Loft mould treatment in Cardiff is a job we see more and more, and in almost every case the mould is a symptom rather than the problem. Black spots on the rafters and roof felt are caused by condensation: warm, damp air from the house reaching the cold underside of the roof and turning back into water. Clean the mould off without fixing that and it will be back the following winter.
So we do both. We treat the mould with a fungicidal treatment, then find and fix why your loft is getting damp, usually through better ventilation and stopping moist air getting up there in the first place.
What causes loft condensation
A typical loft is a cold space by design, with the insulation on the floor keeping heat in the rooms below. That only works if moisture can escape. Loft condensation happens when more damp air gets in than can get out. The common causes:
- Warm, moist air from the house. Steam from showers, cooking and drying clothes rises and gets into the loft through gaps around the hatch, downlights, pipes and cables.
- Bathroom extractor fans venting into the loft. A fan should duct the air outside. We regularly find ducts that stop in the loft, have fallen off, or were never connected, pumping damp air straight onto the roof timbers.
- Blocked eaves vents. Insulation pushed hard into the eaves, often during a previous install, blocks the air that should flow in at the edges of the roof.
- Insulation without ventilation. Adding more insulation makes the loft colder. That is the right thing to do, but without enough ventilation a colder loft is more prone to condensation.
- Non-breathable roof felt. Older bitumen felt does not let moisture through, so the roof relies entirely on ventilation.
- Spray foam on the roof underside. Foam sprayed onto the back of tiles, slates or felt can restrict ventilation and hold moisture against the timbers. If that is what you have, see spray foam insulation removal.
- Damp from outside. A leaking roof, slipped tile or failed flashing is not condensation, but it can look similar. We check for it.
Our guide to loft condensation and mould goes into the causes in more depth.
Older felt and newer membranes
Many older Cardiff roofs have black bitumen felt under the tiles or slates, and some older properties have none at all. Bitumen felt is effectively waterproof to vapour, so any moisture that reaches the loft has to leave through ventilation. Newer roofs, and roofs that have been re-covered, often use breathable membranes that let some vapour pass through. These help, but they still rely on the loft not being flooded with damp air from below. Knowing which you have tells us how much ventilation the loft needs.
Coastal and hillside homes around the city can also be exposed to wind-driven rain, so we check the roof covering itself as part of the survey to rule out a leak before treating anything as condensation.
Signs of mould and condensation in your loft
- Black or grey spotting on the underside of the roof felt or sarking boards.
- Dark staining or mould on the rafters, especially on the north-facing slope.
- Water droplets on the felt or nail tips on cold mornings.
- Damp, matted insulation or drip marks on stored boxes.
- A musty smell when you open the hatch.
- Water marks on bedroom ceilings in winter with no obvious leak.
Why insulating alone is not the fix
A common assumption is that a cold, damp loft needs more insulation. The opposite is often true. Better loft insulation keeps more heat in the house, which makes the roof space colder, and cold surfaces are where condensation forms. Insulation is still worth doing, but it has to go hand in hand with keeping moist air out and letting the loft ventilate. That is why we deal with mould and ventilation together, and why our insulation installs always keep the eaves clear.
How we treat loft mould
Assessment
We look at where the mould is, how widespread it is and what pattern it follows. Mould concentrated above a bathroom points to the extractor. Mould along the whole cold slope points to ventilation. Damp around a chimney or valley suggests a leak. Finding the cause tells us what needs fixing.
Fungicidal treatment
We apply a fungicidal treatment to the affected rafters, felt or sarking and surrounding timber to kill the mould growth. Where insulation is soaked or mouldy, we remove it. We take care around stored items and cover anything that cannot be moved.
Fixing the ventilation
- Clearing the eaves. Pulling back insulation that is blocking airflow and fitting eaves ventilators or rafter trays to hold a clear gap open.
- Soffit vents where there is no airflow at the eaves.
- Tile vents high on the roof to let damp air escape, fitted by replacing standard tiles with purpose-made vent tiles.
- Extractor ducting. Reconnecting or extending bathroom and kitchen fan ducts so they discharge outside.
Stopping moist air getting in
We insulate and draught-proof the loft hatch, and seal gaps around pipes and cables coming up from below where we can. Less warm, damp air reaching the loft means less condensation.
What affects the cost
Loft mould treatment starts from £300. The price depends on:
- How much of the roof is affected.
- Ventilation work needed: eaves vents, soffit vents or tile vents.
- Extractor duct repairs.
- Insulation replacement if it is soaked or mouldy.
- Access, loft layout and how much has to be moved.
If we find a roof leak or decayed timber that needs a roofer or timber specialist, we will tell you rather than treating around it.
What happens on the day
We protect the landing, set up lighting and move stored items away from the work area. Mouldy insulation is bagged and removed. The fungicidal treatment is applied to the affected timber and felt, then the ventilation work and duct repairs are carried out, and the hatch is insulated and sealed. If the insulation needs replacing, we lay new wool to 270mm with the eaves kept clear. We tidy up and show you before and after photos.
Aftercare and what to expect
- Staining may remain. Treatment kills the mould, but dark marks can stay on the timber. That is cosmetic.
- Use your extractor fans and leave them running for a while after showers.
- Keep the hatch closed and avoid drying washing near it.
- Do not block the eaves with stored items or extra insulation.
- Check after a cold spell. Glance up after the first frosty week to make sure the felt is dry.
We cover Cardiff and the surrounding towns. Have a look at the areas we cover, or call 0800 229 4668 to book a free survey.