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 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.

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. Glass types LUNEKS offers for retractable roofs

Clear polycarbonate also lets plenty of light in: the Spanish Wikipedia entry on polycarbonate 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.

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. 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.

Thermal conductivity is what separates them. According to Wikipedia, soda lime window 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 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.

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 or cantilevered terraces it is worth checking what load the support can take. How LUNEKS sizes the roof frame

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.

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. Maintenance LUNEKS recommends for retractable roofs

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.