# Retractable roof: glass, polycarbonate or sandwich panel

> Retractable roof in glass, polycarbonate or sandwich panel: light, heat, insulation, weight and upkeep of each material for a terrace in Alicante.

## Which material should you choose for a retractable roof: glass, polycarbonate or sandwich panel?

Glass gives the most light and a view of the sky, but lets the sun's heat through and weighs more. Polycarbonate is light and diffuses the light, though it scratches more easily and expands in the heat. Sandwich panel is opaque and insulates against both heat and cold. In Alicante the direction the terrace faces usually settles it.

A [retractable terrace roof](https://luneks.es/en/retractable-roofs/) is made of panels that slide along aluminium rails and stack over one another. The mechanism works the same way with all three materials: in good weather you open up the sky, and in rain or cold you close it. What differs is the infill of each panel, which decides how much light gets in, how much heat passes through and how much the whole roof weighs.

So the useful question is what happens under the roof when it is closed. Clear glass over a south facing terrace in July behaves very differently from sandwich panel on the same terrace. Think about the hours when the roof will be shut, because that is when the material is doing its job; open, there is hardly any difference.

## How much light does each material let through?

A clear pane shows the sky exactly as it is. From inside you barely notice there is a roof, which is why it is the usual pick when the terrace opens onto a living room you do not want to darken. Solar control or screen printed glass cuts the light and heat a little while keeping the view. (to be confirmed)

Clear polycarbonate also lets plenty of light in: the [Spanish Wikipedia entry on polycarbonate](https://es.wikipedia.org/wiki/Policarbonato) gives a total light transmission of 86%. Opal or tinted sheets diffuse that light, so the terrace stays bright without direct sun. Sandwich panel is opaque: with the roof closed, light only comes in from the sides, much as it would under a solid porch roof.

## Which material gets hottest underneath in summer?

Alicante averages 2,851 hours of sunshine a year and an average August high of 30.8 °C, according to the [AEMET climate averages for Alicante](https://www.aemet.es/es/serviciosclimaticos/datosclimatologicos/valoresclimatologicos?l=8025&k=val). With that much sun, a clear roof closed at midday works like a greenhouse: the radiation passes through the pane, warms the floor and the furniture, and the heat stays in unless there is ventilation.

Opal polycarbonate, or sheet with a reflective treatment, blocks part of that radiation, so the terrace warms up less than under clear glass. Sandwich panel stops nearly all direct sun, because it is opaque and has an insulating core between two metal skins. Whichever material you choose, opening the roof a few centimetres or leaving the sides open lets the hot air out.

## Which one keeps the cold out best in winter?

Thermal conductivity is what separates them. According to Wikipedia, [soda lime window glass](https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass) conducts between 0.7 and 1.3 W/(m·K), and polycarbonate between 0.19 and 0.22 W/(m·K). The [core of a sandwich panel](https://es.wikipedia.org/wiki/Panel_s%C3%A1ndwich) goes down to roughly 0.024 W/(m·K) with polyurethane and about 0.05 W/(m·K) with mineral wool.

On the terrace, that means sandwich panel insulates many times better than glass of the same thickness, with polycarbonate somewhere in between. Even so, a retractable roof has seals between sliding panels and is never as airtight as a built roof. If you want to use the terrace in January as an extra room, the side glazing matters as much as the roof material.

## How do the three materials compare in figures?

Base material values from Wikipedia: [soda lime glass](https://en.wikipedia.org/wiki/Soda%E2%80%93lime_glass), [polycarbonate](https://es.wikipedia.org/wiki/Policarbonato) and [sandwich panel](https://es.wikipedia.org/wiki/Panel_s%C3%A1ndwich). How the roof actually performs also depends on thickness, seals and the manufacturer.

| Feature | Glass | Polycarbonate | Sandwich panel |
| --- | --- | --- | --- |
| Light | Clear, you see the sky | Total light transmission of 86% for clear sheet | Opaque |
| Density or weight | 2.53 g/cm³ (flat glass) | 1.20 g/cm³ | 10 to 35 kg/m², depending on core and skin |
| Thermal conductivity | 0.7 to 1.3 W/(m·K) | 0.19 to 0.22 W/(m·K) | Core from 0.024 (polyurethane) to 0.05 W/(m·K) (mineral wool) |
| Thermal expansion | 9.5 × 10⁻⁶ per K (flat glass) | 65 to 70 × 10⁻⁶ per K | Depends on skin and core |
| Surface | Hard, difficult to scratch | Scratches with abrasives, suffers under UV without protection | Powder coated metal |

## Why do weight and expansion matter?

Flat glass has a density of 2.53 g/cm³ and polycarbonate 1.20 g/cm³, so at the same thickness glass weighs more than twice as much. That weight is carried by the rails, the carriages and the motor, if there is one. A glass roof needs a frame designed for it, and for [penthouse enclosures](https://luneks.es/en/penthouse-enclosures/) or cantilevered terraces it is worth checking what load the support can take. (to be confirmed)

Expansion works the other way round. Using the same Wikipedia figures, polycarbonate expands about seven times more than glass for the same temperature change. On a terrace that goes from a cool morning to a hot sunny afternoon, the sheets grow and shrink every day, so they are fitted with clearance in the profiles to stop them rubbing or warping.

## How do you clean each material, and how does it age?

Glazing is the easiest to clean: water and an ordinary glass cleaner do the job, and the surface puts up with wiping. Polycarbonate needs more care. Wash it with water and mild soap, without scourers or solvent based products, because it scratches. The UV protected face has to point outwards, since the material itself has poor resistance to ultraviolet light, as Wikipedia notes.

Sandwich panel is looked after like any powder coated metal: a rinse with fresh water, especially near the sea, to wash off salt and dust. With all three, the parts that need most attention are the rails and seals. Keep them clear of leaves and sand so the panels go on running smoothly for years. (to be confirmed)

## Which roof works best for what sits underneath?

The same terrace can call for a different material depending on what you use it for. Before looking at brochures, think about what will be under the roof, at what times of day you sit there and whether you want to use it once the evenings turn cool. These are the combinations that tend to work best in homes on the Costa Blanca:

- Outdoor dining area next to the living room: glass, so the house keeps its light.
- Penthouse terrace in sun all day: opal polycarbonate or solar control glazing.
- Seating area by the pool: polycarbonate, which filters the sun and weighs little.
- Porch you want to use in winter: sandwich panel, which insulates better.
- Summer kitchen or barbecue area: sandwich panel, which does not heat up the way glazing does.

## How do you choose the material in five steps?

1. **Check which way it faces.** A south or west facing terrace with strong afternoon sun calls for opal polycarbonate, solar control glass or sandwich panel.
2. **Think about winter.** If you want to use it in the cooler months, sandwich panel insulates better; glass also needs closed sides.
3. **Decide how much sky you want.** If seeing the sky matters, only glass keeps the view intact.
4. **Check the support.** On penthouses and cantilevered terraces, the weight of glass can shape the choice of frame.
5. **Add the sides.** With [glass curtains](https://luneks.es/en/glass-curtains/) along the sides, the retractable roof closes the terrace completely.

> When you get a quote for a polycarbonate roof, ask how much clearance the sheets have in the profiles. Polycarbonate grows in the August heat, and if it is fitted tight, it rubs and creaks every time you open it in summer. With glazing that problem hardly ever comes up.
>
> Mikhail, founder of LUNEKS

## What else do people ask about retractable roofs?

### What is the best roof for a terrace?

It depends on how you use it and which way it faces. If you want to open up the sky in good weather, a retractable roof. For permanent cover, a [fixed roof](https://luneks.es/en/fixed-roofs/). If adjusting the shade is what matters, a bioclimatic pergola. Among retractable roof materials, glass gives light, panel gives insulation and polycarbonate sits in between.

### Does polycarbonate go yellow in the sun?

Unprotected polycarbonate copes badly with ultraviolet light and loses clarity over time. That is why roofing sheets have one face with UV treatment, which must be fitted facing outwards. Installed the wrong way round, the sheet ages much faster, especially under the strong sun of the Costa Blanca.

### Is a glass retractable roof safe?

Roofs use safety glass, usually laminated, which keeps the fragments stuck to the interlayer if it breaks, so nothing falls onto the terrace. The thickness and make up depend on the size of each panel and on the frame that carries it, so they are chosen for each roof. (to be confirmed)

### Does a retractable roof close watertight?

It keeps the rain off, but the panels slide over one another and there are seals between them, so it is not as watertight as a built roof. In strong wind with driving rain, some water can get in at the edges. A lot depends on the seals and the quality of the installation.

### Can a retractable roof be motorised?

Yes, it can be operated by hand or by motor, and a rain sensor can be added to the motor so the roof closes on its own. A glazed roof is heavier, so the motor has to be sized for that weight or it will run at its limit. (to be confirmed)

### How much does a retractable terrace roof cost?

It depends on the area, the panel material, manual or motorised operation and the aluminium finish. We do not publish prices: the quote follows a visit to measure the terrace, because the frame and the material change a great deal from one job to the next.

## What else is worth reading before choosing a roof?

- [Bioclimatic pergola, awning or tensioned fabric](https://luneks.es/en/blog/bioclimatic-pergola-awning-or-fabric/): Three ways to get shade and how each copes with wind and rain.
- [Glass curtains or insulated curtains: which keeps the heat in](https://luneks.es/en/blog/glass-curtains-vs-insulated/): Single or double glazing on the sides and what changes in winter.
- [Enclosing a penthouse without building work](https://luneks.es/en/blog/enclose-penthouse-without-building-work/): How to cover and close a penthouse terrace without putting up walls.

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https://luneks.es/en/blog/retractable-roof-glass-polycarbonate-sandwich/