A Smear of Sealant Is Not a Repair: How to Spot a Patch-Up That Won’t Last

Quick answer: Sealant is a component of a roof repair, not the repair itself — if the only thing holding water out is a bead of silicone smeared on top of the roof, it will crack, lift and leak again, usually within a couple of winters.

The job that started this one

We got called to a place on the eastern side of Levin last winter. Brown stain on the kitchen ceiling, right where the lower roof met the wall of a lounge extension. The owners had already had it “fixed” twice. Both times, someone had run a bead of silicone along the top of the apron flashing.

When we got up there, the story was easy to read. The first bead had gone hard and pulled away from the paint, leaving a hairline gap you could slide a business card into. The second bead had been laid straight over the top of the first. Underneath both of them, the flashing was too short and had no upstand behind the cladding. Water wasn’t getting in through the flashing. It was running behind it, exactly as it had from day one.

No amount of silicone was ever going to fix that. The flashing had to come off and a properly sized one go on, tucked in behind the cladding where it belonged.

Why sealant on its own fails here

There’s nothing wrong with sealant. We use it every week. But it’s designed to be compressed inside a joint — squeezed between a lap, under a flashing, out of the sun. That’s where it lasts.

Smeared on the surface of a roof, it’s on its own against a Horowhenua summer, and it loses:

• It won’t stick to a chalky roof. An older painted steel roof has a powdery, oxidised surface. Sealant grabs that powder, not the steel. A year later it peels off like a strip of skin.
• UV cooks it. Roof surfaces get seriously hot. Cheap silicone goes brittle, shrinks and splits.
• The wrong type eats the roof. Standard acetic-cure silicone — the stuff that smells like vinegar — is corrosive to metal. It should never go on a steel roof. Neutral-cure is what belongs up there.
• It traps water and salt. Out at Waitārere, Hōkio and Foxton Beach, the roof is carrying salt. A patch of sealant holds moisture and salt against the steel and rust quietly gets on with it underneath, out of sight.

What a patch-up looks like from the ground

You don’t need to get on the roof — and please don’t. Stand back with a pair of binoculars, or look from an upstairs window or the neighbour’s driveway. You’re looking for:

• Grey or clear blobs along ridge caps, valleys, or where the roof meets a wall. Especially blobs on top of older blobs.
• Silver bituminous tape (flashband). It shines. It also peels in the sun and leaves a gummy mess that has to be scrubbed off before anything proper can be done.
• Black tar smeared around a pipe or vent. A penetration should have a purpose-made rubber flashing boot, not a lump of goo.
• A row of extra screws in the flat pan of the sheet, each with a dot of sealant. Screws belong in the ribs. Pan fixings sit in the water path.
• Sealant over rust. Painting or sealing over corrosion doesn’t stop it. The rust keeps going, just where nobody can see it.

When sealant is actually the right call

Being straight with you: sometimes it is. If it’s Friday afternoon, rain’s coming, and the correct flashing is a week away, sealing a joint to get you through is sensible. We’ve done it. The difference is that everyone knows it’s temporary and there’s a date in the diary for the real fix.

Sealant is also correct as part of a repair — in a sheet lap, under a flashing, in a designed detail. The test is simple: if you can see it from the ground, it’s probably doing a job it wasn’t made for.

Questions worth asking before anyone starts

• What’s causing the water to get in — not where is it dripping, but why?
• What are you replacing, and what are you sealing?
• Will I be able to see the repair from the ground when it’s done?
• If this doesn’t hold, what’s the next step?

A roofer who can answer those clearly is worth having. And if the honest answer is “this roof is past repairing” — that’s worth hearing too, before you spend money patching a roof that needs replacing.

When you don’t need to call us

If your roof is ten years old and one flashing is leaking, that’s a repair, not a re-roof. Don’t let anyone talk you into replacing a sound roof because of one bad detail. And if the ceiling stain only appears on cold, still mornings and there’s no rain involved, read our post on leaks versus condensation first — you may not have a leak at all.

If you’ve got a mystery leak

We do a paid Leak Assessment — a proper look for the actual source, with photos, rather than a guess and a tube of silicone. If it turns out the roof is due for replacement, we’ll do a free aerial measure and quote (we measure from the air, so nobody’s tramping around on your roof to price it), and complete roof replacements carry our 5-year workmanship guarantee.

We’re Levin-based and cover the Horowhenua, Himatangi down to Te Horo. Give us a call on 0800 706 337 or email contact@aquashieldroofing.co.nz — no pressure, just a straight answer.

Quick questions

Can I just seal a small roof leak myself? We’d rather you didn’t get on the roof at all — it’s the most common way people get hurt around a house. And surface sealant on a chalky older roof rarely lasts, so you’d likely be doing it again next winter.

How long should a proper roof repair last? A repair that replaces the failed component — flashing, sheet, valley, boot — should last as long as the surrounding roof. A surface smear often fails within a season or two. Note that our 5-year workmanship guarantee applies to complete roof replacements only.

Is silicone ever okay on a metal roof? Yes, when it’s neutral-cure and used the way it’s designed to be — compressed inside a lap or under a flashing, protected from the sun. It’s the visible smear on top that’s the problem.

— Slade, Licensed Building Practitioner · Aquashield Roofing, Levin & the Horowhenua