash window is a Georgian invention that the Victorians perfected, and for three hundred years the only way to get one was in timber — which meant accepting the paint, the rot and the sticking.

An aluminium sash window keeps the proportion and drops the maintenance. No repainting every five years. No swollen bottom rail that will not lift in February. No rotten cill turning up in a survey. What it does need is the right system, because the two on the market do genuinely different jobs.

We fabricate both of Smart's vertical sliders in our own Slough factory and install nationwide, so we have no reason to steer you to one over the other. Here is the honest split:

  • Conservation area, period terrace, or a planning officer who has written "authentic"? The VS600. Square-cut sash joints and a deep bottom rail are what a real timber sash looks like, and that is what gets checked.
  • Need PAS 24 security, a better U-value, thicker glass, or a clean modern frame line? The VS400. Mitred and crimped corners, PAS 24:2022 as standard, and U-values down to 1.0 W/m²K.

Both open flush with no projection into or out of the building, and both tilt inwards so the outside of the glass is cleaned from inside the room — which is why they get specified in flats and high-rise as often as on Victorian terraces.

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Rows of off-white aluminium vertical sliding sash windows on a red brick period facade with wisteriaRows of off-white aluminium vertical sliding sash windows on a red brick period facade with wisteria
Diagram comparing the VS400's 45 degree mitred sash corner with the VS600's square cut joint and deep bottom railDiagram comparing the VS400's 45 degree mitred sash corner with the VS600's square cut joint and deep bottom rail
Georgian townhouse terrace with tall white vertical sliding sash windows in six-over-six pattern

Smart VS400 — the modern one, PAS 24 as standard

Smart's newest vertical slider, and the first aluminium sash window in the range tested to PAS 24:2022 as standard rather than as a chargeable upgrade. That single fact is what makes it work on new build under Approved Document Q, on Secured by Design sites, and on any accessible ground-floor opening an insurer will ask about.

Its sash corners are mitred and crimped — a clean 45° joint with the profile line running unbroken round the sash, rather than the visible butt joint of a traditional sash. On a contemporary elevation that is the detail you want.

  • PAS 24:2022 as standard — not an option
  • U-values of 1.0 – 1.4 W/m²K — against 1.6 on the VS600
  • 24, 28, 32 or 36mm sealed units — room for triple glazing or acoustic laminate
  • Hardware developed with ERA
  • Tilt-in sashes, flush opening, optional sash horns
  • Up to 1600mm wide × 2800mm high, 50kg per sash
Full VS400 specification
Anthracite grey aluminium vertical sliding sash windows in a contemporary brick extensionAnthracite grey aluminium vertical sliding sash windows in a contemporary brick extension
Close-up of the 45 degree mitred and crimped corner joint on an anthracite aluminium sash window frame

Smart VS600 — the timber replicate

The VS600 exists to look like joinery. Its sash joints are square cut and it carries a deep bottom rail — the two details that make a timber sash read as a timber sash from the pavement.

That matters more than most people expect. A conservation officer is not checking the material; they are checking the proportion and the corner. A mitred aluminium sash can be the right colour, the right size, carry the right glazing bars, and still be refused, because the 45° corner gives it away. The VS600's square cut does not.

  • Square-cut sash joints — the traditional timber detail
  • Deep bottom rail — the weighted proportion of a real sash
  • Optional sash horns for full Victorian replication
  • U-value 1.6 W/m²K, WER band C, 28mm sealed units
  • Concealed drainage and an optional concealed coupling
  • Up to 1600mm wide × 2800mm high, 50kg per sash
Full VS600 specification
White aluminium vertical sliding sash windows with Georgian glazing bars on a Victorian London terraceWhite aluminium vertical sliding sash windows with Georgian glazing bars on a Victorian London terrace
Close-up of a sash horn and deep bottom rail on a cream aluminium sash window against London stock brick

Replacing timber sash windows: what actually changes

Most people arriving here already have sash windows. They are timber, they are somewhere between tired and failing, and the question is whether aluminium is a step down.

On appearance, with the VS600, it is not — that is the entire design brief of the system. On everything else it is a step up, and the gap is wider than most quotes explain:

  Timber sash Aluminium sash
Repainting Every 5 years, plus scaffold Never
Rot & swelling Cills and bottom rails, eventually Does not absorb water
Sticking in winter Common No
Typical U-value 2.0 – 2.6 W/m²K 1.0 – 1.6 W/m²K
Cleaning upper floors Ladder or cradle Tilt in, from inside
Security certification Rare PAS 24:2022 on the VS400
Expected life Depends entirely on upkeep Decades, unmaintained

The one place we will talk you out of it: a listed building where the consent requires genuine like-for-like replacement. That is specialist joinery, not aluminium, and we would rather say so than sell you a window that gets refused. In a conservation area, where the test is preserving character rather than reproducing material, aluminium very often gets through.

Read the full comparison
Weathered timber sash window with flaking white paint, the maintenance problem aluminium avoids
Georgian townhouse terrace with tall white vertical sliding sash windows in six-over-six pattern

Specification Comparison

Both systems are thermally broken aluminium polyamide, Document L compliant and certified to BS 6375-1. They share identical maximum sizes. Everything that differs is below.

Smart VS400

  • PositioningThe modern one
  • Sash constructionMitred & crimped
  • Bottom railStandard
  • PAS 24:2022Yes — as standard
  • U value1.0 W/m²K (0.5 centre)
    1.4 W/m²K (1.0 centre)
  • Glazing24, 28, 32 & 36mm
  • Frame depth130mm
  • Air permeabilityClass 3, 600Pa
  • Water tightnessClass 5A, 200Pa
  • Wind resistanceClass C3, 1200Pa
  • HardwareDeveloped with ERA
  • DrainageConcealed
  • Max width1600mm (at 1300mm frame height)
  • Max height2800mm (at 750mm frame width)
  • Max sash weight50kg per sash
  • Tilt-in operationYes — clean the outside from inside
  • Sash hornsOptional
  • Dual colourNot available on either system

Smart VS600

  • PositioningThe timber replicate
  • Sash constructionSquare cut
  • Bottom railDeep
  • PAS 24:2022Not published
  • U value1.6 W/m²K
  • WER ratingC
  • Glazing28mm
  • Frame depth126mm
  • DrainageConcealed
  • Concealed couplingOptional
  • Max width1600mm (at 1300mm frame height)
  • Max height2800mm (at 750mm frame width)
  • Max sash weight50kg per sash
  • Tilt-in operationYes — clean the outside from inside
  • Sash hornsOptional
  • Dual colourNot available on either system

Colours & Finishes

Every aluminium sash window we make is powder coated in-house to a Qualicoat-certified process, in the full RAL palette — over 200 standard colours, plus bespoke matching against a heritage scheme or a planning condition.

Standard Colour Options

  • Anthracite Grey (RAL 7016) The most specified colour on contemporary residential and commercial work
  • Jet Black (RAL 9005) Architectural, and the closest read to a painted steel or dark-stained timber sash
  • Pure White (RAL 9010) The conventional sash colour, and the safest choice on a render or stucco elevation
  • Cream / Off-white (RAL 9001) Warmer than pure white, and usually the better match on a Georgian or Victorian terrace

Dual Colour Finishes

Dual colour is not available on VS400 or VS600. The system is finished in a single colour inside and out. Where a scheme needs a dark exterior and a light interior, our casement and tilt & turn ranges will do it — tell us at quote stage and we will say plainly which systems can and cannot meet the specification.

Anodised and Bespoke

Anodised finishes are available for coastal and high-exposure sites where a marine-grade specification is called for. Bespoke colour matching is available against a RAL, BS 4800 or a physical sample.

Aluminium Sash Windows — Your Questions Answered

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What are aluminium sash windows?

A sash window is one where the opening panes — the sashes — slide vertically past each other in the frame, rather than swinging open on hinges. 'Sash' describes the style, not the material: the design is Georgian and was traditionally built in timber, but the same window is now made in aluminium with a thermal break, modern double glazing and weather seals. You keep the proportions and lose the painting.

Which is better, the Smart VS400 or the VS600?

Neither — they suit different buildings. The VS600 has square-cut sash joints and a deep bottom rail, which is how a real timber sash is made, so it is the stronger case in a conservation area. The VS400 has mitred and crimped corners, is PAS 24:2022 tested as standard, reaches 1.0 W/m²K and takes glass up to 36mm, so it is the one for security specifications, Part L and contemporary elevations. Maximum sizes are identical on both.

Are aluminium sash windows allowed in a conservation area?

Often, yes. In a conservation area the test is whether the character of the area is preserved, not whether the original material is reproduced — and the VS600 is specifically designed to pass that test. It varies by local authority, so send us the address and the planning reference and we will give you a straight answer. On a listed building requiring genuine like-for-like replacement, the answer is usually specialist joinery rather than aluminium, and we will tell you that.

Do aluminium sash windows need painting?

No. The finish is a powder coat applied to a Qualicoat-certified process, not a paint sitting on the surface waiting to peel. There is nothing to sand, seal or repaint — which over the life of a ten-window Victorian terrace removes a scaffold, a decorator and several thousand pounds every five years.

How energy efficient are they?

The VS400 reaches an overall U-value of 1.0 W/m²K with a 0.5 centre-pane unit, or 1.4 W/m²K with a 1.0 centre pane. The VS600 achieves 1.6 W/m²K and a WER band C. For comparison, a typical single-glazed timber sash sits around 4.8 W/m²K and a modern double-glazed timber sash around 2.0–2.6. Both systems are Document L compliant.

How big can you make them?

Up to 1600mm wide (at 1300mm frame height) and up to 2800mm high (at 750mm frame width), with a maximum of 50kg per sash — the same on both systems. Larger openings are handled by coupling units together, and a concealed coupling option is available on the VS600 so the join does not show.

How do you clean the outside of an upstairs sash window?

Both sashes tilt inwards, so the external face of the glass is cleaned from inside the room. No ladder, no cradle, no cherry picker. It is one of the main reasons vertical sliders get specified in flats, student accommodation and high-rise as well as on period houses.

Do you fit them, or supply only?

Both. We fabricate in our own factory in Slough and install nationwide with our own teams, and we also supply trade and developer quantities with drawings and certification. Tell us which you need when you request a quote.

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Cortizo logo FENSA logo BFRC logo Accreditation logo for Direct Trade Windows company. Constructionline Gold Member badge for trusted building industry standards. Close-up of a window with a focus on the frame and glass. Aluminium window with sleek design and modern finish.