Guide · Roof health
Roof leaks and insulation: what to check, what to do, and how cellulose helps you find them.
We never insulate over an active leak. Wherever we can, we get up on the roof and check before we start. If we find a problem we report it — and if it’s minor we’ll often fix it while we’re already up there at no charge.
This page covers what we look for on the roof, why cellulose insulation acts as a built-in leak detector, the 20-point inspection checklist, and the most common SE-QLD leak sources. It also covers whether cellulose grows mould in a damp situation. To book the free roof inspection, follow that link.
Rule one
We check the roof before we insulate.
A lot of insulation companies send a truck, pump product in through the manhole and leave. Wherever we can, we get up on the roof first for a look before we start. If we find an active leak we won’t insulate over it, because insulating over a live leak traps moisture in the ceiling structure — the plaster stays wet, the timber joists absorb water and eventually rot, and mould sets in behind walls where you can’t see it. A job done properly over a dry roof lasts the life of the house; a job done over a leaking one makes things worse.
One caveat on access and weather. Some jobs are done entirely from inside through the manhole, and on those we may not get onto the roof at all — so we’d only flag a problem we can see from inside. And if it’s raining on the day we won’t climb onto the roof to inspect it, for obvious safety reasons. If we can’t get up top, we’ll tell you what we could and couldn’t check.
Minor repairs — a broken tile, a hole where a pipe enters the roof — we’ll usually fix on the spot while we’re already up there, at no cost. If you have a spare tile we replace broken ones for free. Where there’s active damage — a rare case — we don’t just walk away: we might insulate the whole roof except the damaged section, or lay polyester batts in that spot instead of cellulose so a roofer can make the repair later without us having to vacuum the cellulose out first. We talk it through with you — whether it’s recent or an old leak, and whether you’re planning to fix it soon — and decide together.
Ceiling stains do not always mean a current active leak. A previous owner may have fixed the roof but painted the ceiling with ordinary paint — the old stain keeps bleeding through. We check from inside the roof to distinguish old stains from live ones and tell you exactly what we found.
Why cellulose is different
Cellulose discolours where a leak runs. Batts hide it.
Cellulose is hygroscopic — it absorbs water rather than shedding it. When water runs across cellulose it leaves a yellow-to-brown discolouration exactly where it travelled. That stain stays visible even after the insulation dries out. So when we go into a roof with existing cellulose and see a discoloured patch, we follow the stain back up to the entry point. The diagnosis is usually straightforward.
Fibreglass and polyester batts do the opposite. They repel water, so it spreads across the ceiling and shows up in multiple rooms. The damp patch in the plaster can be a metre from the actual entry point — and the water sits underneath the batts against the plasterboard, staying wet long after the rain stops. That sustained dampness is what causes mould and rot.
Old cellulose with staining — what to do
The discolouration is cosmetic — the borax treatment means cellulose is not compromised by small amounts of moisture. But the area should be checked before we top up. If there’s an active leak above it we won’t add more insulation on top — we’ll report it and wait for the roof to be repaired first.
Mould on ceilings is most often caused by condensation on a cold ceiling surface — not by water coming through the roof. See our guide on why mould grows on ceilings and how to fix it for the full explanation.
Free roof inspection — every job
The 20-point checklist we run on every roof.
This inspection happens while we’re already on the roof doing the insulation — it costs nothing extra. In my experience at least half the roofs we go onto have something worth flagging, even on relatively new homes. Anything we find gets reported. One note on asbestos in your roof: we check for it and note the condition, and if there’s any concern we talk it through before we start. If you already know there’s asbestos on the property, you’re obliged to tell us where it is and share any report that mentions it during the quoting stage — it keeps our crew safe and the job compliant.
- 1Tile or iron surface condition
- 2Broken tiles
- 3Damaged roofing iron (temporary repair on site)
- 4Ridge cap bedding and pointing
- 5Asbestos roof condition
- 6Cracked or incorrectly installed flashings
- 7Tile clips and roof screws
- 8Sarking or anticon blanket condition
- 9Roof vents — spinning and not damaged
- 10Toilet vent flashings
- 11Foxtel / antenna brackets and open pipes
- 12Gutters and downpipes — blocked or overflowing
- 13Valley condition — rust or leaf blockage
- 14Debris on the roof — balls, branches
- 15Skylight dome — cracked or UV-degraded
- 16Vermin — rats, mice, possums, snakes
- 17Solar panels — clean and undamaged
- 18Trees causing leaf build-up in gutters
- 19Termites in roof joists or timbers
- 20Evidence of rot or dampness from past leaks
This is a visual observation while we’re up there — it is nota formal builder’s or roof inspection, and we can’t certify structural matters. We aren’t responsible for damage that already exists on your roof, for any repair costs, or for a leak we don’t spot on the day. Roofs are dark places and we’re there to pump insulation, so things can be missed. What our insulation does do is make an existing leak more obvious over time — and we’ll always do our best to find anything like that, photograph it, and report it to you. It’s an extra set of trained eyes from someone who’s been on more than 10,000 SE-QLD roofs, not a guarantee.
SE-QLD leak sources
The most common causes we find.
Tile roofs: ridge capping
The bedding dries out, cracks, and lets water in along the ridge line. Repointing with flexible acrylic pointing fixes it.
Tile roofs: broken or slipped tiles
One broken tile in a valley lets water in during heavy rain. We replace on the spot with a spare, or silicon as a temporary repair.
Tile roofs: flashings
Flashings at walls, skylights, and pipes crack or lift. Valley flashings blocked with leaves are another common culprit.
Iron roofs: rusted screws & lead-head nails
Old lead-head nails work their way up over time and the shaft lets water in; rusty screw heads start rusting the iron sheet around them until it leaks. Lead-head nails are best replaced with roof screws; rusty screws should be painted with cold gal or swapped for proper galvanised screws before they're too far gone to undo.
Tile clip scams
Tile clips almost never cause leaks. I've investigated $5,000 tile-clip quotes and found one broken tile or a blocked valley — and that's most often the cause of the leak, not the tile clips.
The hose test
Wet the roof section by section from the bottom to the top, and watch from inside at the manhole between each section as the hose moves slowly up the roof — the first place water shows is where it starts leaking.
See the hose test, a bad roof-paint job, and the worst roof we’ve insulated.
The first video shows the hose-test method we use on tricky leak diagnoses — section by section while someone watches from the manhole. The second is what a bad roof-painting job looks like and what we flag in the inspection. The third is a real SE-QLD job with termites and all.
Read the transcript
Okay, so we've just been working up this sheet with the water. These sheets here weren't touched by me — as you can see, they're still riveted down. Now, I'll put a bit of silicon on that rivet just in case it leaks, like last time when I came and checked them. But I've just been hosing this sheet. I didn't lift that sheet to install a batt; I actually lifted this one beside it, because it comes out over here, and I lifted it up and poked the batts into these bays underneath this sheet. But the customer's got a leak, and the leak's coming in when we run water in here, and it starts pouring in downstairs. Underneath this flashing here it's all a bit rusty, so when they have a downpour, that started leaking. That's obviously nothing to do with me insulating the roof, because I had this sealed off — as you can see, I hadn't even hosed them until just then. I'll just go downstairs and show you where the water is coming down. You can see a leak in there. My name is Peter Johnson, today's Saturday. So the sheets that I lifted on the roof were over here, one on each side of that flue. But I've isolated this leak — it's actually coming in over here and running along and down. So that's nothing to do with me, but I'll let the customer know, and he can fix that flashing up there, and then it'll fix his problem.
Read the transcript
So you can see here I'm on a tile roof, and all of this roof has been painted previously. But you can see the paint's actually been put on over the top of mould, and because they didn't pressure-clean the roof before they started, it's actually not stuck to the roof. So this is what one of those cheap paint jobs will do: they won't clean it properly, and then your paint will just come off. It'll look good for about a week — well, probably three months or so — and then it'll all look terrible. And now, to get all that paint off, it's going to be a lot more work to fix it.
Read the transcript
Okay, so I've just hopped into this roof. It's an old roof, and you're always amazed at what you see in old roofs — there's lots of rubbish up here, everywhere. We've gone around and done the downlights; there's a couple of downlights we put the batts around. You'll notice there's some foil over one part of the roof, because they put a new roof on but didn't bother putting the roll blanket on the rest — it's pretty pointless putting it on one part and not the rest, even though it is R1.5. And notice these bits of timber that run from the top down to the bottom — they're actually supporting the roof, so they've been put in afterwards. This roof's in really bad condition, so I've got to be very careful going through here. See the snake skins? That's a new extension up in there, so that looks okay, and we'll get up and pump all of that. But you can see the old tongue-and-groove timbers are all warped and bowing in the middle. They've put a new plaster roof underneath this, so you don't see that from below, but this is all in very bad condition. On the ground there's wasps' nests and everything, and all of this timber is just wrecked. So this is the ceiling — anyway, it's all got a second ceiling under it already, so it doesn't really matter. We'll just pump over the top of it, and that'll keep the termites and things out of it, and we go from there.
The clips play right here on the page, or open the playlist to watch them all on YouTube and subscribe.
Some of these were filmed a while back. Our methods, safety standards and products have moved on since. For how we work today, see the rest of this page.
Some of these were filmed a while back. Our methods, safety standards, and products have moved on since then.
Common questions about roof leaks and insulation
Do you insulate over an active roof leak?+
Not over the leak itself. Insulating over a live leak traps moisture in the ceiling structure — the plaster stays wet, the timber joists rot, and mould takes hold in wall cavities where you can't see it. Wherever we can get on the roof, we check it first. Active damage is a rare thing, and rather than just walk away we'll usually insulate the rest of the roof and either leave the wet section or lay polyester batts there instead of cellulose, so a roofer can repair it later without us having to vacuum cellulose out. We talk it through with you first — whether it's recent or an old leak, and whether you're planning to fix it soon.
My ceiling has a water stain but the roof seems fine. Is it a current leak?+
Not necessarily. Old stains are very common in homes where the roof was repaired but the ceiling was painted with ordinary paint instead of stain-blocker — the stain keeps bleeding through every coat. When we inspect, we look at the area from inside the roof. If the timber is dry and there's no fresh mark in the dust above it, it's usually a historical stain rather than a current leak. We'll tell you either way.
How does cellulose help find a roof leak?+
Cellulose is hygroscopic — it absorbs water rather than repelling it. When water runs across cellulose it leaves a yellow or brown discolouration exactly where it travelled. Even after the roof dries out, that stain stays. We follow the stain back up to the source. Batts let water run through and spread — the stain in the ceiling can be a metre from the actual entry point, making the leak much harder to trace.
What does the free roof inspection cover?+
Twenty points: tile or iron surface condition, broken tiles, damaged iron, ridge cap bedding, asbestos condition, flashings, tile clips and screws, sarking or anticon condition, roof vents, toilet vent flashings, Foxtel brackets, gutters and downpipes, valley condition, debris on the roof, skylight domes, vermin, solar panel cleanliness, trees near gutters, termites in the roof timbers, and evidence of rot or dampness. This happens while we're already on the roof — it costs nothing extra.
What are the most common SE-QLD roof leak sources?+
On tile roofs: broken or slipped tiles, cracked ridge capping where the bedding has dried out, and flashing failure at valleys or around penetrations. On iron roofs: old lead-head nails that have worked loose and let water down the shaft, and rusty screw heads that start rusting the iron sheet around them. Tile clip replacement is often pushed by roofing companies but almost never needed — a blocked valley or broken tile is the real cause in almost every case I've investigated.
What happens to batts when a roof leaks on them?+
Batts repel water, so it spreads freely across the ceiling and turns up in multiple rooms. The batts hold moisture against the plasterboard, staying damp long after the rain stops — that's what causes mould and rot. Cellulose absorbs the water and keeps it in one area, limiting the damage and leaving a visible stain you can follow back to the entry point.
Comfort Zone Franchise
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Or call Peter on 0414 586 315 — happy to talk through what you’re seeing on your roof, no pressure.