The two figures and where they apply
The eaves are the lowest line of the roof, past the wall face. The ventilation gap is the continuous opening along it, formed by a fascia vent, a soffit vent or an over-fascia strip, which lets outside air into the roof space.
Marley's advice on ventilating a cold roof is specific. With HR underlay laid, it asks for an opening along the whole eaves, sized by pitch, as the table shows.
| Roof pitch | Eaves vent |
|---|---|
| 15° or less | 25mm along the eaves |
| More than 15° | 10mm along the eaves |
The gap runs the whole length of the eaves, not just at a few points, and it has to stay open once the fascia, soffit and insulation are all in place. A row of small round soffit holes does not add up to the same free opening as a continuous strip, so check how the area is made up as well as how it looks from the ground.
Note which way the figures run: the shallower roof is the one asked for the wider opening.
Why blocked eaves cause condensation
A house makes moisture all day from cooking, washing and breathing. Warm air carries it upward, and some reaches the roof space through ceiling gaps, hatches and light fittings.
With HR underlay, that moisture cannot pass out through the roof covering. Air entering at the eaves and leaving higher up carries it away. If the gap is blocked, the air stops moving, vapour settles on the cold underside of the underlay and the timbers, and the first signs are drips on the felt, damp battens or spots of mould in the loft.
The usual causes of a closed gap are all ordinary ones:
- Loft insulation stuffed hard against the roof edge.
- Paint or debris clogging a vent.
- A bird guard or comb filler fitted wrongly.
- A new fascia board fixed tight to the tiles.
None of these can be seen from the street, and each can usually be put right without stripping the roof. The wider run of symptoms is covered in our page on why a loft is damp.
Standard, underlay type and other routes
BS 5250:2021 is the standard edition on moisture in buildings. The Marley figures sit inside that frame, and they are tied to one particular build, a cold roof under HR underlay, so they are not a universal number.
Change the build and the answer changes. A vapour-permeable membrane comes with instructions from its own maker, which may ask for the same opening, a different one or a different arrangement at the ridge. A warm roof, where the insulation sits at the rafters, is ventilated on different principles again.
So the right size is not one number for every house. Three things set it:
- The pitch of the roof, above or below 15°.
- Whether the underlay is HR or vapour-permeable.
- Whether the insulation lies flat on the ceiling or fills the rafter depth.
Our breathable membrane against felt page covers the underlay choice in more detail, including what a data sheet should say about laps, support and ventilation.
Keep the maker's sheet for whichever underlay goes on, because it is the paper that settles the argument later, and it shows a surveyor or buyer that the roof was built to instructions.
Checking and clearing the gap
From inside the loft with a torch you can look at the eaves. You should see daylight or a clear space where the insulation stops short of the roof edge. If the insulation lies hard against the underlay, the air path is closed.
From outside, a tape measure along the soffit vent or fascia strip gives the opening. Check the pitch too, because the 15° dividing line decides which figure applies, and a digital level on a rafter gives the true angle.
On a re-roof the gap is dealt with when the battens and underlay go on. We set the eaves detail against the maker's instructions, fit eaves support trays where the insulation meets the roof edge and leave the opening clear.
If you plan to add loft insulation, keep the gap in mind first, because the thicker layer is what usually closes it. Read about roof underlay and battens, and see every tiled job under pitched roofing. Photos of your eaves and the loft edge show us quickly what is going on, and a note of any damp patches below helps.
