Loft Condensation: Why It Looks Like a Leak But Isn't
Damp loft timbers, drips from the underlay, mould on rafters — these are the classic signs of a leak, but in modern lofts they're usually condensation. Here's how to tell them apart and what the real fix is.
Symptoms
- Damp patches on rafters or the underside of the underlay, especially in winter
- Black mould spots growing on the timber surfaces inside the loft
- Drips from the felt or membrane that appear on cold mornings but stop by midday
- A 'damp' smell in the loft even when it hasn't rained for days
What's actually causing it
- Bathroom and kitchen extract fans venting into the loft instead of through the roof — the single most common cause
- Insulation laid over the eaves vents, blocking the soffit airflow that's supposed to keep the loft dry
- Non-breathable bitumen felt on an otherwise modern loft, trapping moisture that has nowhere to go
- Recessed downlights or loft hatches without seals allowing warm humid air from the house to rise into the cold loft
Why it matters
Long-term loft condensation rots roof timbers from the inside, ruins insulation by halving its R-value when wet, and grows mould that can spread into the living space below. The frustrating part is that it's often misdiagnosed as a roof leak and 'fixed' with unnecessary roof work — the actual fix is almost always a ventilation correction inside the loft.
What to do right now
Check whether your bathroom and kitchen extract fans actually vent outside or just into the loft space — most under-£100 fans terminate in the loft, which is the cause we find most often. Move any insulation that's lapped up over the eaves vents to restore the soffit airflow.
What a proper fix looks like
A proper diagnosis distinguishes condensation from a true leak — the location, timing and moisture pattern are different. The fix is usually a combination of: properly ducted extract vents through the roof or wall, eaves vents cleared or upgraded to over-fascia vents, ridge ventilation added on non-breathable felt roofs, and any insulation rebalanced so airflow is restored. Most jobs are a half-day, not a re-roof.
What shapes the cost
- Number of extract vents that need re-routing
- Whether ridge ventilation needs adding (requires lifting a section of the ridge)
- Whether the existing felt needs upgrading to a breathable membrane during related work
We don't quote prices online for this category of work — too many variables. A free inspection gives you a written, itemised quote with no obligation.