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Roof Ventilation Explained — Why Lofts Get Damp
A damp loft is usually a ventilation problem wearing a leak's clothing. If the staining is even, widespread and worst in winter, water probably isn't coming in from outside at all.
- Cross-flow from eaves to ridge is the goal
- Deeper insulation often blocks eaves airflow
- Bathroom fans venting into lofts are a common cause
- Ridge, tile and over-fascia vents each have a use
- Breathable membrane is not a substitute for vents
- We diagnose before we sell vents
How it's meant to work
Air enters low at the eaves, rises through the loft picking up moisture, and leaves at the ridge. That gentle continuous flow keeps timbers and the underside of the membrane dry. Break the path at either end and moist air stalls against a cold surface — which in a cold winter is the underside of your roof covering.
Why it fails in modern homes
Loft insulation has got much deeper, and it very often gets pushed hard into the eaves where it seals the inlet completely. Add a bathroom extractor discharging into the loft instead of through the roof, downlighters leaking warm air from the rooms below, and a sealed non-breathable membrane from a 1990s reroof, and you have a loft that soaks itself every winter.
- ▸Insulation blocking the eaves gap
- ▸Extractor fans discharging into the loft
- ▸Downlighters leaking warm moist air
- ▸Old non-breathable felt with no vents
What actually fixes it
Restore the inlet — eaves vents or over-fascia ventilators where soffits are shallow, plus rafter trays to hold insulation clear of the airway. Restore the outlet — ridge ventilation or tile vents high on the slope. Duct any extractor properly through the roof. In most houses that's a one-day job and it ends the problem, which is why we check ventilation before quoting anything bigger.
FAQs
How can I tell condensation from a leak?+
Condensation is widespread, even, worse in cold weather and often shows as droplets on nails and membrane. A leak is localised, follows rain, and usually has a stained track.
Does a breathable membrane mean I don't need vents?+
Not on its own. Breathable membranes help, but guidance still expects ventilation provision in most constructions.
Will vents make my house colder?+
No — the ventilated space is above the insulation. The insulation keeps the house warm; the vents keep the roof dry.
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