How do you check if a roof is ventilated properly?

Most older houses are built with a cold roof. Insulation lies across the loft floor, which leaves the space above it at roughly the temperature outdoors. Moist air from kitchens, bathrooms and bedrooms drifts upward into that space whatever the season, and if it cannot leave it condenses on the coldest surfaces it finds. Ventilation is simply the route that lets it escape.

The path normally starts at the eaves, through gaps in the soffit or vents set into the fascia, passes through the loft, and finishes at vents near the ridge or at ventilated tiles dotted up the slope. The easiest test is to find where air is meant to enter and where it is meant to leave, and then check that nothing has been fitted, stacked or painted over to stop it. A roof with no clear route at either end cannot breathe, however sound the tiles look from the road. Hipped bungalows and houses with deep eaves boxes are worth special attention, because the vent strip is easily hidden behind a boxed-in soffit.

Go up on a cold morning with a torch and a dust mask. Scan the felt from beneath and the roof timbers for beads of moisture, dark staining or black spots of mould. Feel the insulation near the eaves: if it is damp or squashed flat, little air is passing. A musty smell, nail heads that are wet or rusty, and a film of water on the lid of a cold-water tank tell the same story.

Stains on the ceilings below, particularly round the outer edges of the rooms, can have the same cause, and they are often blamed on a leaking tile when there is no hole at all. Our answer on condensation in the loft explains how to separate a moisture problem from a leak. A surveyor reads several signs together, since one damp patch on its own proves very little.

Marley's guidance for cold roofs gives different eaves openings for different underlays. Where the underlay is high resistance and so not vapour permeable, a continuous 25mm gap at the eaves suits pitches of 15 degrees or below, and 10mm suits anything steeper. Where the underlay is the low-resistance breathable kind and the ceiling below is well sealed, Marley accepts the incidental 3mm leakage of ordinary eaves construction, subject to third-party conditions on the product.

BSI lists BS 5250:2021, the moisture code, as current but under review, so published figures can move. Always read the maker's own instructions for the products on your roof. Our answer on underlay and ventilation on pitched roofs shows how the pieces combine, while roof tile vents covers the ridge end of the path.

A sound design usually fails because of something added later. Quilting stuffed tight into the eaves can seal the very gap air depends on. Rolls laid over the wall plate, stored boxes pressed against the felt, and paint carried across a vent strip all do the same job. To check, pull the insulation back gently with a gloved hand and see whether a clear channel runs up between the rafters.

From outside, look up at the soffit. Vents clogged with paint or dirt, or a fascia vent covered by a new gutter bracket, are visible from the ground. If the underside of the roof has been sprayed with foam, the airflow questions change, and that belongs with a surveyor who can see the roof, as it affects how moisture behaves in the timber. Ask for it to be assessed before you add anything else to the loft.

Ask for a visit if the signs return each winter, if damp appears after the roof has been re-covered or newly insulated, or if you cannot reach the eaves yourself. Our inspection takes in the felt and battens from below, the eaves, any vents already fitted and the insulation, and each point is photographed so the explanation can be followed on screen. We also look at where the household moisture comes from, such as extractor fans that discharge into the loft instead of outdoors. The verdict is plain: improve the ventilation, clear the blockage, or leave the roof alone.

Remedies are usually modest, such as a tile vent, a soffit vent or a restored gap, and they cost less than the damage that follows if damp is left to spread. We put the quote in writing after we have seen the roof. For the wider picture, see the hub on roof inspections and surveys and the answer on checking a roof from the loft, which both help you prepare.