Conservatory Full of Flies? Here's What's Drawing Them In
- K&S Bespoke Builds
- 5 days ago
- 5 min read
You open the conservatory door and three flies dive past your head. Or you walk in to find a dozen of them buzzing against the glass. Either way, it's irritating, and it doesn't take much for a conservatory to go from the nicest room in the house to the one you avoid.
Here's why conservatories attract flies more than the rest of the house, the five most common reasons they're getting in, and what actually stops the problem rather than just covering it up.
Why is my conservatory full of flies?
Conservatories attract flies because they're warmer than the rest of the house, full of light, and almost always have small gaps in the seals, vents or polycarbonate roof panels for flies to get in through. Old polycarbonate roofs are particularly bad because cluster flies overwinter inside the flutes. Once they're in, the warm interior keeps them active long after the rest of the house has cooled down.
The five most common reasons your conservatory has flies
In rough order of how often we see them:
Cluster flies living inside a polycarbonate roof. They overwinter in the hollow flutes and emerge in warm weather.
Gaps in seals, end caps or roof flashings. Small entry points around the box gutter or roof bars.
Trickle vents without insect mesh. A direct route in from outside.
The room being warmer than outside. Flies follow heat, especially at dusk and in autumn.
Plants, food and standing water inside. All powerful fly attractants.
Cause 1: Cluster flies in the polycarbonate roof
If you get flies appearing in spring and autumn in particular, and you can never find an obvious entry point, this is almost always the answer. Cluster flies look for warm, hollow cavities to overwinter in. Polycarbonate roof sheets, with their twin or triple-wall flutes, are an open invitation.
End caps that have come away over the years let them in. Once inside the flutes, they're protected from the cold, breed, and emerge in warm spells. You'll often see dozens of them at once on the inside of the glass, especially on a sunny day.
The British Pest Control Association confirms cluster flies routinely colonise loft spaces, wall cavities and conservatory roofs. Once they're established in a polycarbonate roof, there's no sensible way to clear them out. Cleaning the outside doesn't reach them, and chemical treatments inside the flutes aren't viable.
This is one of the strongest practical reasons people replace polycarbonate roofs.
Cause 2: Gaps in seals, end caps or roof flashings
Even on a glass or modern roof, flies can find a way in if seals have failed somewhere. The usual culprits:
Perished sealant at the box gutter where the conservatory meets the house
Cracked or missing end caps on polycarbonate sheets
Gaps around roof bars where the rubber gaskets have shrunk
Loose flashings at the wall junction
Damaged or missing breather tape along polycarbonate edges
Run your eye along the roof bars and the line where the conservatory meets the house. Anywhere you can see daylight from inside is a route for flies. Even a 2mm gap is plenty.
Cause 3: Trickle vents and roof vents
Trickle vents are the small slot vents in the top of window frames that let air in. They're useful for ventilation but they're also a fly motorway if they don't have insect mesh fitted. Older conservatories often have plain trickle vents with nothing covering the outside.
Same goes for roof vents and any opening rooflights. Check that any vent in the conservatory has fine mesh on the outside. If it doesn't, you can buy retrofit insect mesh strips from most DIY shops and fit them in 10 minutes.
Cause 4: The room being warmer than outside
Flies aren't random. They follow heat sources, particularly in autumn when temperatures drop. Conservatories radiate heat back out to the garden, which acts like a beacon for any fly within range. Once they spot a gap or an open door, they're in.
This is also why flies seem worse at dusk - they're looking for somewhere warm to settle for the night, and a lit conservatory window is exactly what they're after.
A warm roof system actually helps with this, counter-intuitive as it sounds. A properly insulated roof means less heat radiates out, the conservatory cools at the same rate as the rest of the house, and the room isn't a thermal beacon for flies.
Cause 5: Plants, food and standing water
The avoidable causes:
Houseplants, especially those with damp soil
Fruit bowls left out in summer
Pet food if the conservatory is where the dog or cat eats
Drinks left out, especially anything sweet
Damp areas under leaking seals or condensation puddles
Compost or bin stores close to the conservatory doors outside
These don't get flies into the conservatory on their own, but they keep flies that have made it in from leaving. Sort these and you give the problem half a chance of going away.
Quick fixes that work
If you've got flies right now and you need them gone today:
Fly screens on doors and opening windows. Magnetic ones fit most standard doors in minutes and roll up out of the way.
Insect mesh on trickle vents and roof vents. Cheap, easy, surprisingly effective.
A fan in the room. Flies struggle in moving air.
Fly traps or fly paper in the corners where they cluster.
A bug zapper if you don't mind the noise.
Move plants outside for a few weeks to see if it changes anything.
Clear food, drinks and pet bowls from the room.
Skip fly sprays in conservatories. The chemicals build up in a confined glass space and can affect pets, kids and your own breathing more than they would in an open room.
The long-term fix: sort the entry points
Surface fixes work as long as you keep them up. But if you're constantly battling flies and the cluster fly problem comes back every spring, the root cause is almost always the roof or the seals.
We replace conservatory roofs for homeowners across Berkshire who've tried everything else first. A common pattern: years of fly screens, sprays and dehumidifiers, eventually a conservatory roof replacement in Woodley or somewhere similar, and the fly problem disappears overnight along with the cold winters and hot summers.
A new warm roof:
Replaces all the perished sealant and seals
Eliminates the polycarbonate flutes that cluster flies live in
Comes with proper insect-meshed vents where needed
Keeps the room at house temperature so it's not a thermal beacon
Carries a 25-year guarantee on the structure and panels [verify: K&S specific guarantee terms]
A homeowner near Camberley we worked with had been dealing with cluster fly emergences every March for six years. The replacement went on in September, the following March came and went with two flies in total. That's the difference between a surface fix and a root cause fix.
When the roof isn't the answer
Sometimes the conservatory itself isn't worth the investment. If the frame's failing, the dwarf walls are cracked, or you've never used the room properly, removal is the cleaner answer. We do that too, and we'll tell you straight which one your conservatory needs.
Talk to us about your conservatory
We're a family-run firm based in Reading covering Berkshire and the bordering areas. Whether you've got a cluster fly problem you can't shake, a leaking roof, or you just want an honest opinion on whether your conservatory is worth keeping, we'll come out and tell you straight.
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