How to spot cracks, UV wear, leaks and when to replace the dome
The external dome of a sun tunnel plays a crucial role in collecting daylight and keeping your roof waterproof. Over time, weather exposure, UV light and general wear can cause the dome to weaken. Cracks, cloudiness and reduced light output are all common signs that the dome may be damaged.
This guide explains how to identify sun tunnel dome damage, what causes it, when repairs are possible and when a full dome replacement is needed.
What does the dome on a sun tunnel do?
The dome sits on the roof and gathers daylight, directing it into the reflective tube. It also forms part of the waterproofing system. When the dome becomes damaged, both daylight performance and weather protection are affected.
A healthy dome should be:
- clear and free of cracks
- firmly attached
- well-sealed
- free of cloudiness or yellowing
When these conditions change, the dome may need repair or replacement.
Most common signs of dome damage
Below are the key symptoms UK homeowners usually notice when the sun tunnel dome starts to deteriorate.
1. Cracks or splits in the dome
Cracks are the most obvious and serious sign of damage. They allow rainwater to enter the dome assembly and sometimes the reflective tube.
What to look for:
- small hairline cracks
- larger visible splits
- crazing (tiny fracture patterns)
- moisture inside the dome
Why it matters:
Cracks usually worsen quickly due to wind pressure and temperature changes.
2. Yellowing, cloudiness or UV fading
Over time, UV radiation causes plastic domes to fade, discolour or turn milky. This reduces the amount of light that reaches the tube.
Signs include:
- cloudy or milky appearance
- uneven patches on the dome surface
- significant drop in room brightness
This is extremely common in older domes that lack modern UV coatings.
3. Water inside the dome or diffuser
If moisture appears inside the dome, it may indicate:
- cracks
- a failing gasket seal
- dome warping
- worn weatherproof joints
Water inside the dome suggests the waterproofing system is compromised.
4. Loose or rattling dome in windy weather
A dome should sit firmly in place. If it becomes loose:
- wind can lift it
- rainwater can enter gaps
- the flashing may fail
Loose screws, warped bases or failed fixings often cause this.
5. Reduced daylight inside the room
Dome damage often results in dimmer light. If the room suddenly feels darker than before, the dome may be obstructed, cloudy or cracked.
Typical reasons:
- UV fading
- debris accumulation
- incomplete seal causing internal condensation
If cleaning the dome does not improve brightness, deeper damage may be present.
6. Debris or insects trapped inside the dome
If insects or debris appear inside the dome, the outer seal is likely deteriorating.
This can eventually lead to water ingress and mould growth.
Causes of dome damage (UK homes)
Dome damage is usually the result of:
- long-term UV exposure
- aging plastic materials
- hail or storm impact
- thermal expansion and contraction
- roof movement
- poor installation
- accidental roof damage during maintenance
Older domes (10–20 years) tend to become brittle, especially on south-facing roofs.
Dome damage vs condensation: how to tell the difference
Use this comparison to diagnose correctly:
| Symptom | Dome Damage | Condensation |
|---|---|---|
| Cracks visible | Yes | No |
| Water during rain | Yes | Usually no |
| Mist or fog inside tube | Sometimes | Yes |
| Moisture mainly in winter | Not typical | Very common |
| Sudden brightness loss | Yes | Rare (unless diffuser fogged) |
If drips appear only in cold weather, condensation is more likely. If they appear during or after rain, dome damage is the usual cause.
Can you repair a damaged dome?
Certain minor issues can be repaired:
Repairs that may work:
- cleaning debris from seals
- tightening screws
- resealing edges
- replacing gaskets
Repairs that usually do NOT work:
- gluing cracks
- patching brittle plastic
- trying to polish UV fade
- sealing large splits
Once a dome has cracked or faded heavily, replacement is the safest solution.
When you should replace the dome
You should replace the dome when:
- cracks or splits are visible
- it has become heavily yellowed
- water enters the dome
- the gasket seal fails repeatedly
- the dome feels loose or unstable
- the model is old and spare parts are available
Replacement domes are available for Velux, Fakro, LiteLite and Solatube systems.
Cost of replacing a sun tunnel dome in the UK
Typical dome replacement costs:
- Dome only (supply): £50–£200
- Dome replacement (installed): £150–£400
- Premium dome with UV protection: £100–£250
- Complete new sun tunnel (if needed): £600–£1,500
Replacing the dome often restores brightness and solves leaks without replacing the full sun tunnel.
Preventing future dome damage
You can extend the dome’s lifespan by:
- choosing UV-resistant domes
- cleaning debris annually
- inspecting after storms
- ensuring good loft ventilation
- replacing seals if they dry out
Most modern domes last longer due to improved acrylic or polycarbonate materials.
Get free quotes for dome repair or replacement
If you notice cracks, fading or reduced light output, you can request up to four free quotes from vetted UK roofers and sun tunnel specialists. They can confirm whether your dome needs repair or full replacement.
