What is sightline and why does it matter?

When comparing aluminium bifold and sliding doors, one term comes up again and again: sightline.

Sightline is one of the most important factors in how modern doors look, feel, and perform — yet it’s often misunderstood.

This guide explains what sightline means, why it matters, and how it affects your choice of doors.

What does “sightline” mean?

Sightline refers to the visible width of the door frame or profile when the doors are closed.

In simple terms:

• A slimmer sightline = more glass, less frame

• A wider sightline = more visible aluminium, less glass

Sightlines are usually measured in millimetres (mm) and apply to:

• Vertical meeting stiles

• Frame sections

• Interlocking profiles on sliding doors

Why sightline matters

1. More glass, more light

Slimmer sightlines allow for:

• Larger glazed areas

• More natural light

• Brighter internal spaces

This is one of the main reasons aluminium doors are so popular in extensions and modern homes.

2. Cleaner, more modern aesthetics

Narrow profiles create:

• A minimalist appearance

• Cleaner lines

• A more contemporary look

Wide frames can make doors feel heavier and more dated, especially in modern builds.

3. Better views outside

With less visible frame, your eye is drawn through the glass, not to the aluminium.

This is particularly important for:

• Garden-facing elevations

• Coastal or countryside views

• Large sliding door openings

Sightline differences: sliding doors vs bifold doors

Sliding doors

Sliding doors typically offer the slimmest sightlines, especially at the interlock where two panels meet.

• Slim interlocks can be as narrow as 20mm

• Ideal for large openings and uninterrupted views

• Panels slide rather than fold, so frames can remain slimmer

Sliding doors are often chosen where maximum glass and minimal frame is the priority.

Bifold doors

Bifold doors generally have slightly wider sightlines due to:

• Multiple panels

• Hinges and folding mechanisms

• Structural requirements

However, modern aluminium bifolds still offer:

• Slim, clean profiles

• Excellent aesthetics

• Flexible opening options

The difference is noticeable, but not a drawback — just a design choice.

Does slimmer sightline mean weaker doors?

No — not when properly designed.

High-quality aluminium systems use:

• Thermally broken aluminium

• Reinforced profiles

• Advanced engineering

This allows manufacturers to achieve slim sightlines without compromising strength, security, or performance.

Poorly designed systems, however, may widen profiles to compensate for weaker engineering — which is why system choice matters.

Sightline vs performance: finding the balance

While slim sightlines look great, doors must also meet:

• Thermal performance requirements

• Security standards (such as PAS 24)

• Structural integrity for large glass panels

The best aluminium systems balance:

• Slim profiles

• Strong frames

• Excellent U-values

• Secure locking

It’s not just about being the slimmest — it’s about being well engineered.

What sightline should I choose?

There’s no single “best” sightline — it depends on:

• Door type (sliding or bifold)

• Opening size

• Desired look

• Budget

• Performance requirements

For large openings and views:

• Slimline sliding doors are often preferred

For flexibility and full opening:

• Aluminium bifold doors are ideal

Final thoughts: why sightline matters

Sightline directly affects:

• How much light enters your home

• How modern your doors look

• How connected your space feels to the outside

Understanding sightline helps you make a more informed decision, rather than choosing doors on appearance alone.

Choosing the right system

At Home Build Doors, we supply a range of premium aluminium bifold and sliding door systems, each with different sightlines, performance levels, and design benefits — and we’ll help you choose the right balance for your project.