Walk-on units in a terrace or balcony deck

Levels

The panel must finish flush with the surrounding deck. Proud is a trip hazard; recessed collects water and dirt.

That is straightforward with a paving system on adjustable pedestals, where the height is dialled in. It is harder with timber decking, where the boards have their own thickness and the joists set the level.

The frame height has to be established from the finished deck level, and that means the deck build up is decided before the unit is ordered.

Drainage

Flush means no upstand protecting the perimeter, so a drainage channel around the panel takes water away before it reaches the frame junction.

With pedestal paving, water drains through the joints into the layer below, and the channel connects to that. With decking, water falls through the gaps and the channel sits beneath.

Falls run away from the panel, not towards it. That sounds obvious and is routinely got wrong where paving was set out from the building rather than from the drainage.

The edge detail

Where paving or decking meets glass is the detail that decides whether it looks designed. A trim gives a clean line and stops the deck material bearing on the glass edge, which it must not.

Where to put it

Against a wall or in a corner rather than in the middle of a terrace, for two reasons. It maximises light to the room below, which is usually deepest at the back. And people are noticeably more comfortable walking around a glass panel than across it.

A panel in the centre of a terrace tends to become an obstacle everyone routes around, which wastes the terrace.

Furniture

If anything will stand on it, say so at survey. Point loads from small feet are the hard case and the rating cannot be increased later.

The situation this solves

A room below, a terrace or balcony above, and no way of getting daylight into the room without giving up usable outdoor space.

That is the classic case: a rear extension with a roof terrace over it, a flat above a shop where the only outdoor space is on the roof, or a converted basement under a courtyard. The walk on unit lets the same square metre be floor above and window below.

It is a genuinely elegant answer, and it is a considerably more demanding piece of construction than a rooflight nobody stands on.

What makes it different from a glass floor over a light well

Two things: the deck around it, and the traffic.

A terrace is a designed walking surface with its own build up, falls and drainage. The glass has to integrate into that build up rather than sit as an object in a paved area, and the level relationship between the glass and the surrounding deck is the detail that decides whether the result works.

And a terrace gets walked on continuously rather than occasionally, with furniture moved across it, planters sitting on it, and in a domestic setting children on it. The wear regime is harder.

Flush or upstand

The central decision, and it is genuinely a trade off rather than a right answer.

Flush means the glass finishes level with the deck. It looks better, it is safer to walk across because there is nothing to trip on, and it is what most people picture when they ask for this.

It is also harder to waterproof, because water running across the deck arrives at the glass rather than being turned away by an upstand. The entire weathering burden falls on the perimeter detail and the drainage.

An upstand raises the unit above the deck. Far easier to weather, because water has to climb to get in, and considerably cheaper to build.

It is also a raised edge in the middle of a terrace, which is a trip hazard and looks like an intrusion rather than a feature.

On a terrace people actually use, flush is usually worth the additional cost and the additional design attention. On a roof that is accessed occasionally for maintenance, an upstand is the sensible economy.

The drainage detail on a flush unit

This is where these installations succeed or fail.

A flush unit needs a drainage channel around its full perimeter, the deck falling towards that channel or towards its own outlets rather than towards the glass, and the channel discharging somewhere that will not block.

The channel needs a removable grating so it can be cleared. A perimeter channel that cannot be accessed will fill with leaves, grit and moss within two years, and then it holds standing water against the perimeter seal permanently.

The waterproofing membrane of the terrace has to be dressed up into the unit upstand and terminated properly, with the sequence right: membrane first, then the unit, then the deck build up over it. A membrane dressed over the top of an installed unit is not a detail, it is a hope.

Structure

A walk on unit is heavy, and the imposed load on a terrace is higher than on a roof nobody uses.

Both of those go to a structural engineer, and the answer usually involves steel around the opening. That steel has to be designed and installed before the unit is ordered, because the manufacturer needs confirmed opening dimensions and the tolerance on those dimensions is small.

The engineer needs to know the real use. A domestic terrace and a terrace that hosts twenty people at a summer party are different design cases, and the honest answer at design stage is cheaper than the alternative.

Deflection matters as much as strength. A structure that is strong enough but flexes will work the perimeter seal every time somebody walks across it, and a seal that is worked repeatedly eventually leaks. The engineer designs for stiffness as well as capacity.

Slip resistance on a terrace

Non negotiable outdoors.

A fired ceramic frit is the durable answer, because it is part of the glass and does not wear. Applied coatings wear off under foot traffic and need renewing, which on a terrace is a recurring job at height.

The pattern is a design decision as much as a safety one. A dense frit gives excellent grip and passes noticeably less light to the room below. A pattern of dots or lines gives grip on the treated areas while leaving clear glass between, which is usually the better balance on a unit that is the main daylight source for the room underneath.

Privacy, which surprises people

Clear walk on glass means the room below can see up and the terrace can see down.

In a domestic setting over a kitchen that is usually fine. Over a bedroom or a bathroom it is not, and it is much better resolved at design stage with a frit or etched pattern than discovered after installation.

Thermal and solar

A horizontal unit takes the full summer sun and the room below can overheat badly, particularly if it is a room with limited other ventilation.

Solar control glazing with a lower g value is the answer, accepting the modest reduction in light. On a terrace with a large glass area over a single room, this is worth taking seriously at specification rather than dealing with afterwards.

For heat loss, ask for the whole unit U value. On a thick walk on build up with a substantial frame, the centre pane figure flatters the real performance significantly.

What it costs to look after

Cleaning, and more of it than people expect. Feet bring grit onto the surface and grit scratches. Regular washing is what keeps the walking layer clear rather than hazed.

Annual clearing of the perimeter drainage channel. This is the maintenance task that prevents leaks.

And a check of the perimeter seal, because that is where an ageing installation gives up first.

The realistic sequence

Structural design, then unit specification and order, then the steel, then the unit, then the waterproofing and deck build up around it, then commissioning.

Lead times on made to measure structural glass run to several weeks and frequently longer. Every one of these units is bespoke, because terrace openings are never a catalogue size, so the order goes in early and the programme is built around the delivery date rather than the other way round.

Getting the levels to land together

The detail that decides whether a flush installation looks right, and it is set long before the glass arrives.

Three planes have to coincide: the top of the glass, the top of the deck finish, and the top of any perimeter channel grating. They are built by different trades at different times, and each works to its own datum unless somebody sets one.

The sequence that works is to set and level the frame first, confirm its finished height, and then work the deck build up to that height. Not to lay the deck and hope the glass lands on it.

An error of a few millimetres is visible. Ten is a trip hazard.

The build up beneath the deck

On a terrace the paving usually sits on pedestals or on a bedding layer above the waterproofing, and that layer has its own falls.

Those falls are the ones that matter, because water travels through the paving joints and moves along the waterproofing below. If the waterproofing falls towards the rooflight upstand, water stands there regardless of how the paving above appears to slope.

This is the distinction most often lost between the roofer and the landscaper, and it is worth stating explicitly to both.

Pedestal paving and the rooflight edge

Pedestal systems are common on terraces because they allow the paving to sit level over a falling waterproofing layer, with drainage running freely underneath.

They work well with a walk on unit provided the perimeter is detailed: the pedestals adjacent to the unit need to bear on something solid rather than on the upstand, and the gap at the edge needs closing in a way that does not trap debris against the frame.

Ask how that edge is being made, because it is the point where a neat system meets a bespoke unit.

Living with it on a balcony

A balcony is smaller than a terrace and every element is more visible, so the perimeter detail is looked at closely and daily.

It is also more exposed, and drainage from a small area is easier to overwhelm with a single blockage. A removable grating and an accessible outlet matter more here than on a large terrace where water has other routes.

The furniture question

Tell the engineer what will sit on it. Planters full of wet soil are heavier than people estimate and they hold water and organic matter against the glass, which grows algae, which is far more slippery than the glass itself.

Keep planters on the deck rather than on the pane. The light below is the reason the glass is there.