Kumar IronUpdated
In this guide
How much of the roof to make translucent
Most guidance points to a small share of the roof. The energy codes we checked cap skylight area at around 3 to 5 percent of roof area. Autodesk's daylighting guidance puts about 5 percent of roof area as skylights for full daylighting from traditional skylights, and 1 to 2 percent for tubular skylights. An older article suggests 2 to 4 percent. We did not find a source that supports 10 percent as a typical figure, so treat 3 to 5 percent as the starting range and check your local building code and energy rules.
A translucent sheet that lets through less light needs more area to do the same job. The product's visible light transmission matters, so ask the supplier for it. Spread the light sheets along the length of the roof rather than grouping them in one corner.
| Source | Stated figure | Note |
|---|---|---|
| Energy code caps (examples) | About 3 to 5 percent of roof area | Depends on controls and location |
| Autodesk daylighting guidance | About 5 percent (traditional skylights) | Lower, 1 to 2 percent, for tubular |
| Older daylighting article | 2 to 4 percent | Trades light against heating and cooling |
Matching the profile
A translucent sheet must match the profile of the metal sheets around it, or the overlaps will not line up and water will find the gap. Palram's SUNTUF range is designed to match existing corrugated metal profiles, and the company says it has more than 400 corrugated profiles, with a custom option called MetalMatch for others. Custom profiles need a minimum order.
Take a piece of your metal sheet to the dealer, or note the pitch, depth and cover width, and ask for a matching translucent sheet. Do not assume that a sheet with a similar name will fit.
Polycarbonate or FRP, and thickness
Polycarbonate (PC) is a tough plastic that holds a lot of impact. Translucent FRP (glass fibre reinforced polyester) is stiffer in some cases, and the Indian standard IS 15224 covers translucent GRP sheets for roofing and cladding. We could not retrieve the full text of IS 15224, so check the BIS copy for purlin spacing and lap values before fixing FRP.
For thickness, Palram says the required sheet thickness depends on wind and snow load, panel size and how the sheet is supported, so there is no single safe number to quote. Ask the maker for the thickness that suits your purlin spacing and wind zone, and make sure every edge of the sheet sits on a purlin.
UV side up, and why it matters
Standard SUNTUF has a co-extruded UV protective layer on one side only. Palram says it is critical to install the sheet with the UV side facing up, and the marked strip shows which side it is. The SUNTUF UV2 version has UV protection on both sides. Remove the protective strip after installation, as the instructions say.
Installed with the unprotected face up, the sheet is exposed to the same UV the protective layer is there to block, and it can degrade faster. Fitting it correctly costs nothing; replacing sheets does.
Fixing and sealing
Pre-drill the fixing holes slightly oversized, so the sheet can expand and contract. Palram says pre-drilled holes give the panels room to move with temperature. Use corrosion-resistant screws with EPDM gaskets and metal washers. For metal purlins, self-drilling screws are standard, and fix through the crest into the purlin below. Do not over-tighten, because it can crack or distort the sheet.
End laps should sit over a purlin. The sources we checked give 150 to 300 mm for metal sheets, and some FRP makers specify at least 200 mm, with sealant or foam tape between the sheets. Side laps are usually one corrugation for Palram's profiles. Palram does not make a sealant for SUNTUF, so check any sealant with its maker for compatibility with polycarbonate.
- Pre-drilled holes, oversized, to allow movement.
- EPDM gaskets with metal washers, and no lightweight washers.
- Fix through the crest into the purlin, and keep the screws straight.
- Lay the first sheet against the main wind and rain direction, and lap each next sheet over the previous one.
Expansion and heat
Polycarbonate expands more than steel. Palram's technical page gives a coefficient of linear thermal expansion of 6.5 x 10 to the power minus 5 per degree C. As a rough check, a 6 metre sheet that warms by 30 degrees C grows by about 12 mm (6000 mm x 0.000065 x 30). Allow for that at the fixing holes and the ends, and check the figure for the product you buy.
Light comes with heat. A clear sheet lets through solar heat along with the light, so the shed under it can get warm. A solar control version, such as Palram's SUNTUF Solar Control, transmits about 20 percent of light and is designed to limit heat gain. Tinted or solar control sheets are the option if the roof gets too hot. See our guide on heat and noise under a tin roof for other fixes.
Cutting and ordering
Cut polycarbonate with a fine-toothed circular saw, a jigsaw, or a fine-toothed hand saw. Advance the saw steadily and do not stop once the cut has started. Once trimmed, a sheet cannot be patched back together, so measure twice. Keep the protective film on until you need the sheet.
Share the shed size, the metal profile and the purpose of the light, and we will help you check the sheet count and accessories. Please call or WhatsApp the store for the current price, since rates change.
Frequently asked questions
- What percentage of the roof should be polycarbonate?
- A small share, usually around 3 to 5 percent of roof area as a starting range. We did not find support for 10 percent as a typical figure. Check your local energy code and the light transmission of the sheet.
- Which way round do I fit polycarbonate sheets?
- With the UV protected side facing up, as marked on the sheet. Remove the protective strip after installation.
- Can I mix polycarbonate with metal sheets on the same purlins?
- Yes, if the translucent sheet matches the metal profile. Fix it through the crest into the purlins, and keep the laps in line with the metal sheets.
- Does polycarbonate make a shed hot?
- Clear sheets let in solar heat along with light. Use a tinted or solar control sheet, keep the share small, and add ventilation at the ridge.