Frame details for a flush walk-on finish

Frame types

Minimal or frameless. The glass is supported around its edges by a concealed frame beneath, so from above almost nothing is visible except a narrow line. The best appearance and the most demanding to install, because the supporting structure has to be genuinely flat and true.

Slim exposed frame. A visible aluminium edge, typically 20mm to 50mm. More forgiving of tolerance, and a legitimate look in its own right.

Tolerance

Tighter than on any other rooflight. A flush panel with a 5mm reveal on one side and 15mm on the other is obvious in a way it never would be on a roof nobody looks at closely.

That is why the frame is set out from the measured structure rather than from a drawing, and why the glass is ordered afterwards.

Bearing

The glass bears on a continuous setting material around its full perimeter, never on hard points. Point bearing on glass is how panels crack, and it is the failure that follows an uneven frame.

This is the practical reason deflection limits are tighter on walk-on structure.

The deck junction

Deck material must not bear on the glass or on the glass edge. A trim or a small gap separates them, and that gap is part of the drainage path rather than something to fill with sealant.

Thermal break

Where the unit is over a heated room, the frame needs a thermal break like any other glazing frame. A metal frame bridging from a cold terrace to a warm ceiling will condense on the inside in winter.

Access for replacement

Worth thinking about once. If the panel ever needs replacing, can it be lifted out without dismantling the terrace? Designing that in costs nothing now.

What flush actually means

The walking surface of the glass finishing level with the walking surface of the deck around it, with no step, no upstand and no raised trim between them.

It sounds simple. It is the most demanding detail in this part of the trade, because everything that would normally keep water out has been deliberately removed.

Where the frame goes

In a flush installation the frame is below the walking plane. The glass sits on it, the deck build up comes up around it, and the finished surfaces meet at the glass edge.

That means the frame is buried, permanently, in a wet zone. It sits below the deck surface where water collects, and it will spend its life in contact with moisture.

So the frame material matters more here than in almost any other rooflight application. Thermally broken aluminium with a durable coating, or stainless steel, and detailing that does not create a place for water to sit against an untreated edge.

The thermal break

An aluminium frame in this position is a straight thermal path from the cold outside surface to the warm room below.

A thermally broken frame interrupts that path with a low conductivity insert between the outer and inner sections. Without it the internal frame face runs cold, and a cold surface in a warm humid room grows condensation. On a flush walk on unit over a kitchen or a bathroom, an unbroken frame will run with water in winter.

This is worth checking specifically on a quotation. A structurally adequate frame is not automatically a thermally adequate one, and the difference is not visible in a photograph.

How the glass is held

Structural silicone bonding rather than a mechanical clamp, on most flush systems.

A clamped edge needs a pressure plate on top of the glass, and a pressure plate stands proud, which defeats the point. So the glass is bonded to the frame with a structural silicone that carries the load and forms the seal simultaneously.

That bond is doing two jobs, and it is why the fabrication quality of these units matters so much. A structural silicone joint is made in factory conditions with controlled surface preparation and cure. It is not something that can be remade on a roof.

The practical consequence for the owner: the unit is a sealed assembly, and a failure of the bond means replacing the unit rather than repairing it. That is an argument for buying a well made one from a manufacturer that will still exist in fifteen years.

The perimeter joint

Between the glass edge and the deck material there is a joint, and that joint has to do several things at once.

It has to accommodate movement, because glass, aluminium, and whatever the deck is made of all expand and contract at different rates across a temperature range that on a roof runs from below freezing to very hot.

It has to stay watertight through that movement.

And it has to survive being walked on, cleaned, and scraped by furniture.

The joint is normally a sealant in a properly sized gap with a backing rod, sized so the sealant can stretch and compress within its capability. A joint that is too narrow puts the sealant beyond its movement range and it tears. A joint that is too wide sags and collects dirt.

This is the detail worth asking a contractor to explain, because a vague answer here predicts the failure.

Drainage, again, because it is the whole game

Flush means water arrives at the glass rather than being turned away.

The deck falls have to take water to outlets that are not at the glass perimeter. Where that cannot be arranged, a perimeter channel takes it instead, and that channel needs a real outfall and a removable grating.

Below the visible detail, the frame should have a drained and ventilated chamber: a route for any water that does get past the surface seal to escape back out rather than accumulate. Good systems have this designed in with weep paths. It is the difference between a seal failure that produces a damp patch and one that produces a flood into the room below.

Ask whether the system is drained and ventilated. It is a standard term and a supplier who cannot answer is not selling a system designed for this.

Tolerances

Flush detailing has very little tolerance. The glass has to land at exactly the right level relative to a deck that has not been laid yet.

That means the structural opening has to be built accurately, the frame set precisely, and the deck build up calculated to arrive at the same plane. An error of a few millimetres is visible and an error of ten is a trip hazard.

Practically, this is why the sequence matters: the frame is set and levelled, its finished height confirmed, and the deck build up worked to that height rather than the reverse.

What goes wrong

Deck falling towards the glass. The commonest error, and it puts standing water against the perimeter for the life of the installation.

A perimeter channel with no outfall. Looks correct, holds water permanently.

A channel that cannot be cleared. Blocks within two seasons, then behaves like the previous item.

Membrane terminated over the unit rather than dressed into it. Wrong sequence, and it cannot be corrected without lifting the deck.

An untreated or thermally unbroken frame. Condensation inside, corrosion outside.

A sealant joint sized by eye. Tears within a few years of thermal cycling.

Who should build it

A flush walk on installation involves structural engineering, specialist glazing, roofing waterproofing and hard landscaping, and the failures nearly always happen at the boundaries between those trades.

The single most useful thing an owner can do is make one party responsible for the whole detail, in writing, rather than assuming the glazier and the roofer will resolve the interface between them on the day.

Thermal movement, and why the joint width is calculated

A flush installation brings four materials together at one line: glass, the frame metal, whatever the deck is made of, and the sealant between them. Each expands and contracts at a different rate, across a temperature range that on an exposed terrace runs from below freezing to very hot in direct sun.

Aluminium moves considerably more than glass. Stone or concrete paving moves differently again. Over a long perimeter those differences add up to real millimetres.

The sealant joint has to absorb all of that, repeatedly, for the life of the installation.

Sealants have a stated movement capability expressed as a percentage of the joint width. Which means the joint width is calculated from the expected movement and the sealant capability, not chosen by eye.

A joint too narrow puts the sealant beyond its range and it tears, usually at a corner. A joint too wide sags under its own weight, collects dirt, and looks wrong.

The backing rod matters as much. It sets the depth of the sealant so that it bonds to two faces rather than three, allowing it to stretch. A sealant bonded to the back of the joint as well as the sides cannot move and will fail early.

Corrosion in a buried frame

A flush frame spends its life below the walking surface, in a zone that is wet for much of the year and where salts from de icing or from marine air concentrate.

Marine grade anodising or a high specification powder coat on aluminium, or stainless steel, and attention to what the frame is in contact with. Dissimilar metals in a wet joint set up galvanic corrosion, so fixings and brackets need to be compatible with the frame material rather than merely convenient.

Cut edges are the vulnerable point on any coated section, and they need sealing rather than leaving exposed.

Access for the future

The question nobody asks at design stage and everybody asks in fifteen years.

When the sealed unit eventually fails, or the perimeter seal needs renewing, how is the unit removed? On a flush installation the deck comes up to reach it, and if the deck was laid over the frame flange rather than up to a defined edge, that means lifting a substantial area of paving.

A detail that allows the unit to be removed without destroying the surrounding surface is worth designing in, and it costs nothing at the time.

Who is responsible for the line

The failures on flush installations happen at the boundaries between trades: glazier, roofer, engineer and landscaper all meet at the same fifty millimetres.

The most useful thing an owner can do is name one party as responsible for the whole detail, in writing, before the work starts. Not the glass, not the waterproofing, but the junction between them.

Setting blocks and even bearing

Structural glass has to bear evenly on its supports. A hard point or a high spot concentrates stress in the glass at that point.

Setting blocks and bearing strips are specified for that reason, and the frame has to be level and true before the unit is set on it.