Plastic and Polycarbonate Skylight Installation

What a dome is

A plastic rooflight is a thermoformed sheet: a flat panel of acrylic or polycarbonate heated until it softens, then drawn over a mould to give it shape. The shape is structural. A flat panel of any useful size will sag, drum in the wind and hold water, whereas a dome, a pyramid or a barrel vault is stiff for its weight and sheds water by geometry rather than by falls. That is the whole engineering idea, and it is a good one.

The unit comes either with its own moulded kerb, usually glass-reinforced plastic or rigid PVC bedded onto a timber upstand, or as a bare dome with a flange to sit on a builder’s kerb formed and weathered on site. Domes are single, double or triple-skin, in clear, opal or bronze tints. On commercial roofs the same materials appear as long runs of profiled or multiwall sheeting.

How they became the default

Plastic rooflights arrived with the post-war building boom and the flat-roofed extension. Glass was heavy, needed a substantial frame, needed a structure sized to carry it, and needed careful handling at roof level. A moulded dome weighed a fraction of the same area in glass, arrived in one piece, and could be carried up a ladder by one person. It went into the builders’ merchant catalogue and stayed there, and garages, side returns, stairwells, workshops and a great many rear additions took one.

None of that history is a criticism. A plastic dome was the sensible answer to the question being asked at the time, and in many settings it still is. What has changed is the standard the building around it is held to. We install new plastic and polycarbonate units where they are the right specification, and replacements where an existing unit has finished.

Multiwall against solid sheet

The two constructions behave differently enough that they are effectively different products, and the words are used loosely in sales material.

Solid sheet is one continuous thickness of plastic, thermoformed into a dome. It can be optically clear, so you can see the sky through it, and it gives the cleanest appearance from below. Its weakness is thermal: a single skin insulates very little, so a solid single-skin dome is close to a hole in the roof as far as heat is concerned. Double and triple-skin versions hold two or three moulded domes apart in a common frame, which introduces an air gap and, with it, the sealing problem discussed further down this page.

Multiwall sheet is extruded rather than moulded flat: two or more thin skins held apart by continuous internal webs, so the section looks like corrugated card in profile. The cavities between the webs are the insulation. Multiwall is light, strong across the flutes and cheap for the area covered. It cannot be optically clear, because the webs refract everything passing through, so it always gives diffused light with visible striping. It is the standard material for canopies, lean-to roofs and industrial rooflighting.

Polycarbonate against acrylic

Both are clear thermoplastics, both have been used for rooflights since the trade existed, and they fail in different ways. Neither is the premium option.

Acrylic is harder at the surface, so it resists scratching and abrasion from wind-blown grit. It has excellent optical clarity and holds it, because the polymer is naturally stable under ultraviolet light rather than relying on an added layer, and it is cheaper. Its weakness is brittleness: acrylic cracks rather than deforms, so a dropped tool or a stone from a mower will break it cleanly. It also moves a good deal with temperature, so an acrylic dome screwed down tight through undersized holes will crack at the fixings in its first hot summer.

Polycarbonate is the tough one. Its impact resistance is in a different category, which is why it is specified where glazing may be struck, walked near or vandalised: it bends and absorbs energy rather than shattering. The trade-off is a softer surface that scratches more readily, and a polymer that is not naturally stable under ultraviolet light. Every quality polycarbonate rooflight therefore carries a co-extruded ultraviolet-resistant layer bonded to the weather face at manufacture. That layer is the product’s service life. Polycarbonate also costs more and moves more.

In practice, domestic domes on a garage or a rear addition are often acrylic, and anything within reach of a person, a ladder or a football is polycarbonate.

Why plastic goes cloudy

This is the question we are asked most about existing units, and the answer is more reassuring than people expect, because nothing has gone wrong.

Ultraviolet radiation carries enough energy to break the chemical bonds in a polymer chain. Every day a rooflight is exposed, a small proportion of the molecules at the surface break and recombine into new compounds, some of which absorb visible light in the blue part of the spectrum. Absorbing blue makes the material look yellow. That is all yellowing is: a chemical change accumulating at the exposed face, slowly and irreversibly. Acrylic resists it inherently. Polycarbonate resists it through its co-extruded layer, and once weather and grit have eroded that layer, the base polymer is exposed and degradation accelerates.

Alongside yellowing, three other things happen. Crazing is a network of fine surface cracks, produced by ultraviolet degradation, thermal cycling between hot afternoons and cold nights, and stress locked into the sheet at moulding or by over-tightened fixings. Crazing scatters light, which is why an old dome reads as milky rather than merely yellow. Surface erosion is mechanical: rain, hail, grit and years of movement roughen a face that started out smooth, and a rough face holds dirt and algae that no cleaning fully lifts. Embrittlement is the serious one. As the polymer chains shorten, the material loses the toughness it was specified for, so a sheet that would once have taken a falling tool can break under far less.

All four are the normal ageing of an organic material left outdoors facing the sun. A dome that has gone cloudy has not been badly fitted and has not developed a fault. It has done the work it was made to do and reached the end of its service life. Nothing can be done to the sheet itself: polishing removes what is left of the protective layer, coatings do not bond to a degraded surface, and cleaning a crazed dome makes it briefly cleaner and no clearer. The remedy is a new unit.

Condensation inside the skins

The complaint that brings people to this page most often is water sitting inside the rooflight, where it cannot be reached.

In a multiwall sheet, the flutes between the skins are long, narrow, closed voids. At the factory the cut ends are taped: solid tape at the top edge against dirt and water, breather tape at the bottom to let vapour escape while filtering dust. The system works only while the tapes and end profiles hold. Over years of thermal movement, ultraviolet exposure and roof-level weather, the tape lifts at a corner, a profile loosens, or a fixing hole is drilled into a flute. Moist air then enters, and when the outer skin cools at night below the dew point of the air trapped inside, that vapour condenses on the inner face, runs down the flute and collects at the bottom edge. Each cycle brings in a little more than escapes.

The same happens in a double or triple-skin dome, where the skins are held apart at the perimeter by a spacer and a gasket rather than the hermetic, desiccated seal a glazed unit uses. Once that gasket has hardened and shrunk, the cavity breathes, and the cavity is where the condensation forms.

It cannot be cleared. The cavity is not accessible without destroying the sheet, the internal faces cannot be dried in place, the dirt and algae arriving with the moisture stain them, and any hole drilled to drain it becomes the route by which the next lot of moisture arrives. Once a sheet or a multi-skin dome has water between its skins, that unit is finished as a piece of glazing whatever condition the plastic is in otherwise. A new unit is the answer.

The thermal performance gap

A rooflight faces the sky, the coldest surface it will ever see, and it does so at the highest point of the room, where the warmest air collects. It is the hardest-working element of the roof in thermal terms, and plastic domes were designed before that was taken seriously.

Several causes act together. The skins are thin, and thin plastic conducts heat readily. The air gaps are narrow, so they do very little. There is no low-emissivity coating, which in a modern glazed unit reflects long-wave heat back into the room and does more for the figure than the gap itself, and there is no inert gas fill. The perimeter is usually a plastic or fibreglass section with no thermal break, so heat short-circuits around the edge. And underneath it, a kerb built in an era when kerbs were not insulated runs cold from the deck up to the frame.

What that means for the room below is specific rather than abstract. The internal face of the unit and its frame run cold in winter, so warm moist air from the room meets a cold surface and forms water on it. People see that water and conclude the roof is admitting rain. It is not. It is condensation, and it appears on the coldest nights, which are exactly the nights when nothing is falling from the sky. The room also loses heat steadily through the unit, so a converted outbuilding with a large dome is expensive to hold at temperature. In summer the same unit admits solar heat freely, because plastic offers no solar control glazing of the kind a modern sealed unit can carry.

Where a rooflight is replaced in a heated part of a dwelling, thermal performance is not merely a comfort question. It is assessed against the standards current at the time of the work, and a replacement is the moment the building is brought up to those standards rather than the moment they are avoided. We deal with the Building Control notification as part of the job.

Diffused light or view

Clarity is treated as the goal in most glazing conversations, and for a plastic rooflight that is often the wrong instinct.

An opal or textured dome, and any multiwall sheet, scatters incoming light rather than transmitting an image. The room below gets illumination without a beam: no sharp square of sunlight tracking across the floor, no hard shadows, no glare on a screen or a workbench, and far less difference between a bright morning and a dull afternoon. That is genuinely valuable in some rooms. A bathroom gains daylight with no possibility of being overlooked from an upper window next door. A workshop gains flat, even working light over a bench, which is easier to work under than direct sun.

What diffusion costs is the sky. A clear unit gives a view of moving cloud, of rain arriving, of the changing colour of the light through the day, and it is that connection to the weather rather than the raw quantity of daylight that people describe when they say a room feels open. A diffused dome gives brightness and nothing to look at. It also conceals its own condition, because it already looks white.

The useful question at survey is therefore not how much light the room needs but what kind. Over a bathroom, a utility room, a workshop or a store, diffusion is often the better answer and plastic supplies it cheaply. Over a kitchen, a sitting room or a loft conversion, a clear glazed unit is usually what the client wants once the difference is described honestly.

Where polycarbonate still belongs

Plastic rooflighting is not obsolete. There are settings where specifying glass would be spending money to make something worse.

Garages and detached outbuildings. Unheated, not habitable, not assessed for thermal performance, and often reached only by ladder. The thermal gap is irrelevant because there is nothing to keep warm, and the cost difference is real.

Workshops, studios, covered ways and canopies. Even, diffused, shadowless light delivered cheaply over a large span is what multiwall does best, and its light weight suits the slight structures these roofs are built from. Impact resistance matters too, because they sit at low level where things hit them.

Commercial and light industrial roofs. Long runs of in-plane rooflighting integrated with profiled metal sheeting, where the daylight is doing a functional job and the roof is designed around the sheet profile.

Concealed domestic flat roofs where budget is the constraint. A rear outbuilding, a store, a side return that nobody will sit in.

Where it is not the right answer is equally clear. A kitchen extension, a loft conversion, or any position where the rooflight is part of what the room is for, all argue for glass. So does any position visible from a public vantage point in a designated setting, and any situation where the unit’s contribution to the thermal performance of a dwelling has to be demonstrated.

Roofs that get walked

On garages, outbuildings and every commercial roof, somebody eventually goes up there: to clear a gutter, to service a flue, to inspect a covering. Where a rooflight sits in that roof, its behaviour under a person’s weight stops being a detail.

Roof assemblies are classified as fragile or non-fragile by drop testing the whole assembly, not the sheet alone. That matters more than any product description, because the result depends on the sheet, its thickness, the span between purlins, and the fixing type and spacing. The same sheet can form a non-fragile assembly at one spacing and a fragile one at another. Polycarbonate can be specified into a non-fragile assembly precisely because it deforms and absorbs energy instead of breaking. Acrylic domes are generally fragile and should be treated as such.

Two points are worth stating plainly. A non-fragile classification means an assembly should resist a person falling onto it, not that it is a walkway: nobody should walk on a rooflight of any material. And classification describes a unit when new. An aged sheet that has lost its protective layer and gone brittle no longer behaves the way the original test says, and there is no way to tell by looking, so an old translucent panel should be assumed fragile.

When a dome fails

This is the real decision, and it deserves the arguments on both sides rather than a recommendation dressed up as advice.

The case for replacing like for like

A new plastic unit costs less than a glazed one, sometimes considerably less. It weighs a fraction as much, so the existing structure and trimmers carry it without question. It usually suits the existing kerb dimensions, because dome sizes have been broadly standardised for a long time, which keeps the roof open for less time. On a commercial roof it matches a run of neighbouring units, so the roof continues to read as one thing, and it preserves a non-fragile assembly where that was designed in. On an unheated garage or store, the performance a glazed unit would add is performance nobody in that building will ever notice.

The case for moving to glass

Glass does not yellow, does not craze and does not lose clarity, so the appearance you take delivery of is the appearance you keep. A modern sealed unit is hermetically closed with a desiccated spacer, which is why it does not fill with condensation the way a multi-skin dome does. Thermal performance is in a different class, so the room below is warmer in winter, less prone to surface condensation, and controllable in summer if solar control glass is specified. Rain noise is markedly lower, and over a bedroom or a sitting room that alone changes how the room is used. A clear unit gives a view of sky rather than a bright blur. And where the building is designated or the roof is seen, a flush glazed rooflight is very often the only specification with a route through the planning position.

The split falls where common sense puts it. On a detached garage, an unheated store or a workshop, replacing like for like in polycarbonate is usually the better use of the money and we will say so. On a rear addition that is now a kitchen, a bathroom or a room somebody sits in, the glazed unit repays the difference. Between those two it depends on how the room is used, what the roof is seen from, and what the budget is doing elsewhere.

The opening underneath

Whichever way that decision goes, taking an old dome off exposes something nobody has looked at since it was fitted. The kerb is the raised timber or moulded upstand the unit sits on. On an older installation it is frequently too low, which puts the base of the unit and the termination of the covering into the zone where water stands on a flat roof. It is very often uninsulated, so it runs as a continuous cold bridge around the opening and produces exactly the condensation the owner blames on the rooflight. Timber sitting under an aged covering may have taken moisture at an edge or a corner, and the covering dressed up the kerb under the old flange may have hardened, shrunk or split.

None of that is discovered halfway through if the survey has been done properly, which is why we establish kerb height, squareness, insulation and covering condition before anything is ordered. Where the kerb is sound, the new unit goes onto it and the covering is dressed and terminated correctly. Where it is not, the kerb is rebuilt to the right height, insulated as a continuation of the roof insulation, set out square to the new unit, and the covering taken up and over it so the unit closes over the waterproofing rather than acting as it. Making the opening good is part of the installation, priced in the quotation.

Conservation and the visible roof

Oxford makes this question sharper than most places, and the honest position has two halves.

A plastic dome is rarely acceptable on a roof that can be seen. It sits proud of the plane, it is bright white or bronze against slate, stone or clay, and it reads unmistakably as a modern component. On a listed building, on a prominent roof slope in a conservation area, or on any elevation visible from a street, a footpath or an open green, a dome is very unlikely to be the specification an officer accepts. The route in those positions is a flush glazed rooflight detailed to sit in the plane of the roof, dark finished, and where the setting calls for it carrying the single vertical bar that makes it read as a traditional cast-iron light.

The other half is equally true and gets said less. On a concealed flat roof, a plastic dome is frequently unremarkable. A rear outbuilding behind a parapet, a store in a back garden, a garage roof invisible from any public vantage point: nobody is being asked to look at the unit, and specifying a glazed conservation rooflight there is spending the client’s money on a problem that does not exist.

The Oxford context changes the odds. The central conservation area is unusually large, North Oxford is a substantial designated Victorian suburb in its own right, and village cores absorbed as the city grew carry their own designations. The listed building count is high and includes ordinary terraces and cottages, not only landmark stone. Article 4 directions removing permitted development rights are applied street by street. And where an outbuilding stands within the curtilage of a listed building, it can be covered by the listing itself.

We are installers, not the local planning authority, and we do not present ourselves as the people who decide. What we can tell you is the sort of position your building is likely to be in, what an officer usually looks for, and which specification tends to satisfy it. Where a determination is needed, that belongs to the council. Where you are working with an architect, we specify to their drawings.

What the price follows

A figure published without seeing the roof is close to meaningless, particularly on replacement work where what sits under the old unit governs the job. What is useful is knowing which factors move the number.

The unit itself. Acrylic against polycarbonate, single, double or triple skin, and whether a stock size will do. A standard dome onto an existing kerb is the cheapest outcome available. Cost scales with area, and a run of units on one visit is cheaper each than a single one.

What is under it. Rebuilding an undersized or uninsulated kerb is a real item of work, and dressing an aged covering to a new upstand takes time and material. A new opening rather than a replacement brings trimming, deck work and structural checks that a like-for-like swap does not, and moving to a glazed unit usually means the kerb is rebuilt to suit the weight.

How the roof is used and reached. A non-fragile requirement sets sheet thickness, span and fixing pattern. Access covers height, what stands below the elevation, and whether the unit can be handled up by ladder. Where an application for consent is required, that is time and fees before any work starts.

The sequence we work in

The building first. What it is, what designations apply, and whether the roof is seen from anywhere. Much of this is established before anybody visits.

The roof second. Structure, deck, covering, insulation build-up, and on a replacement the height, squareness, insulation and condition of the existing kerb. A survey at roof level, not a look from the garden.

The specification third. Plastic or glass, and if plastic then acrylic or polycarbonate, solid or multiwall, clear or diffused, and what the assembly needs to be if the roof gets walked.

The consent position fourth. Whether the work sits within permitted development, whether listed building consent is engaged, and whether the visible position of the roof changes what can be fitted.

The price last. A number quoted before the first four steps is a guess wearing a suit, and usually a low one that gets revised once the old unit is off.

Our workmanship carries a ten-year guarantee, with its coverage, exclusions and how to claim set out on its own page. We have been installing rooflights for fifteen years, we cover Oxford and up to twenty five miles by road, and we install new units and replacements only. If you are weighing a polycarbonate dome against moving to glass, the useful first conversation is about what the roof is, what the room underneath is for, and what sits under the unit you already have.