When a brief calls for “ultra slim” or “narrow frame” sliding doors, what’s actually being asked for is a system with a small interlock sightline — the strip of aluminium visible where two sliding panels meet. This is a manufacturer’s guide to what those numbers mean in practice and how to specify the right system for the brief.
What counts as a slimline sliding door?
There’s no British Standard for it, so the term gets used loosely. In practice, “slimline” means the visible aluminium between two sliding panels is below 35mm. “Ultra slim” means below 25mm. The slimmest aluminium sliding systems available on the UK market sit at around 20mm.
The reason it matters is geometric: every millimetre of aluminium interlock is a millimetre of glass you don’t see. On a 6m elevation with two interlocks, the difference between a 50mm system and a 20mm system is 60mm of additional visible glass — enough to noticeably change the architectural read.
Standard sliding systems: 50–90mm interlock. Slim sliding: 30–35mm. Ultra-slim sliding: 20–27mm. Below 20mm: structural glazing territory, not a regular sliding door.
Why sightlines aren’t the only number that matters
Sightlines get the marketing attention, but three other figures matter as much for premium residential and commercial projects:
Maximum panel size
An ultra-slim system that maxes out at 2m wide panels is worse on a 6m opening than a 30mm system that takes 3m panels. Fewer panels means fewer interlocks visible at all. Cor Vision Plus takes panels up to 3m wide; many slimmer-sightline systems do not.
Sash weight capacity
Big glass is heavy glass. A 3m sash in argon-filled double glazing comes in at 300–400kg. The roller geometry, frame extrusion, and threshold detail all have to be engineered for that load. This is where domestic-grade “slimline” systems fall apart at scale — the slim sightline is achievable, but only by limiting glass size and weight.
U-value at the interlock
The thinnest interlock is also typically the thinnest thermal break. Premium ultra-slim systems compensate with multi-chamber profiles and warm-edge spacers; cheaper imitations don’t, and their whole-window U-values reflect that. For Building Regulations compliance on a high-spec project, ask for the Uw figure at maximum panel size, not the Ug figure on a test panel.
The systems we manufacture
Three systems cover most premium residential briefs we’re asked to fabricate from our Slough workshop:
| System | Interlock sightline | Max panel width | Best fit |
|---|---|---|---|
| Cortizo Cor Vision Plus | 20mm | 3.0m | Premium architectural — minimal aluminium read |
| Cortizo Cor Vision (standard) | 27mm | 3.0m | Architectural with marginally lower cost than Plus |
| Cortizo 4700 | ~35mm | 2.4m | Mid-tier slim, robust for everyday domestic use |
Smart Systems’ Visoglide and Visofold sliding ranges sit in the mid-tier — we fabricate these too on briefs where Cortizo isn’t the right specification or budget fit.
How to specify well
1. Lead with the opening size
If the opening is over 4m, you need a system rated for 3m panels — otherwise you’re forced into three or four panels and lose the minimal aesthetic you presumably wanted in the first place. View the systems we manufacture for openings 2–12m+.
2. Then pick the sightline
For openings up to 5m, 27–35mm sightlines often look right. For 6m+ or where the brief explicitly calls for “ultra slim,” 20mm is the move. Anything thinner than 20mm is no longer a sliding door — it’s a structural glazing assembly with different engineering, different cost, and different lead times.
3. Confirm U-value at full panel size
System manufacturers sometimes publish a Uw at small test sizes. Ask for the figure at the panel size you’re actually specifying, with the glass spec you’re actually using. The difference can be material on the Building Regulations side.
4. Consider the threshold
Slim sightline systems often have shallower thresholds. Coordinate with the floor build-up early — level thresholds are achievable but they need to be designed in, not retrofitted.
Common terms, decoded
- Interlock sightline — the visible aluminium where two sliding panels overlap.
- Frame sightline — the visible aluminium at the perimeter of the entire opening (often thicker than the interlock).
- Sash — the moving panel of a sliding door.
- Uw value — thermal transmittance of the whole window assembly, including frame.
- Ug value — thermal transmittance of the glass only. Always higher (better-sounding) than Uw.
- Lift-and-slide — mechanism where the sash lifts onto rollers before sliding. Common on heavy ultra-slim systems for ease of operation.
