Can a walk-on unit replace a normal rooflight?

What would have to change

The structure. Sized for a standard rooflight, meaning its own weight plus wind and snow. A walk-on unit imposes several times that, and to tighter deflection limits because glass needs even bearing.

Over a habitable room that generally means steel where timber may have been adequate.

The kerb and levels. A standard rooflight sits proud on an upstand. A walk-on unit sits flush with the surface around it, so the whole level relationship changes and the deck build up has to be designed around it.

Drainage. A flush unit has no protecting upstand, so a perimeter drainage channel has to be formed and connected.

Weight handling. A square metre of 50mm laminated glass is roughly 125kg. Mechanical lifting, which needs space beside the building.

So it is a rebuild

Effectively a new installation rather than a replacement, priced accordingly.

When it is worth it

Where the use of the roof above has genuinely changed: a flat roof becoming a terrace, or a light well becoming a route.

That is a real change of purpose and it justifies the work.

When it is not

Where the appeal is the flush appearance. A flush glazed standard rooflight looks very similar from below, delivers the same light, and costs a fraction.

We are asked for this fairly often by people who want the look rather than the function, and we would rather say so.

Planning ahead

If a terrace above is a possibility one day, mention it when the roof is being built. Designing the structure for it now is far cheaper than upgrading later.

Rarely a simple swap

The two products differ in almost every respect that matters for an installation: weight, thickness, how they are supported, and how they sit relative to the surrounding surface.

Dropping a walk on unit onto a kerb built for a standard rooflight is not something to attempt.

What changes

Weight. A walk on unit is a laminated stack of three or more toughened plies over a sealed cavity. It is several times heavier than a standard rooflight of the same size, and the structure around the opening has to carry both it and the people on it.

Support. A standard rooflight sits on a kerb, supported at its perimeter and designed to resist wind and snow. A walk on unit is a floor, supported to a stated imposed load with tighter deflection limits, so the frame and the structure beneath it are engineered rather than adapted.

Level. A standard unit sits on a 150mm kerb, proud of the roof. A walk on unit sits flush with the surrounding surface, because a raised kerb in the middle of a terrace is a trip hazard. That is a different construction rather than a different fixing.

Drainage. Because it is flush, the usual defence against standing water is gone. A perimeter drainage channel discharging into a working outlet has to be built, and the surrounding surface has to fall away from the unit.

Thickness. A walk on build up is considerably deeper, which affects how the perimeter is formed and how the thermal detail works.

What it amounts to

Rebuilding the opening rather than changing a unit: structural check, new supporting frame, new drainage, new thermal detail, and reworking the surrounding surface to sit flush.

That is a project rather than a replacement, and it should be priced as one.

The question worth asking first

Whether anyone will actually walk on it. Where the answer is only occasionally, or only for maintenance, a non fragile rooflight is the right specification and it costs a fraction of walk on glazing.

Non fragile means a person falling onto it will not go through, which is a life safety requirement rather than a traffic rating, and most flat roof rooflights should be that.

What we will not do

Fit a walk on unit into an opening that has not been checked structurally, or certify an existing standard unit as trafficable because it looks robust.

What has to change

More than the glass, in almost every case.

A walk on unit is far heavier than a standard rooflight of the same size, and it carries an imposed load the original opening was never designed for. The structure around the opening was sized for a light unit, wind and snow. It now has to carry people.

So a structural engineer looks at the existing trimmers and the deck, and the answer usually involves new steel. That is the main cost, and it is frequently larger than the unit itself.

Deflection as well as strength

The structure has to be stiff, not merely strong enough. A deck that flexes underfoot works the perimeter seal every time somebody crosses it, and a seal worked repeatedly eventually leaks.

An engineer designs to a deflection limit for exactly this reason, and it is why an apparently adequate structure sometimes still needs strengthening.

The surrounding surface

A rooflight sits in a roof. A walk on unit sits in a floor, and it needs a floor around it: a deck at a compatible level, falls that take water away rather than towards it, and a perimeter drainage channel with a real outfall on a flush installation.

Replacing a rooflight with a walk on unit in the middle of a membrane roof with no deck leaves you with glass you can stand on surrounded by a surface you cannot.

Whether it is worth it

Where the roof is becoming a usable terrace, yes, and the whole build up is being reconsidered anyway. Where the only motivation is being able to clean the rooflight, no, and a water fed pole costs a tiny fraction of it.