Structural support beneath a walk-on unit

Usually steel

Over a habitable room, the perimeter of a walk-on opening generally needs steel rather than timber. Two reasons: the load, and the deflection limit.

Timber deflects more than steel for a given depth. A frame that sags even slightly under load will not suit a glass panel bearing on it, because glass does not tolerate uneven support.

Deflection matters more than strength

The usual failure people imagine is something breaking. The real design constraint is movement.

A walk-on panel needs continuous, even bearing on all sides. If one edge deflects more than the others under load, the panel is being twisted rather than supported, and glass is unforgiving about that.

Engineers therefore design walk-on openings to tighter deflection limits than an ordinary opening, which usually means a deeper or heavier section than strength alone would need.

Sequence

Engineer designs the structure, structure is installed, its actual position is measured, the frame is set out from that, and only then is the glass ordered.

A made to size walk-on panel is expensive and it cannot be adjusted. Ordering it before the steel is in place is how a very costly piece of glass ends up not fitting.

Bearings and cold bridging

Steel bears onto padstones sized to spread the load into the masonry. And a steel through the insulation line is a substantial cold bridge, so it is insulated while it is accessible.

Why the price surprises

Because people price the glass. On a walk-on installation over a habitable room, the glass is often the smaller half.

The largest line on the quotation

On a retrofit walk on rooflight, the structure below is frequently more expensive than the glass above it, and it has nothing to do with the glazing specification.

That surprises people who have been comparing glass prices. It is worth understanding early, because it decides whether a project is viable at all and it decides the size and position of the opening more than preference does.

What the engineer is calculating

Dead load. The weight of the glazing itself. A walk on unit is a laminated stack of three or more toughened plies over a sealed cavity and an inner pane, commonly 50 to 70mm thick in total. A large pane runs to several hundred kilograms and it is permanent.

Imposed load. The people on it, at the rate appropriate to the use. A domestic terrace is one figure. A commercial or public route is higher and brings different safety requirements. Vehicle loading is an entirely different product with wheel loads an order of magnitude higher, applied dynamically and repeatedly.

Span. Cost and member depth rise steeply rather than proportionally, because a larger opening needs a stronger or deeper member to carry the same load over a longer distance.

Deflection. As important as strength here, and the reason a walk on opening is designed more tightly than a roof opening.

A structure that sags slightly under load is entirely acceptable for a roof. It is not acceptable for a glazed floor, because the glass has to sit on a support that does not move. Movement at the perimeter loads the panel unevenly and stresses the seal.

Which way the joists run

Changes the answer more than the size of the unit does, and it is one of the first things a survey establishes.

An opening set within the span of existing joists, running parallel to them rather than across, frequently needs far less work: the joists either side of the opening are doubled up and the load is carried without cutting anything.

An opening cutting across joists means the cut members have to be picked up by trimmers, and those trimmers have to span the opening and carry that load into the joists either side. Beyond a certain span, timber trimmers get impractically deep and steel is the sensible answer.

On a rectangular unit that sometimes means simply rotating the design, which costs nothing at the drawing stage and can save the entire steel line.

New build against retrofit

The difference is substantial and it is worth stating.

On a new extension the opening is designed in from the start. The joists are sized and positioned knowing where the glazing goes, and the additional cost over an ordinary roof is modest.

On a retrofit, the joists were sized for the roof as built with no allowance for a large hole in it, still less for people standing on that hole. Cutting them means a designed trimmer and frequently steel, plus an engineer’s fee, plus the disruption of installing steel into an existing structure.

If a walk on unit is a possibility on a project that has not been built yet, say so at design stage. It is one of the cheapest decisions to make early and one of the most expensive to make late.

The perimeter frame, which is structure too

Frequently overlooked in the conversation about joists and steels.

A walk on unit sits flush with the surrounding surface, because a raised kerb in the middle of a terrace is a trip hazard. So the supporting frame is at the same level as the finished surface rather than standing above it.

That frame has to be continuous, level and rigid. It carries the glass, it carries the perimeter drainage channel, and it carries the thermal detail. Getting it right is as much of the job as the trimmer beneath it, and a frame that is out of level will not take the panel evenly.

Bearing and support conditions

How the glass sits on the frame matters as much as what the frame is made of.

Structural glazing is normally supported continuously around its edges on a resilient bearing rather than sitting on hard points. Point loads at the edge of a laminated panel are exactly what it is not designed for.

The bearing width, the setting blocks and the edge cover are all part of the manufacturer’s specification, and they are not details to improvise on site. This is one of the areas where using the system’s own components rather than a fabricated equivalent genuinely matters.

Getting it there

Not structure, and it belongs in the same conversation because it constrains the same decisions.

A large pane cannot be manhandled. Crane or glass robot access, and a route to the position for it, have to be established before anything is ordered.

A property with no side access and no frontage close enough for a lift is a genuine constraint on what size is realistic, and finding that out after the glass is made is an expensive way to learn it.

On a terraced house with a rear terrace, this is frequently the binding constraint rather than the structure.

What the loading does not include

Worth stating plainly because it comes up afterwards.

A domestic terrace specification assumes people walking across: a distributed load plus a point load representing a person, applied and removed.

It does not assume a heavy planter, a water feature, a hot tub or a stone bench standing permanently in one place. A sustained point load in a fixed position is a different case and it has to be designed for rather than assumed.

If anything heavy is going to live on the terrace, say so before the unit is specified. A higher loading can be designed for and the cost of doing that at the outset is far less than the cost of a failure. Where a hot tub is contemplated, that is a structural design exercise in its own right regardless of what the glass can do.

Building Control

Forming a structural opening engages Part A, and that is notifiable.

Treat the inspection as a benefit rather than a burden. An independent check of the structural work while it is open is exactly what you want on a glazed floor, and it produces a certificate that a buyer’s solicitor will ask for.

Keep the engineer’s calculations with the house paperwork alongside it. In fifteen years, somebody asking whether the terrace can take a new use will need them.

The question worth asking first

Whether the glazing needs to be in the walking route at all.

A terrace rarely needs glazing across its whole width. Moving the panel to the edge, or to a part of the terrace outside the through route, changes the specification from walk on to non fragile and removes most of the structural work with it.

Non fragile means a person falling onto the unit will not go through, which is a life safety requirement rather than a traffic rating, and it costs a fraction as much.

That is half an hour with the plan, and it is worth doing before half a day with an engineer.

Two loads, not one

The structure has to carry the dead load of the unit, which on a thick laminated assembly is far heavier than a standard rooflight of the same size, and the imposed load of everything that will cross it.

It also has to carry the roof area that was previously supported by whatever joists the opening interrupted, redirected around the hole.

Between them, on anything but a very small unit, that means steel.

Deflection governs more often than strength

The point that separates a working installation from one that leaks after three years.

A beam can be strong enough and still move more than the glazing tolerates. A deck that flexes underfoot works the perimeter seal every time somebody crosses it, and a sealant joint flexed thousands of times fails.

It also feels unsafe, which determines whether the terrace actually gets used.

Engineers design walk on openings to tighter deflection limits than a plain ceiling for exactly this reason, and it is why an apparently adequate existing structure sometimes still needs strengthening.

Bearings and padstones

A steel concentrates load at its ends, and that load has to be spread into whatever it bears on.

On masonry that usually means a padstone sized by the engineer. Sitting a beam directly on ordinary blockwork crushes it over time, and the resulting settlement puts the unit out of level.

The bearing for the glass itself

Separate from the structure, and equally important.

Structural glass has to bear evenly on its supports. A hard point, a high spot, or a support that is not continuous concentrates stress in the glass at that point.

That is why setting blocks and bearing strips are specified rather than the glass sitting directly on metal, and why the frame has to be level and true.

The sequence this forces

Structural design first. Steel installed and the opening formed to the designed dimensions. Frame set and levelled. Then the unit surveyed and ordered to confirmed dimensions.

Tolerances on structural glass are small, and a bespoke unit has weeks of lead time. Building the opening and ordering the glass to fit whatever resulted is how these jobs go wrong.

Tell the engineer the truth

The honest use, including the occasional one. A domestic terrace and one that hosts twenty people at a summer party are different design cases, and the honest answer at design stage costs far less than the alternative.