Sheets & Panels

Polycarbonate Sheet Installation: Thickness, Spans & Sealing

Most polycarbonate roofs that fail early were not sold badly — they were fitted badly. Thickness matched to span, oversized fixing holes, the UV face outward and the right tape on the right edge account for nearly every long-lived installation.

Load-span specified | UV face verified | Fixings and tapes supplied together

If you are still deciding between polycarbonate and fibre sheet, start with our polycarbonate vs fiber sheet comparison. This guide assumes the decision is made and covers what actually determines whether the roof lasts fifteen years or three: specification and installation.

Multiwall polycarbonate sheet with flutes running down the roof slope — Flutes run down-slope

Oversized fixing hole in polycarbonate sheet with neoprene washer — Oversized hole, washered fixing

Vented tape at lower edge

Start With Thickness and Span

Thickness is not a preference, it is an output. It follows from how far the sheet must bridge unsupported and what load it has to carry while doing so. Get this wrong and everything downstream — fixings, sealing, warranty — is irrelevant.

Three inputs determine the answer:

Span

The clear distance between purlins that the sheet must cross with no support beneath it. The single biggest driver of required thickness.

Load

Wind uplift for your location and building height, plus monsoon water load, plus any maintenance access loading on the roof.

Pitch

Flatter roofs shed water more slowly and pond more readily, so they need more conservative spacing than a steep pitch.

Specification decision	What it depends on
Sheet type	Solid for clarity and impact; multiwall for span, light weight and insulation; corrugated to match an existing metal roof profile
Thickness	Read from the manufacturer's load-span table for that specific product — not carried over from another job
Purlin spacing	Set together with thickness; closer spacing allows thinner sheet and vice versa
Colour / grade	Clear, opal, bronze or heat-blocking, chosen for the space below rather than for looks
UV faces	One face as standard; both faces on request for high-exposure roofs
Fixing centres	Per manufacturer detail, with edge distances respected

Read a quotation this way. If you are offered the thinnest sheet in the range for a wide span, someone is pricing to win the order rather than specifying to carry the load. Ask which load-span table the thickness came from.

Heat: The Specification Most People Skip

Daylight is why you are buying a translucent sheet. Heat arrives with it, and in Delhi NCR that decides whether the covered space is usable between April and July.

Grade	Best used for
Clear solid	Entrance canopies and glazing where optical clarity is the point. Highest heat gain.
Clear multiwall	Carports and utility covers. Diffused light, moderate heat.
Opal / white multiwall	Roofs over seating and work areas. Soft even light, lower heat, no hotspots.
Bronze / grey tinted	West-facing exposure and glare control.
Heat-blocking / IR grades	Terraces, balconies and restaurant seating that people actually occupy.

Local advice. For any covered space people sit in, clear sheet is almost always the wrong specification — it builds a greenhouse. An opal or heat-managing grade costs slightly more per square foot and changes whether the space gets used in summer.

The Mistake That Cannot Be Seen on Handover Day

Install the UV face outward. Polycarbonate carries its UV protection on one specific face, identified by a printed protective film. That film is the instruction, not the packaging. Fitted upside down, the roof yellows and turns brittle within a couple of seasons — and no warranty covers it, because the failure is installation rather than product. The roof looks perfect on handover day. It looks perfect for about eighteen months.

This is the strongest practical argument for buying sheet and installation from one source. When supply and fitting are split between two parties, nobody owns this mistake and nobody can be held to it.

Thermal Movement: Oversized Holes and Washered Fixings

Polycarbonate expands and contracts noticeably with temperature, and across a Delhi year that movement is substantial over a long sheet. Two rules follow, and both are routinely broken:

Oversize every fixing hole so the sheet can slide as it moves. A hole drilled to match the screw diameter converts every fixing into a stress point.
Snug fixings, never torque them. Use a neoprene or EPDM washer and stop the moment it seats. Over-tightened screws crack the sheet around the hole, and cracks in polycarbonate travel.

Respect the manufacturer's minimum edge distance, and on multiwall never place a fixing through a flute wall — always through the flat between ribs.

Multiwall Edge Sealing

Multiwall has open channels running through it. How those channels are closed determines what the roof looks like in five years.

Lay sheets so flutes run down the slope, allowing condensation to drain rather than pool inside
Seal the top edge with solid aluminium tape to exclude dust and insects
Close the bottom edge with vented breather tape, which lets moisture escape while filtering dust
Fit a U-profile or F-profile over both taped edges to protect the tape and finish the edge

Never seal both edges solid. You permanently trap moisture and dust inside the flutes, and there is no way to clean it out afterwards. Those streaked, grubby multiwall roofs on older buildings are almost always a taping error, not a product failure.

Installation Sequence

Confirm orientation first. Identify the UV-protected face by its printed film and confirm it will face outward before a single sheet is lifted.
Check the substructure. Purlins level, spaced as specified for the chosen thickness, free of sharp edges or protruding fasteners that will abrade the sheet.
Cut on the ground. Fine-tooth blade, sheet fully supported, protective film left on. Clear all swarf out of multiwall flutes — dust sealed inside is permanent.
Tape the edges before lifting. Solid tape to the top edge, vented breather tape to the bottom.
Drill oversized holes. Larger than the screw shank, set back from the sheet edge, through the flat and never a flute wall.
Set the sheets. Flutes down-slope, correct side laps, gaskets or glazing bars as specified. Work up the slope so each sheet laps over the one below.
Fix with washers. Neoprene or EPDM washer under every screw head. Snug until it seats, then stop.
Fit closures and flashings. End closures, ridge, side flashings, U or F profiles over the taped edges.
Remove the film last. Peel only after all work is complete, and within the manufacturer's stated window — left too long it bakes on and becomes very hard to remove.

Accessories Checklist

More roofs are compromised by missing accessories than by the wrong sheet. Confirm every item below is on the quotation rather than left for you to source:

Item	Why it matters
Screws with sealing washers	Neoprene or EPDM washers; the washer perishes long before the sheet does and a perished washer leaks
Solid aluminium tape	Closes the top edge of multiwall against dust and insects
Vented breather tape	Closes the bottom edge while letting condensation escape
U-profile / F-profile	Protects taped edges and gives a finished appearance
Glazing bars with gaskets	Weatherproof the joint where two sheets meet, while allowing movement
Ridge, side and end flashings	Complete the weatherline at every roof junction
Profiled foam closure strips	Seal the corrugation profile at eaves and ridge
Compatible silicone	Many general-purpose sealants attack polycarbonate and cause stress cracking

Do not let a random tube of silicone near the roof. Incompatible sealants and solvents cause stress cracking around joints. The sheet does not fail where it was hit — it fails where it was sealed.

Polycarbonate Installation Checklist Thickness and span reference, fixing detail, edge sealing sequence and the full accessories list in one printable sheet for your site team.

Download PDF

Where This Applies

CarportsMultiwall over parking bays and driveways

Terrace coversOccupied space — specify a heat-managing grade

Balcony coversRain protection without losing daylight

PergolasCurved or flat, cold-curved on site where needed

Walkway canopiesSolid sheet where vandal resistance matters

Factory roof lightsCorrugated grade dropped into a metal roof run

Entrance canopiesClear solid sheet for clarity at close range

Skylights & light wellsDiffused multiwall to avoid hard shadow lines

Machine guardingSolid sheet where impact resistance is the requirement

Cleaning and Annual Maintenance

Cleaning

Rinse with plenty of clean water first to float grit off the surface
Wash with a mild soap solution and a soft cloth or sponge, never a brush
Rinse thoroughly and allow to air dry
Never use ammonia-based glass cleaner, solvents, thinners, scouring pads, or a pressure washer at close range

Before Each Monsoon

Clear leaves and debris from valleys, gutters and side laps
Check fixings and replace any perished sealing washers
Look for hairline cracks radiating from fixing holes — the early signature of an undersized hole or an over-tightened screw
On multiwall, confirm the breather tape is intact and not blocked

What Voids a Warranty

Warranties on translucent roofing are narrower than buyers assume, and every common exclusion is an installation decision:

UV face installed downwards
Fixing holes not oversized, or screws over-tightened
Incompatible sealants or cleaning chemicals
Walking directly on the sheet instead of using crawl boards
Both multiwall edges sealed solid
Purlin spacing wider than the load-span table permits

Ask for the warranty document before you order, not after a problem appears.

Frequently Asked Questions

What thickness of polycarbonate do I need for my roof?

It depends on the clear span between purlins, the roof pitch, and the wind and monsoon loading at your location. Thickness must be read from the manufacturer's load-span table for the specific product being supplied. Send us your span, pitch and exposure and we will specify it rather than guess.

Which side of a polycarbonate sheet faces up?

The UV-protected face goes outward, toward the sun. It is identified by a printed protective film — keep that film on through installation and read it. If both faces carry plain film with no printing, ask the supplier before fitting anything.

Why must fixing holes be oversized?

Polycarbonate expands and contracts significantly with temperature. A hole sized to the screw prevents that movement and turns each fixing into a stress point, which cracks the sheet. Oversized holes plus a washered, snug-only fixing let the sheet move freely.

Which way should multiwall flutes run?

Down the slope, always, so condensation inside the channels can drain out at the lower edge. Flutes running across the slope trap water permanently.

Can I seal both edges of multiwall sheet?

No. Solid tape closes the top edge, vented breather tape closes the bottom. Sealing both solid traps moisture and dust inside the flutes with no way to clean it out — this is the cause of most streaked, dirty-looking multiwall roofs.

Can polycarbonate be cut and drilled on site?

Yes. Use a fine-tooth blade, support the sheet fully, and leave the protective film on while cutting. Clear all swarf out of multiwall flutes before taping the edges.

Can polycarbonate sheets be curved?

Both solid and multiwall can be cold-curved on site, but only to the minimum radius the manufacturer specifies for that thickness. Forcing a tighter curve locks in stress that later shows up as cracking. Multiwall must be curved in the direction of the flutes.

Can I walk on a polycarbonate roof?

No. Use crawl boards spanning the purlins to spread the load. Walking directly on the sheet is also a standard warranty exclusion.

Does polycarbonate reduce heat?

Multiwall reduces heat transfer better than single-skin sheet thanks to the trapped air, and opal, tinted or IR-reflective grades cut solar gain substantially. Clear solid sheet does the least. For occupied spaces in Delhi summers, specify a heat-managing grade.

Do you supply the fixings and tapes as well as the sheet?

Yes — screws with sealing washers, solid and vented tapes, edge profiles, glazing bars and flashings are quoted with the sheet. Missing accessories are the most common reason a cheap quotation turns into an expensive roof.

Do you deliver to Noida, Faridabad, Gurgaon and Delhi?

Yes. We supply cut-to-size polycarbonate across Noida, Greater Noida, Faridabad, Delhi, Gurgaon and the wider NCR.

Send Us Your Span and We Will Specify the Sheet

Give us the purlin spacing, roof pitch, area and what the space below is used for. You will get a thickness taken from real load-span data, the right grade for heat, and every fixing, tape and profile listed on one quotation.

Call Now WhatsApp Enquiry

Spans, thicknesses and fixing details vary between manufacturers and product ranges. Always work from the load-span and installation data supplied with the specific sheet being used.