Question answered
Signs of poor roof ventilation
The short answer
The main signs of poor roof ventilation are black mould spotting on rafters and the underside of the roof, damp or darkened timbers, water dripping from the underlay, a musty smell in the loft and a layer of frost on nails and felt in cold weather. These come from condensation, not rain. Look in the loft on a cold morning, and check that the eaves are not blocked.
A roof can leak without a single tile out of place. When warm, damp air from the house has nowhere to go, it condenses in the loft, and the signs look a lot like a leak.
Mould and dark staining on the timbers
The first thing to look for is black or grey spotting on the rafters (the sloping timbers holding up the covering) and on the underside of the sarking or boarding where there is any. Mould needs damp, and a loft that stays damp is a loft with a ventilation problem. The spots often start on the north-facing slope and near the eaves, where the air is coolest.
Darkened, tide-mark staining on the timbers tells you the water has been arriving for a long time. A single leak leaves a patch around the point of entry. Condensation leaves a wide, even pattern across several rafters, often following the coldest line of the roof. That difference is one of the first ways a surveyor tells a leak from a ventilation fault.
Do not scrub the mould off and carry on. Cleaning removes the evidence and leaves the cause in place, so the staining returns within a season or two. Photograph it first, then deal with where the moisture is coming from and why it cannot escape.
Dripping underlay, frost and rusting nails
On a cold morning, glance up at the inside face of the roof. A layer of frost on the felt, on the nail points that pass through the roof or on the insulation edge is a plain sign of warm air meeting a cold surface. When the day warms, the frost melts and drips onto the loft floor and sometimes through the ceiling below, which is why people assume a tile has slipped.
Rust on nail tips poking through the boarding, damp insulation that has gone flat and a cold, clammy feel to the loft all point the same way. Look too at the lowest part of the slope. Moisture runs down the underlay and collects at the eaves, so wet timber at the wall plate, the beam the rafters sit on, matters more than a damp patch near the ridge.
Frost is a winter clue; the damage it points to continues all year. If you can only visit in summer, look for the dry-weather evidence: staining, rust, flat insulation and a musty smell in the loft.
What the standard asks of a cold roof
A cold roof is the usual loft arrangement, with insulation on the ceiling and the roof space left cold above it. BS 5250 sets the ventilation for it. Marley's summary for a high-resistance (HR) underlay is a continuous opening of 25 mm at the eaves where the pitch is 15° or less, and 10 mm where it is steeper.
Marley also lists an extra continuous 5 mm opening high on the slope where the pitch is above 35°, where the span is over 10 m, and on any single-slope roof. A low-resistance (LR) underlay works differently: with a tightly sealed ceiling and a covering that lets air through, the 3 mm that ordinary eaves construction gives can be enough, so long as the maker's independent certification conditions are met.
These are the figures to check against what you find, and the roof's own underlay type decides which applies. Fitting the wrong ventilation to the wrong underlay causes as many problems as having none.
Everyday causes that block the air
Faults usually trace back to airflow rather than the covering. Insulation shoved right up to the eaves can close the gap that lets air in. Boxes and stored items stacked against the eaves do the same. An extractor fan from a bathroom or kitchen that ends in the loft, rather than outside, pours warm, wet air straight into the roof space.
Gaps around the loft hatch, recessed lights and pipes let warm air leak upwards from the rooms below, and a hatch that does not seal is a common reason a loft is damp in winter. Newly fitted insulation, new windows that make the house tighter and a change in how the family dries washing all raise the moisture that the loft has to cope with.
Roofs altered over time can lose ventilation without anyone noticing. A new covering with a different underlay, for example, may need different eaves detail from the old one. So we read the roof and the ceiling below it together.
Telling it from a leak, and what to do next
A leak shows up after rain and follows a path from one point. Condensation tracks the temperature: worse in cold, still weather, worse when the heating is on, and spread across the whole roof space. Check the loft in a dry spell as well as a wet one. If it is damp when it has not rained, ventilation is the likelier cause.
Photograph what you see and note the dates. We trace the cause from the loft first, then the outside, and you receive photographs from your roof and a written quote once the cause is clear. Sometimes the fix is as small as clearing the eaves or re-routing a fan. Sometimes it means opening up the eaves detail during a re-roof, which is a decision to make with the survey in hand.
Bats may roost in eaves gaps, so read our note on bat surveys for roof work before blocking or opening any. A new roof can arrive with its own faults, which our new-build roof snagging page covers, and the drone or ladder survey guide explains how the outside gets checked. Everything else we check is listed under roof inspections and surveys.
Loft condensation: two further questions
Will more loft insulation fix a damp, mouldy loft?
Not by itself. Extra insulation keeps the roof colder, which can increase condensation if warm air still reaches it, and it must never be pushed into the eaves where it blocks the ventilation gap. Seal the ceiling first, keep the eaves clear and then add insulation.
Is a flat-roof leak the same as a ventilation fault?
They can look alike. Flat roofs can also suffer condensation inside the build-up, and the ceiling below stains in the same way. The difference is the pattern: a fault in the covering marks one spot after rain, while condensation spreads and appears on cold, dry days.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
10-year workmanship guarantee, in writing.