Why Ventilation Is a Roofing Issue
Roof ventilation is not simply a loft comfort question — it's a structural protection issue. Moisture from everyday household activity (cooking, bathing, breathing) rises through the building and enters the cold loft space. Without adequate air movement to carry it out, this moisture deposits on the cold surfaces — insulation, timbers, felt underside — and accumulates over months and years.
The result: insulation that becomes saturated and loses its thermal value; batten rot that isn't visible until tiles are lifted during a re-roof; rafter decay that affects structural integrity; and conditions that accelerate mould growth on the underside of the roof deck. A poorly ventilated roof can fail structurally from within while looking perfectly serviceable from outside.
The Chippenham-Specific Problem
Hardenhuish, Stanley and Cocklebury's 1960s–70s housing was built with original ventilated soffit boards as standard. Many of these properties subsequently had loft insulation upgraded — mineral wool laid to the ceiling joists — without the installer ensuring the eaves ventilation remained clear. Insulation packed against the eaves blocks the soffit vent and eliminates the cross-ventilation the roof needs.
We find this pattern consistently during nail sickness survey loft inspections: rotten battens on properties where the tiles are still holding, the felt looks reasonable, but the moisture damage has been accumulating for 15–20 years because the eaves vents were blocked when someone upgraded the insulation in the 1990s.
Quick check: In your loft, look toward the eaves. You should be able to see daylight (or at least light) at the lowest point of the roof structure. If the insulation is pressed against the rafters all the way to the wall plate with no gap, your eaves ventilation is compromised.
Cold Roof vs Warm Roof Ventilation Requirements
Cold Roof (insulation at ceiling level)
The most common configuration in Wiltshire domestic properties. Insulation sits on the ceiling joists, leaving the roof void above as a cold, ventilated space. BS5250 requires a minimum 25mm clear airspace between the insulation and the underlay throughout the slope, with ventilation openings equivalent to a 25mm continuous gap at the eaves on both sides.
Warm Roof (insulation between or above rafters)
Used where loft space is habitable (loft conversions) or where thermal performance takes priority. Insulation fills the rafter depth, with different ventilation strategies depending on whether breathable underlay is used. Warm roof configurations require careful design to avoid interstitial condensation within the insulation layer — a more complex condensation risk than the simple loft condensation in a cold roof.
What Correct Ventilation Looks Like
For a typical cold-roof Wiltshire semi-detached, correct ventilation involves: ventilated soffit panels at eaves (not solid fascia boards that block airflow); a clear 25mm+ airspace maintained across the whole roof slope; and on longer slopes or where ridge ventilation is provided, additional ventilation at high level to promote air movement through the space. We assess ventilation adequacy during every loft inspection and note any deficiencies in the survey report.