How To Seal Leaking Aluminium Roof Panelling

What should you use to seal leaking joints in aluminium caravan roof panelling?

Selleys Roof & Gutter Silicone is by far the best sealant for this job. But the product is only half the answer: the success and longevity of the repair depends on surface preparation. The surfaces need to be clean and free of all dirt, dust, mould and old sealants — particularly if a different type of sealant has been used previously.

If the last roof and gutter sealant did not last long, that is almost always a preparation problem rather than a product problem, and the section below sets out how to avoid repeating it.

Silicone or flexible mastic?

The question of whether a flexible mastic is better than silicone has a clear answer here, and it comes down to what an aluminium caravan roof actually does.

An aluminium panel in the weather all the time, hot and cold, expands and contracts constantly. Aluminium has a high coefficient of thermal expansion, and a caravan roof goes from freezing overnight to searing by mid-afternoon. On top of that, the whole structure flexes on the road.

A non-skinning mastic stays soft but has no real elastic recovery — it deforms and stays deformed. A cured silicone is a rubber: it stretches and returns. Roof & Gutter Silicone's published movement capability is ±25%, with elongation at break of 330% (ASTM D412), and it carries a 25-year guarantee against cracking and peeling. Applied correctly it will remain flexible and last for many years.

Published figures

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
Shore A hardness	32
Elongation at break	330% (ASTM D412)
Modulus at 100% elongation	0.6 MPa (ASTM D412)
Application temperature	+5°C to +30°C
Service temperature	−60°C to +150°C
Shelf life	18 months
Suitable substrates	Aluminium, steel, stainless steel, mild and galvanised steel, powder-coated steel, PVC, timber, fibre cement, brick, concrete, masonry
Colours	Translucent, white and a range of Colorbond-matched colours

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

Aluminium is a listed substrate, which is the first thing to check for any sealant on a caravan. Neutral cure matters too: the product cures without releasing acetic acid, which is why it is specified for coated metal and Colorbond sheet rather than an acetic cure product.

Service temperature to +150°C covers a metal roof in full sun with a wide margin.

Preparing the joints — the part that decides the outcome

1. Cut the old sealant out. Use a sharp knife such as a Stanley knife or similar retractable trimming blade. There are also special tools that can help cut old sealant out. Nothing new bonds to old sealant, and this matters most where a different type of sealant was used before.
2. Brush everything clean. An old toothbrush or a small wire brush is good for getting into the joint profile and clearing dust, oxide and debris.
3. Wipe over with a rag soaked in a little turps or metho, then wipe dry. This lifts the oily film that collects on a road-going roof. The technical data sheet's own preparation for non-porous surfaces is a cloth dampened with IPA or white spirit, wiped in one direction only, followed immediately by a clean dry cloth.
4. Deal with mould. Mould and mildew growth in the old joints has to be removed, not sealed over.
5. Check the joint is dry before you start sealing.

Applying the sealant

1. Cut the cartridge tip, screw on the nozzle and trim it to the bead size you want. Use a good-quality gun — a high thrust ratio helps with a high-viscosity product.
2. Apply a continuous bead along the joint.
3. Force the sealant into the joint rather than smearing it over the top. This is the single most important application detail. A bead sitting on the surface has almost no depth of bond and peels; one pressed into the joint grips both faces and flexes with the panel.
4. Tool the bead with a clean spatula dipped in soapy water before the skin forms — you have around 5 minutes.
5. Work in sections you can gun and tool inside that window. Skin time is 5–10 minutes.
6. Clean excess off tools and equipment with turps.

Joint sizing, if you are rebuilding a joint rather than repairing one

• Optimum width-to-depth ratio: 2:1.
• Minimum joint width and depth: 5mm.
• Widths up to 12mm: use a 1:1 ratio.
• Widths greater than 12mm: use 2:1.
• Backer rod, where used, should be 25%–50% larger than the joint width.

Limitations — check these before you start

• Do not apply if the ambient or substrate temperature is below +5°C or above +40°C. A metal roof passes 40°C early on a summer day, so work in the morning or on an overcast one.
• Not paintable. Allow paint to dry at least 7 days before applying sealant over it.
• Not recommended for continuous submersion or below-waterline use.
• Not for use on polyethylene, polypropylene or PTFE. Pre-test compatibility with other substrates before use.
• Avoid contact with materials containing high levels of plasticisers, such as rubber and bituminous substrates — components can migrate into the sealant and cause staining or reduced performance.
• Not for structural glazing.
• The sealant requires atmospheric moisture to cure. On non-porous substrates like aluminium, or in low humidity, curing is slower.

Safety: solvents on a roof, with the ignition sources underneath you

Roof & Gutter Silicone carries the signal word Warning, with eye irritation Category 2A (H319, causes serious eye irritation) and skin sensitisation Category 1 (H317, may cause an allergic skin reaction).

H317 and the five-minute tooling window. Step 4 above gives you about five minutes to tool the bead — the tightest window of any product in this part of the range — and a tight window is exactly what makes people tool with a wet finger instead of fetching the spatula. Sensitisation is not irritation: it is permanent once acquired, then triggered by far smaller exposures, and built by repeated casual contact — panel joint after panel joint. Use the soapy spatula, or a gloved finger.

H319 on a roof. A high-viscosity product in a high-thrust gun spits when the trigger releases, and you are above the work looking down into the joint. Safety glasses, and a splash gets a proper flush rather than a rub with a dusty sleeve.

The preparation step is where the serious classification is. Selleys Sealant Remover, for a stubborn residual film, is classified Danger, Poison Schedule S5, and is more than 60% petroleum naphtha: H304 may be fatal if swallowed and enters airways, H226 flammable liquid and vapour — Dangerous Goods Class 3, UN 1133, Packing Group II — H336 may cause drowsiness or dizziness, and H315 causes skin irritation. The turps or metho that step 3 uses for the wipe-down is a flammable solvent too.

On a caravan the ignition sources are below you and out of sight: the gas bottles on the drawbar, the fridge flue, the hot water vent. Hydrocarbon vapour is heavier than air, so it runs off the roof toward all three. Turn the gas off at the bottle before solvent goes on the roof, and no smoking anywhere near the van.

The temperature limit has a safety edge as well as a performance one. This page says not to apply below +5°C or above +40°C, and notes that a metal roof passes 40°C early on a summer day. Solvent on hot metal evaporates much faster than you expect, and H336 dizziness at roof height is a fall hazard. Work in the morning or on an overcast day, and get off the roof for breaks.

H304: lid on between uses, never decanted into another container, out of reach of children. If any is swallowed, do not induce vomiting — rinse the mouth, give a glass of water and seek medical advice immediately. And follow step 1 first: cut the old sealant out with a blade, because the remover is meant for a residual film under 5mm rather than a whole bead.

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

Common questions about sealing aluminium roof panels

Why did the last sealant fail so quickly?

Most often because the joint was not clean, or because the new bead was applied over old sealant, or because it was smeared across the surface rather than forced into the joint. All three are preparation and technique problems rather than product problems.

Is a flexible mastic better than silicone here?

For a joint that moves with heat and road flex, a cured silicone with ±25% movement capability and elastic recovery is the better performer. Applied correctly, Roof & Gutter Silicone will remain flexible and last for many years.

Will it stick to aluminium?

Yes. Aluminium is listed among its suitable non-porous substrates, along with steel, stainless steel, galvanised steel, powder-coated steel and PVC.

Which colour should I use?

Translucent is the least conspicuous on an unpainted or light roof, and the range includes white and a set of Colorbond-matched colours.

How do I remove the old sealant?

Cut it out with a sharp retractable blade, then brush the joint clean and wipe with turps or metho. Selleys Sealant Remover can lift a stubborn residual film — but it is not suitable for rubber, EPDM or polycarbonate, so keep it off any rubber trim.

How long before the van can be used?

The bead is tack-free in about 30 minutes and cures at 2mm per 24 hours. Give it time before subjecting it to road flex or heavy rain.

Can I paint over the bead to match the roof?

No — the product is not paintable. If you want a painted finish, choose a colour to match instead.

What about the centreline seam under the beading?

That is a different repair with a different product — see resealing a seam in a caravan.

Are there other caravan joints to think about?

Yes — see the best sealant for caravan joints.