What is a walk-on rooflight?

How it differs

A standard rooflight is glazed for wind and snow, which are distributed loads. A walk-on unit is glazed for concentrated point loads as well, and it is built as a laminated stack of several toughened plies, typically 40mm to 60mm thick against about 28mm for a standard unit.

The top surface is treated for grip, usually with a ceramic frit fired into the glass.

The safety case

Not that the glass will not break. It is that if the top ply breaks, the plies beneath still carry the load. That redundancy is why the build up cannot be reduced.

What it sits on

Structure designed for the load, which over a habitable room usually means steel rather than timber, and designed to tighter deflection limits than an ordinary opening because glass needs even bearing.

It also sits flush with the surface around it, so there is no upstand protecting the perimeter and the drainage has to be designed.

Where it belongs

Where the floor area above genuinely cannot be given up. A terrace over a kitchen extension, a light well that is the only route to a garden, a basement that would otherwise have no daylight.

Where the roof is not walked on, a standard rooflight does the same lighting job for a fraction of the cost. We would rather say so. Detail on the walk-on page.

Glazing you can stand on

A walk on rooflight is structural glass specified to carry foot traffic, set flush into a terrace, balcony or roof surface, bringing daylight into the space below.

It is a different product from an ordinary rooflight in almost every respect: the glass build up, the structure supporting it, the surface treatment, the drainage detail and the price.

The glass

A laminated build up, typically three or more plies of toughened glass bonded with structural interlayers, sitting over a sealed cavity and an inner pane where the unit also has to insulate.

Lamination rather than one thick pane is about failure behaviour. Toughened glass, when it breaks, breaks completely and instantly into fragments. A laminated stack that loses one ply retains the others and stays in place as a surface you can walk off. That is the entire safety case, and it is why nobody makes a monolithic walk on rooflight.

The surface

The top face carries an anti slip treatment, either a ceramic frit fired into the glass during toughening or an etched pattern. Fritted is more durable and slightly more visible from below.

It is a wearing surface, and it is what makes the difference between a rooflight and a skating rink after rain. Expect the pane to read as slightly textured and slightly obscured from below; that is the trade for a floor you can stand on.

What it is not

Not a standard rooflight. Those are designed to keep weather out and let light in. You should not stand on one.

Not the same as non fragile. Non fragile means a person falling onto the unit will not go through it, which is a life safety requirement and a much lower and much cheaper specification. Most flat roof rooflights should be non fragile; very few need to be walk on.

Not rated for vehicles. A driveway is a completely different loading case needing a different engineered product.

The detail that matters

Because a walk on unit sits flush rather than on a 150mm kerb, the usual defence against standing water is gone. Everything depends on the surrounding surface falling away from the unit and a perimeter drainage channel discharging into a working outlet.

The second detail is thermal: a flush unit over heated space is a cold bridge at the perimeter unless it is insulated and the vapour control layer is continuous and sealed to the frame.

When it is worth it

Where you have a terrace over habitable space and no other route for daylight. There it is the answer and it is worth what it costs. Where nobody will actually walk on the glass, a non fragile rooflight does the job for a fraction of the price.

The definition

A structural glass unit designed and certified to carry foot traffic, installed in a floor, terrace, courtyard or light well, giving daylight to the space below while remaining a usable surface above.

It is not a standard rooflight in thicker glass. It is a different product with a different structural basis.

How the glass is built

A laminated assembly in layers, each doing a distinct job.

The top layer is the walking surface, usually with an anti slip treatment, and it is sacrificial in design terms. It takes the grit, the scratches and the dropped objects.

Below it sit structural plies bonded with interlayers. The interlayer is the safety component: if a ply breaks, it holds the glass together so the pane stays in place and keeps carrying load rather than dropping through.

Where the unit also has to insulate, an insulating cavity and an inner pane are added, making the assembly thicker and considerably heavier.

What distinguishes it from non fragile

A common confusion worth clearing up.

Non fragile is a fall protection classification. It means the unit will not let a body fall through in an accidental one off event, which protects somebody working on a roof. It is not a permission to use the unit as a floor, and a non fragile rooflight walked on regularly will fail eventually.

Walk on means designed and certified for repeated foot traffic as a floor.

What always comes with it

A structural engineer, because the load case has to be calculated for the actual use. Usually steel around the opening. A bespoke unit with a lead time of weeks. And a drainage strategy, because the unit sits in a horizontal surface where water collects.