Frameless flat rooflights: the frame hidden in the upstand

No rooflight is truly without a frame. A frameless flat rooflight simply moves the frame out of sight: the glass unit is bonded or seated so that the supporting structure sits beneath and behind the upstand rather than around the edge of the pane.

A conventional flat rooflight has a visible perimeter. A metal or plastic frame wraps the glass, rises above the kerb and stands in the rain. A frameless flat skylight reverses that. The glass overlaps the upstand, or sits on top of it, and the frame is a thin structural carrier concealed below. Seen from the garden, the top pane appears to float a few millimetres above the roof. Seen from the kitchen, the reveal is a flat plane of plasterboard running to a small shadow gap and then glass.

Three ideas are folded into the name, and it helps to keep them apart:

• The frame is the supporting carrier. It still exists, and it still has to be strong enough for the glass, for snow and for a person cleaning it.
• The upstand is the raised kerb on the roof that the unit sits on. On a frameless build, it does extra work, because it hides and protects the frame.
• The glass package is the sealed unit, normally two or three layers, bonded at its edge so that no outer bead or clip is needed.

If you are still deciding between flat glass and other forms, the comparison of flat glass and dome rooflights sets out the basic differences, and our flat roof rooflight installation hub covers the whole family of products.

The upstand is the structure that makes the look possible. It carries the load, keeps water off the joint and hides the metalwork, so it has to be built before the glass ever arrives.

The sequence, from the roof deck upwards, runs like this. A timber or insulated kerb is framed around the opening, tall enough above the finished roof surface for the waterproofing to turn up it with room to spare. The roof membrane is dressed up the outside face and over the top edge. A thermal break and a compressible seal go on the top of the kerb. Then the carrier frame is fixed, which is where the frameless build parts company with the usual one: the carrier is set back from the outer face, so the glass can overhang it and shield it.

The glass unit itself is bonded to the carrier with a structural sealant, or held by concealed fixings, and the exposed edge is finished with a slim profile that is barely wider than the pane is thick. Nothing rises above the glass. Because nothing rises above the glass, nothing stops water from leaving it.

This is why the height and insulation of the upstand matter more here than on other products. A shallow or poorly insulated kerb leaves the concealed frame cold, and a cold frame is where condensation forms. Our page on flat roof rooflight upstands: height, insulation and finish goes through the build-up layer by layer, and the answer on what a builder's upstand is is a good place to start if the word is new to you.

A frame takes up glass. Remove the bulky perimeter and more of the opening becomes clear pane, so a frameless rooflight delivers more daylight than a framed unit cut into the same hole.

The difference is easiest to see by thinking about a modest opening. On a framed unit, each side loses a band of the opening to the frame, the bead and the cill, and those bands are a fixed width whatever the size. On a small opening they add up to a noticeable share of the light. On a frameless unit the loss at the edge is a thin strip. The gain is greatest on small and narrow units, which is where a designer is usually fighting for every millimetre of glass.

Daylight is also about what happens to light once it is inside. A shallow upstand with a hidden frame leaves a short, wide reveal. Sunlight can spread across the ceiling and walls rather than being shaded by a deep frame on one side. A narrow edge also means no heavy line of a different material against the white ceiling, which keeps the room calm in the hours when the sun is low.

A few practical points follow from this:

1. The reveal below is now the main visual edge, so it deserves proper attention. Lining the reveal under a flat rooflight is covered in its own guide.
2. Glass choice shows more, because nothing distracts from the pane. Tint, coatings and the spacer line are all easier to notice.
3. Low winter sun gets in at a flatter angle, so a north or east edge of the ceiling is often the brightest.

The brightest room is rarely the one with the biggest hole. It is the one where the glass, the reveal and the ceiling have been thought about together.

A framed unit has a lip around its top edge. Leaves collect against it and puddles form where the bead sits proud of the pane. A frameless unit has no such lip, so the water has a clear path from the glass to the roof.

This is the practical argument for the build, and it is stronger than the aesthetic one. Rain running down a flat pane meets a continuous surface all the way to the edge and drops onto the roof beyond the upstand. The same goes for melting snow and for the fine grit that settles on glass through the summer. Because the top face is flush, rinsing from the rain carries the dirt away rather than depositing it along a ledge.

The glass is still laid to a shallow angle. A flat rooflight is never quite flat, and the pane is usually set at a small fall so that water moves to one edge, which is the low one. You may also notice that the surrounding roof has its own fall, and the upstand interrupts that, so water coming down the roof meets the high side of the kerb and has to divide round it. Both ideas are explained in falls, drainage and why flat rooflights are never dead flat.

If you have seen standing water on a rooflight and wondered why, why water sits on a flat rooflight explains the cause, which is usually a fall that is too shallow rather than anything in the glass itself.

The two builds are close cousins, and the right one depends on the room, the roof and the finish you want. This table sets the main differences side by side.

A framed unit has its place. On a rough existing roof, or where the upstand cannot be rebuilt, a framed product tolerates the roof as it finds it. A frameless one rewards good preparation, and punishes poor preparation more visibly, because there is no frame to cover an uneven edge.

Where the choice is between a fixed unit and something that opens, the page on fixed flat rooflights for extensions explains why a fixed pane suits the usual room under a flat roof.

With the frame out of the way, the glass is the product. Its make-up decides how warm the room is, how safe it is overhead and how it sounds in rain.

A typical frameless unit is a sealed double or triple glazed package. The outer pane is often toughened for strength against hail and thermal stress. The inner pane over a room is laminated, which means two sheets of glass bonded to a plastic interlayer. Laminated glass holds together if it breaks, which is what good practice asks of overhead glazing, and it also does something useful for sound. The interlayer damps vibration in the glass, so rain and the hum of traffic ring less in the room.

Coatings add a further choice. A low emissivity coating keeps heat in during winter. A solar control coating cuts heat gain for south facing roofs. Either can be added to a frameless unit, and the choice should follow the aspect of the room, not a default choice. What glass a flat rooflight should have goes through the options in plain terms.

Because the unit is bonded at its edge, the spacer, the sealant and the cavity need to be specified with the same care as the coatings. Thin glass and tired seals are where a rooflight first begins to fail, and the edge is where they show. That is why a frameless unit should be bought from a maker who states the build and is happy to have it written down.

Rain on flat glass is the noise that gets noticed first. A frameless rooflight changes how that noise is made, because the pane is not clamped in a rigid, ringing frame. The sound still depends on what is in the glass.

A few parts of the build decide how it sounds:

• A laminated inner pane damps the pane, so the drumming is duller and further away.
• A wider cavity between the panes, or panes of different thickness, stops the two layers ringing together.
• Compressible seals under the carrier frame stop the structure passing vibration into the ceiling.
• A well insulated upstand keeps hard surfaces from sounding like a drum skin.

Over a loft bedroom or a nursery, these choices are more important than the choice of frame. Over a kitchen, where the pan extractor and the radio already mask much of the weather, they are still worth making. We write them down in the quiet spec that comes with every quote, along with what we heard at the survey. For a bedroom, quieter glazing for a flat rooflight over a bedroom goes through the layers in more depth.

In Crawley and the towns around it, the noise that reaches a flat roof is seldom only rain. Aircraft on the Gatwick routes pass over parts of the borough, and the A23, the M23 and the Brighton main line carry sound across the neighbourhoods beside them. A frameless rooflight lets in the sky, and the glazing build decides how much else comes in with it.

A frameless unit leaves a clean slot of glass at the top of the reveal. There is no frame to hide the line where plaster meets light, so the finish inside is more visible and needs more care.

The reveal, which is the sloping or vertical lining between the ceiling and the glass, is usually formed in plasterboard over the upstand timbers. The insulation in the upstand runs down behind it. The aim is a crisp, straight edge with a narrow shadow gap, and a colour that does not compete with the glass. A white reveal reflects the greatest amount of light into the room. A slightly angled reveal, splayed outwards towards the ceiling, spreads light more widely than a square one.

Some points that make the difference between a neat finish and a disappointing one:

1. The upstand inside face must be straight, because the plasterboard follows whatever the timber does.
2. The lining should be fully supported, since it is visible along its whole length.
3. The junction between plaster and glass is usually finished with a thin trim or a formed bead, never left to paint alone.
4. Blinds, if wanted, need space decided before the lining goes up. Fitting a blind to a flat rooflight explains what works under a flush pane.

The internal finish is made good as part of the job, so the room is left ready for decoration.

The work on the roof comes before the glass. A frameless unit depends on a true, level, well formed upstand, so the bulk of the time on site goes into building that, not into lifting the glass into place.

The sequence is straightforward. The opening is marked and cut, and the joists are trimmed around it so that the load is carried properly. The upstand is built and insulated. The waterproofing is dressed up and over it, in whichever system the roof already has, and fitting a flat rooflight into an EPDM rubber roof covers the details that change for one common system. Then the carrier frame goes on, then the glass, and lastly the inside is lined and finished.

The glass is heavy, so lifting and placing it is a two or three person job, often with a small crane or hoist on a larger unit. The work is planned around a dry day, because the upstand and the membrane joint must be protected until the unit is on. A frameless unit is fitted to current Building Regulations, and where the work is notifiable, we handle the Building Control notification.

A flush pane is easier to clean than a framed one. Nothing stands proud at the edge, so a soft brush or a water fed pole reaches every part of the glass and the dirt rinses clear.

Cleaning is gentle work. Use clean water and a non abrasive cloth or soft brush, and avoid anything harsh on coatings. A little regular attention stops a grey film forming, and leaves the glass looking clear and bright. Leaves and debris can lodge on the roof beside the upstand, rather than on the glass, so a quick look from time to time is enough. How to clean a flat rooflight has the simple routine.

It suits a roof where the upstand can be built to the right height and insulation, and a room where a clean edge is worth having. It is well placed over a kitchen or a living space, a stairwell, a hallway or a bedroom where a slim edge and more glass help the light.

It is less obviously the right answer if the roof is in poor condition, if the fall is too shallow to drain well, or if the budget has to be stretched over the smallest possible list of changes. In those cases, the roof comes first. A framed unit on a sound roof is better than a frameless one on a poor one.

Cost depends on size, glass, the upstand work and the roof system, and the costs page gives the general picture. A frameless unit asks for more preparation, and that is where the difference in cost usually sits, rather than in the glass.

These are the points that come up when someone is comparing a frameless flat skylight with the framed kind.

Is a frameless flat rooflight weaker than a framed one?

No. The frame is still there, only hidden, and it is sized for the glass and the loads on it. A properly built frameless unit carries snow, wind and a person cleaning it. The upstand and the bonding do the work that a visible frame would otherwise do.

Can a frameless rooflight go into an existing flat roof?

Often, yes, provided the upstand can be built or rebuilt to suit, and the roof deck is sound. The answer on putting a rooflight in an existing flat roof covers what is checked at the survey.

Will a frameless rooflight be louder in rain?

Not by reason of the frame. Sound depends on the glass, the cavity, the seals and the upstand. A laminated acoustic inner pane does the larger part of the work, and we specify it for the room in the written quiet spec.

Does a frameless rooflight need a bigger upstand?

It needs a well formed one, tall enough for the waterproofing and insulated enough to keep the hidden frame warm. The exact height depends on the roof build, and the survey confirms it. To talk it through for your roof, get a quote and we will write the spec down.