Hollow Polycarbonate Sheets

Hollow polycarbonate sheets — also called multiwall, twinwall or cellular sheets — are extruded with internal flutes running the length of the panel, forming air chambers between two or more skins. That structure is what makes them the most economical way to roof a large area in daylight, and it is why they cover most of the carports, walkways, greenhouses and pergolas you see across Kenya.

BHD Kenya stocks hollow polycarbonate in Nairobi, in clear and tinted finishes, with delivery countrywide.

What the air chambers actually do for you

The flutes are not just a way of using less material. They change how the sheet performs in three ways that matter on a Kenyan site:

• They carry load. The cellular structure gives the sheet far more stiffness per kilogram than a solid panel, so it spans further between purlins for the same weight. That means lighter framing and fewer supports.
• They insulate. Trapped air is a poor conductor of heat, so a hollow sheet transmits noticeably less solar heat than a single-layer panel. Under a hollow roof a carport or walkway stays cooler than it would under iron sheets or clear solid glazing.
• They quieten rain. The chambers break up impact noise. A hollow polycarbonate roof in a downpour is dramatically less loud than iron sheeting, which is why it is chosen for restaurant terraces, hotel walkways and covered seating.

The trade-off is the view. Internal walls diffuse light and obscure what is behind the sheet, so hollow is not the choice where genuine transparency is required.

Twinwall, triple-wall and multiwall

Hollow sheet is described both by its overall thickness and by the number of skins in its cross-section. A twinwall sheet has two skins and one row of chambers. Triple-wall and heavier multiwall sheets add more skins and more rows, which increases stiffness, improves insulation and allows longer spans — at greater weight and cost.

Thicker is not automatically better. The right specification is the lightest sheet that will span your purlin spacing safely while giving the insulation the space needs. Over-specifying wastes money; under-specifying produces a roof that flexes, ponds water and eventually cracks at the fixings.

Sealing the flutes is not optional

This is the detail most often skipped, and it is the reason so many hollow polycarbonate roofs look dirty within a year. The flutes are open tubes at both cut ends. Left open, they fill with dust, insects, rainwater and condensation, and because the chambers are sealed from view there is no practical way to clean them out afterwards. The roof develops dark streaks visible from underneath and stays that way.

Done properly, the top edge of each sheet is closed with solid aluminium tape and the lower edge with breathable (vented) tape, which lets condensation escape while keeping dust and insects out. The flutes must run down the slope so water can drain, and the ends are then capped with U-profile. Getting this right at installation costs very little and is the single biggest determinant of how the roof looks in five years.

UV protection determines the lifespan

Polycarbonate degrades under ultraviolet light unless a UV-resistant layer is co-extruded onto the weather-facing skin during manufacture. It is part of the sheet, cannot be applied afterwards, and is the specification that separates a roof lasting years from one failing in a season. In Kenya's sun an unprotected sheet yellows, clouds and turns brittle, and the flutes make the discolouration especially obvious.

Protected sheets are marked on the masking film to show which face goes outward. Fitting a sheet upside down defeats the protection entirely, which is a genuinely common and entirely avoidable site error. If UV protection is not confirmed in writing, assume it is absent.

Where hollow polycarbonate is used

Carports, car shades and porches

The most common application in Kenya. Hollow sheet covers the span economically, keeps the vehicle out of direct sun, and stays quiet in rain. Tinted sheet is worth considering where the parking area faces west.

Greenhouses and nurseries

Hollow sheet is preferred over clear glazing for growing because it diffuses light, spreading it more evenly across the crop instead of casting hard shadows, while its insulation moderates the temperature swing between day and night.

Walkways, corridors and pergolas

Covered links between buildings, school corridors and garden pergolas, where the point is shelter and daylight rather than a clear view of the sky.

Restaurant terraces and hotel walkways

Chosen specifically for the rain-noise reduction, which makes covered outdoor seating usable during a downpour rather than abandoned.

Industrial roof lights

Translucent hollow panels set into a warehouse or factory roof bring in daylight and cut the daytime lighting load, without the heat gain of clear glazing.

Clear or tinted

Clear hollow sheet passes the most light and is the default. Tinted, bronze and opal sheets pass less, which is an advantage more often than people expect — under the equatorial sun a clear roof over a west-facing terrace or a north-facing shopfront can make the space uncomfortably bright and hot. Where the priority is shade rather than maximum light, a tint is the better specification.

Specifying and fixing

Beyond thickness and wall structure, what determines the specification is your purlin spacing, the span you need to cover, and whether the sheet will be laid flat, pitched or curved. Hollow sheet can be cold-curved along the flute direction, which is how barrel canopies are formed, within a minimum radius set by its thickness.

Fixings must pass through oversized holes fitted with bonded washers, never driven tight, because polycarbonate moves measurably with temperature. Screwing a sheet down hard is the usual cause of cracking around the fixings in the first hot season.

Tell BHD the span, the purlin spacing and whether the space needs maximum light or shade, and the team will confirm current stocked thicknesses, wall structures, sheet sizes and tints, along with the tapes, H-connectors, U-profiles and screws the installation requires.

Cleaning and care

Rinse with plenty of water and a mild detergent using a soft cloth or sponge. Never use an abrasive pad, a dry cloth or a solvent-based cleaner — all of them will scratch or craze the surface. Avoid walking directly on the sheets; use a crawl board spanning the purlins.

Buying hollow polycarbonate sheets in Kenya

BHD Kenya is a supplier and stockist rather than a manufacturer, so the range is selected for Kenyan conditions rather than set by a single factory. Sheets are held in Nairobi for collection or countrywide delivery. What you pay depends on thickness, wall structure, sheet size, tint and quantity, so send your dimensions and the team will confirm current rates and availability.