UK · Q2 2026
20mm Interlock Sliding Doors UK
What 20mm interlock actually means
The interlock is the vertical aluminium bar that runs floor-to-ceiling at the point where two sliding panes meet. It's the part of the frame that sits at eye level when the door is closed — and visually the most dominant. On a standard aluminium slider it's 50–60mm wide. On uPVC it's 100mm or more. A 20mm interlock is roughly the width of a finger. The difference, looking out across a kitchen-extension to the garden, is the difference between seeing a frame and seeing a window.
As of 2026 only two systems in the UK truly hit 20mm: Cortizo Cor Vision and APX20 Plus. Schüco ASE 60 RC at 30mm and Reynaers MasterPatio at 35mm are often described as 'slimline' but they're not 20mm.
The 20mm vs near-slim spec table
Like-for-like comparison of the two true 20mm systems against the closest runner-up specs available in the UK. Pane size and weight ratings are the manufacturer's published limits for double-glazed configurations.
| System | Interlock | U-Value | Max pane | Max weight |
|---|---|---|---|---|
| Cortizo Cor Vision | 20 mm | 1.3 W/m²K | 3000 × 2200mm | 320 kg |
| APX20 Plus | 20 mm | 1.3 W/m²K | 2700 × 2000mm | 280 kg |
| Schüco ASE 60 RC | 30 mm | 1.0 W/m²K | 3000 × 2700mm | 400 kg |
| Reynaers MasterPatio | 35 mm | 1.1 W/m²K | 3000 × 2600mm | 400 kg |
| Schüco ASE 80.HI | 35 mm | 0.84 W/m²K | 3000 × 3000mm | 450 kg |
| APX47 (large-pane) | 45 mm | 1.2 W/m²K | 3500 × 1800mm | 400 kg |
The trade-off — what 20mm can't do
A 20mm interlock is structurally lighter than a 30–45mm meeting stile. The aluminium has less material in section, so it can't carry as much glass. That shows up in the published pane and weight limits. If your opening pushes against these limits, 20mm is not the right system.
| Constraint | 20mm systems | 30–45mm systems |
|---|---|---|
| Max pane height | 2700–3000mm | 3000–3500mm |
| Max pane weight | 280–320kg | 400–700kg |
| RC2 security available | No (PAS 24 only) | Yes (Schüco ASE 60 RC) |
| Lift-and-slide HS portal | Limited | Yes (Schüco ASE 80.HI) |
| Triple-glazed compatible | Yes (0.9 Uw) | Yes (0.84 Uw on ASE 80.HI) |
When to specify 20mm
The brief that justifies 20mm:
| Scenario | Why 20mm matters |
|---|---|
| Architect-led contemporary build | Specified almost universally — anything thicker undermines the brief. |
| Garden-view rear extension | The interlock is at eye level and dominates the view across the kitchen-diner. |
| 'Almost frameless when closed' brief | 20mm is the closest mainstream alu slider to a frameless aesthetic. |
| Low ceiling height (under 2.5m) | Frame width matters more proportionally — slimmer interlock = larger relative glass area. |
| Premium spec fit-out | Cor Vision is a recognised brand among high-end developers and designers. |
When NOT to specify 20mm
The brief that pushes you to 30–45mm:
| Scenario | Better choice |
|---|---|
| Opening larger than 3.5m × 3m | Schüco ASE 80.HI — 450kg per pane, 35mm interlock. |
| RC2 / Secured by Design required | Schüco ASE 60 RC — only RC2-rated slider option. |
| Standard suburban patio replacement | 20mm vs 50mm rarely visible at small sizes — APX35 saves money. |
| Listed building or front elevation | Aluminium not approved — timber sliding sash instead. |
| Heavy traffic / commercial use | APX47 or Schüco ASE 80.HI — heavier-duty hardware. |
20mm interlock fitted prices
UK Q2 2026, fitted, including VAT, frame, double glazing, fitting, removal of existing door, and FENSA self-certification. London prices typically sit at the upper end; regional prices 5–10% lower.
| Configuration | System | Low | High |
|---|---|---|---|
| 2-pane (1 fixed, 1 sliding) | APX20 Plus | £4,500 | £6,500 |
| 2-pane (1 fixed, 1 sliding) | Cortizo Cor Vision | £5,500 | £7,500 |
| 3-pane (2 fixed, 1 sliding) | APX20 Plus | £6,000 | £9,000 |
| 3-pane (2 fixed, 1 sliding) | Cortizo Cor Vision | £7,200 | £10,800 |
| 4-pane straight | 20mm systems | £8,500 | £12,500 |
| 4-pane corner (90°) | Cortizo Cor Vision | £10,500 | £14,000+ |
| Hidden floor track (Cor Vision only) | — | +£600 | +£1,200 |
| Triple glazing upgrade | — | +20% | +25% |
Frequently asked
What is a 20mm interlock sliding door?
The interlock is the vertical bar where two sliding panes meet at eye level. On a 20mm-interlock system, that bar is just 20mm wide — about the width of a finger. Standard aluminium sliders are 50–60mm; uPVC sliders are 100mm+. The 20mm spec maximises the glass-to-frame ratio when the door is closed, giving the cleanest possible sightline through the meeting rail.
Which 20mm interlock sliding doors are available in the UK?
Two true 20mm-interlock systems are widely available in the UK as of 2026: Cortizo Cor Vision (Spanish-engineered, premium spec, 3000mm panes, 320kg leaves) and APX20 Plus (value alternative, 2700mm panes, 280kg leaves). Schüco ASE 60 RC at 30mm and Reynaers MasterPatio at 35mm are runners-up but not strictly 20mm.
How much do 20mm interlock sliding doors cost?
In London 2026: 2-pane fitted £4,500–£7,500 (APX20 Plus low end, Cortizo Vision high end), 3-pane £6,000–£10,800, 4-pane corner £10,500–£14,000+. Prices include VAT, frame, double glazing, fitting, removal and FENSA self-certification. Triple glazing adds 20–25%; hidden floor track adds £600–£1,200 per opening.
What are the limits of a 20mm interlock?
The 20mm interlock is structurally lighter than 30–45mm meeting stiles. Max pane sizes are capped accordingly: APX20 Plus 2700 × 2000mm at 280kg, Cor Vision 3000 × 2200mm at 320kg. By comparison, Schüco ASE 80.HI at 35mm interlock handles 3000 × 3000mm at 450kg; APX47 at 45mm reaches 3500 × 1800mm at 400kg. For the very largest panes, you trade 10–25mm of interlock for the structural capacity.
When should I specify 20mm interlock?
Specify 20mm when sightline is the brief. Architect-designed contemporary builds, garden-view rear extensions, projects where the meeting rail at eye level dominates the view. Specify 20mm where you want the door to read as 'almost frameless' when closed. The premium over 30mm is typically 15–25%.
When should I NOT specify 20mm interlock?
Skip 20mm if your opening is over 3.5m wide × 3m tall (move to Schüco ASE 80.HI at 35mm with 450kg pane capacity), if you need RC2-rated security to EN 1627 (Schüco ASE 60 RC is the answer), or if the configuration is a standard suburban patio replacement under 2.4m tall — at that scale, the 20mm vs 50mm difference is rarely visible. Save the 20–25% premium.
What's the U-value of a 20mm interlock door?
Both Cortizo Cor Vision and APX20 Plus achieve 1.3 W/m²K whole-door U-value with double glazing, 0.9 W/m²K with triple glazing — comparable to standard alu casement windows and better than uPVC patio doors. The slim interlock doesn't compromise thermal performance because the polyamide thermal break runs the full frame depth.
Cortizo Cor Vision vs APX20 Plus — which is better?
Both have 20mm interlocks and 1.3 Uw. Cor Vision supports 3000mm panes, 320kg leaves, hidden floor track, and full RAL dual-colour. APX20 Plus tops at 2700mm and 280kg, no hidden track, but is typically 15–20% cheaper fitted. For openings under 2700mm tall, APX20 Plus delivers the same 20mm sightline at lower cost. Above 2700mm or where a hidden track is the brief, Cor Vision is the right call.
