Joining Polycarbonate Sheeting

Which sealant can you use to join overlapping polycarbonate sheets on a greenhouse?

Start with the fixings. You need sufficient manual fixings in place to hold the polycarbonate sheets together — the sealant is there to weatherproof the overlap, not to hold the structure. With the sheets mechanically fixed, Selleys Roof & Gutter Silicone can be used to aid adhesion and to provide a weathertight seal along the joins.

One caution comes with that recommendation: polycarbonate is prone to crazing under stress and heat, whether a sealant is used or not. That is a property of the material, not a fault of the sealant, and it is worth knowing before you build a greenhouse out of it.

Why the fixings have to do the structural work

A profiled polycarbonate sheet on a greenhouse expands and contracts a great deal. It is a large, thin panel in full sun, and its thermal movement across a run of several metres is significant. Sheeting systems are designed around that: the fixings are placed to hold the sheet while allowing it to move, usually through slightly oversized holes.

A sealant bead cannot substitute for that. Ask a bead of silicone to hold two sheets together against thermal movement and one of two things happens — the bead tears, or it holds and the stress goes into the polycarbonate instead. The second outcome is worse, because stress is one of the two things that crazes polycarbonate.

So the sequence is: fix the sheets properly, then seal the overlap.

What Roof & Gutter Silicone brings to this job

Property	Roof & Gutter Silicone
Technology	Neutral cure, 100% silicone
Joint movement	±25%
Skin time	5–10 minutes
Tooling time	5 minutes
Tack-free time	30 minutes
Cure rate	2mm / 24 hours
Elongation at break	330% (ASTM D412)
Application temperature	+5°C to +30°C
Service temperature	−60°C to +150°C
UV and weather resistance	Yes; 25-year guarantee against cracking and peeling
Colours	Translucent, white and a range of Colorbond-matched colours

Indicative data only, not intended for the preparation of specifications.

Neutral cure is the relevant feature. Roof & Gutter Silicone cures without releasing acetic acid, which is why it is the product used against coated metal and Colorbond sheet. On a greenhouse it means the sealant is not putting an acid on the polycarbonate surface as it sets.

UV resistance matters more here than almost anywhere else. A greenhouse roof takes full sun year-round, and Roof & Gutter Silicone is specified for exactly that exposure — roofs, gutters, flashings and skylights.

The 5–10 minute skin time sets your working rhythm. Tooling time is 5 minutes. On a long overlap, work in sections you can gun and tool before the surface skins, rather than running the full length and coming back.

Polycarbonate compatibility — what the data sheets say

Polycarbonate is not one of the plastics Roof & Gutter Silicone excludes. Its stated exception is polyethylene, polypropylene and PTFE (Teflon), and it adds that compatibility with substrates should be pre-tested before use. Do that: run a short test bead on an offcut, leave it to cure, and check both adhesion and the appearance of the sheet underneath.

Other Selleys products are more explicit about polycarbonate, and the pattern is worth knowing:

• All Clear — may damage some plastics, polycarbonate and polystyrene named specifically. Do not use it on a polycarbonate greenhouse.
• Sealant Remover — not suitable for rubber, EPDM or polycarbonate. Keep it off the sheets if you ever need to strip an old bead.
• Marine Flex — lists polycarbonate among the surfaces it can be used on without a primer, with light sanding recommended on plastic surfaces to improve adhesive strength.

What crazing is, and how to reduce the risk

Crazing is a network of fine surface cracks in the plastic. In polycarbonate it is driven by stress and heat, and once it starts it does not reverse — it clouds the sheet and weakens it.

The practical steps that reduce it are all about not building stress into the panel:

• Do not over-tighten fixings. A fixing pulled hard into a sheet creates a permanent stress concentration around the hole.
• Use the fixing holes and washers the sheeting system specifies, sized to let the sheet move.
• Do not force a sheet into position. If it has to be bent to line up, the alignment is wrong.
• Do not use a sealant bead as a clamp between sheets.
• Pre-test any product on an offcut before committing to a full run.

Limitations to check before you seal

• Do not apply if the ambient or substrate temperature is below +5°C or above +40°C. A polycarbonate roof in the sun is easily above 40°C by mid-morning — seal early or on an overcast day.
• Not paintable. Allow paint to dry for at least 7 days before applying the sealant over it.
• Not for use on polyethylene, polypropylene or PTFE.
• Not recommended for continuous submersion or below-waterline use.
• Not for structural glazing. This is directly relevant: the sealant is weatherproofing the join, not glazing the panel in.
• Avoid contact with materials containing high levels of plasticisers, such as rubber and bituminous substrates.
• The sealant requires atmospheric moisture to cure. On non-porous substrates — and polycarbonate is non-porous — curing is slower than on a porous surface.

Safety: sealing overhead, in a glasshouse

The product this page recommends is the mildest of the three named on it, and the two it tells you to keep away from the sheets are also the two with the more serious hazard profiles. The material rule and the safety rule point the same way.

Product	Classification	Why it matters here
Roof & Gutter Silicone — the recommended product	Warning. H319, causes serious eye irritation. H317, may cause an allergic skin reaction (skin sensitisation, Category 1)	Overhead gunning; long runs tooled by hand
All Clear — not for polycarbonate	Danger. H225, highly flammable liquid and vapour. H315, H319. Dangerous Goods Class 3, UN 1133, PG II	A greenhouse in sun is a hot, closed box
Sealant Remover — not for polycarbonate	Danger. H304, may be fatal if swallowed and enters airways. H226, H336, H315. Poison Schedule S5	Never decant it; if swallowed, do not induce vomiting

H317 is the one that fits this job. Skin sensitisation is not irritation that settles when you wash your hands — it is the immune system learning to react to the material, generally permanently, after which far smaller exposures trigger it. It builds from repeated casual contact, and a greenhouse gives you metres of overlap to seal, worked in sections because of the 5 to 10 minute skin time. Tooling that much bead with a bare wet finger, section after section, is precisely the exposure pattern that acquires it. Use a tool or a gloved finger, not skin. Once the silicone is fully cured it is inert; the hazard belongs to the uncured bead.

H319 matters because of where the gun is. Sealing an overlap on a greenhouse roof puts the cartridge above your head, with the nozzle and the bead directly over your eyes, often while you are looking up into full sun. Wear eye protection, and be aware that a cartridge that has warmed in the sun extrudes faster than a cool one.

The enclosed-space point is worth making for the structure itself. A greenhouse is a small sealed volume that gets hot — the reason this page tells you to seal early or on an overcast day is that the substrate goes over 40 °C by mid-morning. Prop the door and any vents open while you work and while the bead cures.

The low-hazard choice is already the recommended one. Roof & Gutter Silicone is signal word Warning with two classifications, neither of them flammability. Selleys Storm, for wet-weather work, carries the same pair — H317 and H319 — so switching between them changes nothing you need to plan for. Keeping All Clear off the greenhouse, as this page instructs for crazing reasons, also keeps a Class 3 flammable liquid out of a sealed glass box.

Poisons Information Centre — 131 126 in Australia, 0800 764 766 in New Zealand.

Common questions about sealing polycarbonate sheeting

Can the sealant replace the fixings?

No. Manual fixings hold the sheets; the sealant weatherproofs the overlap. Roof & Gutter Silicone is also explicitly not for structural glazing.

Which colour should I use on a greenhouse?

Translucent keeps the join as unobtrusive as possible and lets light through, which is usually what you want on a greenhouse.

Will the sealant craze the polycarbonate?

Polycarbonate is prone to crazing under stress and heat whether a sealant is used or not. Pre-test on an offcut, avoid building stress into the sheet, and do not use All Clear, which may damage polycarbonate.

How long before it is rain-safe?

Skin time is 5–10 minutes and tack-free time is 30 minutes; the bead cures through at 2mm per 24 hours. The skin is what sheds a shower.

What if I need to remove a bead later?

Not with Sealant Remover — it is not suitable for polycarbonate. Cut and scrape the bead off mechanically instead, carefully.

Can I clean the sheets with turps first?

The data sheet's preparation for non-porous surfaces is a clean cloth dampened with IPA or white spirit, wiped in one direction, followed immediately by a clean dry cloth. Test on an offcut first — solvents and polycarbonate are not always compatible.

What joint size should I aim for?

Minimum joint width and depth is 5mm, with an optimum width-to-depth ratio of 2:1. On an overlap you are sealing a lap rather than a formed joint, so the practical aim is a continuous bead in full contact with both sheets.

Is there a related answer for polycarbonate roofing?

Yes — see which silicone is compatible with Laserlite polycarbonate and how to fix a hole in polycarbonate roofing.