Yes, a pitched roof space is normally designed to be ventilated, but how much air they need depends on what lies beneath the tiles. Under a non-breathable underlay, damp air from the house has nowhere to go unless the roof is built with a gap at the eaves. Under a breather membrane, some of that moisture can pass through the underlay itself, and the ventilation rules change.
Underlay is the sheet laid over the rafters and under the battens (the timber rows the tiles hang from). It catches wind-driven rain, and it also decides how the loft deals with the steam that every kitchen, shower and drying rack sends upwards. A wrong match between underlay, insulation and ventilation shows up years later as black mould on timber, damp insulation or dripping nail points.
This guide explains the difference between HR and LR underlays, the eaves figures that Marley publishes from BS 5250, what Approved Document C asks of a roof, and what we check on a re-roof near Farnborough before any new membrane goes down.
HR and LR underlays: what the letters mean
Underlays are sorted by how easily water vapour passes through them. HR stands for high resistance, and an HR underlay is non-breathable: vapour stays in the loft. LR stands for low resistance, while an LR underlay acts as a breather membrane, letting vapour pass through yet still turning back liquid water.
The old bitumen felt
A roof laid before breather membranes became common usually carries a bitumen felt, which is an HR underlay. It does a decent job against rain, but it holds vapour back. That is why older roofs were built with gaps at the eaves and often a vent at the ridge, so that moving air could carry the moisture out before it condensed.
The breather membrane
A modern breather membrane is a woven or spun plastic fabric. Because vapour can pass through it, less of the loft's moisture is trapped under the tiles. That does not mean a loft can be sealed up and forgotten. The ceiling below still needs to be well sealed against steam rising from the rooms, and the manufacturer's data sheet sets out what the membrane has been tested for.
Why the label matters
The two types are not interchangeable at the eaves. If a re-roof swaps an old felt for a breather membrane, the ventilation allowance can change. If a roofer replaces a breather membrane with plain HR felt and leaves the old eaves detail, the loft can start to hold damp. For that reason we name the underlay on the survey sheet, and we say which type is already there before we say what goes on.
HR underlay is not breathable. If your roofer says a high resistance felt is a breather, ask for the data sheet.
Eaves gaps for an HR underlay
In a cold roof the insulation sits on the ceiling joists and the loft above is left cold and aired. BS 5250:2021, the BSI standard on moisture management in buildings, is listed by the BSI as current and under review. Marley turns it into two simple eaves figures when the underlay is HR (non-breathable).
The two published figures
| Roof pitch | Ventilation along the eaves |
|---|---|
| 15 degrees or less | 25 mm, all along the eaves |
| More than 15 degrees | 10 mm, all along the eaves |
The gap runs the whole length of the eaves on both sides, so air can enter low and leave high. It is usually formed with an over-fascia vent or with vents in the soffit, the board under the overhanging eaves. Our guide to soffit ventilation covers those parts.
Where the gap gets blocked
The most common fault we see in a loft is not a missing vent but a blocked one. Insulation pushed into the eaves to fill a cold corner, or a loft board laid hard against the felt, closes the path that the air should follow. The figures above only work if the gap is open in the roof as it stands today, so the survey includes the eaves, seen from inside the loft and photographed.
Where the pitch is a low one, such as over a porch or a rear extension, the larger 25 mm gap applies, so a roof can need more ventilation exactly where it is hardest to fit.
What changes with a breather membrane
For an LR (breather) underlay, Marley accepts what it calls fortuitous ventilation, meaning air that enters through ordinary eaves construction, counted as 3 mm if the ceiling beneath is well sealed. That allowance carries a condition, because the membrane needs independent third-party certification. Without that certificate, a breather membrane does not earn the relaxed treatment, and the full eaves gap is the safer design.
Three conditions to look for
- A certified product: the membrane's data sheet or certificate should state that it can be used on that basis.
- A well-sealed ceiling: gaps around loft hatches, light fittings and pipes let steam into the roof space faster than a membrane can pass it out.
- A matching eaves detail: the membrane has to be laid and supported as the maker specifies, and not allowed to sag into the gutter or tent over the rafters.
Warm roofs and loft conversions
Where a loft is converted, the insulation moves to the rafters and the roof space is no longer a cold void. That changes the ventilation design again, and the answer turns on the build-up chosen. Our guide to insulation when re-roofing sets out the thermal side of the same decision.
Questions to put to any roofer
Ask which underlay is going on, and what its maker allows at the eaves. Ask how the eaves gap will be kept open once insulation is topped up. A roofer who can answer both from the data sheet has thought about the loft, not just the tiles.
A breather membrane is a good choice on many re-roofs, but it is not a licence to close every vent. If we propose one, the written price covers the membrane named on the data sheet and the eaves detail that goes with it, and we explain why before work starts.
What Approved Document C asks of a roof
The law here is the Building Regulations, and the relevant part is Approved Document C, which deals with moisture. Its roof section sets two plain aims. Paragraph 6.2 says a roof should keep rain and snow from getting in, and that heat and structure should not suffer from "interstitial condensation".
Condensation inside the roof
Interstitial condensation is moisture that forms within the layers of a roof, not on a visible surface. It cannot be seen from the room below, which is why a loft can look dry and still have soaked insulation. Paragraph 6.10 says a roof meets the condensation requirement when built to Clause 8.4 of BS 5250:2002 together with BS EN ISO 13788:2002. The document still cites the 2002 edition of BS 5250, and the current one is the 2021 edition, so the figures we use come from the manufacturers' guidance on the current code.
Cold decks, small roofs and loft hatches
In paragraph 6.11, ventilation is the accepted answer for a cold deck, with a note that a tiny roof, such as a porch canopy, can sometimes go unventilated. Paragraph 6.12 wants loft hatches sealed against draughts, which links back to the sealed ceiling described above.
These are not abstract points. Whether a new covering needs an application follows the Planning Portal test: under 25 per cent of the building envelope and under 50 per cent of the roof affected, normally no application. Your building control body, the Hart and Rushmoor Building Control Partnership for Farnborough addresses, on 01252 398715, can say how it applies to your own job.
Signs your loft is not breathing
You do not need instruments to spot a loft that is not ventilating properly. A torch, a dry day and a few minutes on the loft boards will show most of what matters.
Look at the timber and the underlay
- Black or grey mould on the underside of the underlay or on rafters near the eaves.
- Dark, damp-looking battens or rafters, especially on the north slope.
- Drips from nail tips on frosty mornings, where cold metal collects condensation.
- Damp or compressed insulation pushed into the eaves.
- Rust on nail heads and metal brackets.
Why weather matters, in general terms
Frosty mornings matter in a loft because a cold roof surface is exactly where warm, damp air condenses. Winter is when a loft with poor airflow shows its damp, and spring is when the drips and mould are noticed. None of this says anything about your own roof until someone has looked, which is why we look.
What a roofer can and cannot tell you
From the loft, we can see whether the eaves are open, whether the underlay is HR felt or a breather membrane, and whether insulation has blocked the air path. We cannot tell from a glance how well the ceiling below is sealed, which is why we ask about bathroom and kitchen extractors too.
Ventilation is only one cause of loft damp. A small leak from a failed flashing can look the same, so our guide to battens that need replacing shows how rot from above and rot from below differ.
How we check a roof space on a re-roof
On a re-roof, the old underlay comes off with the covering, which gives a clean chance to put the ventilation right. We do not assume the old arrangement was correct, and we do not copy it onto the new roof just because it was there.
The survey first
We photograph the eaves from inside the loft, the ceiling hatch, the insulation and any vents we can find. We measure the pitch, since the pitch decides which of the two eaves gaps applies for an HR underlay. Those notes go into the written price, which is one total after the survey.
Choosing the underlay
The choice sits between a modern HR underlay with a proper eaves gap and a certified breather membrane. Each is acceptable when it is detailed correctly. What we avoid is a mixture: a breather membrane laid over a deck that still relies on a vent no longer open, or an HR underlay laid with the eaves choked by new insulation.
Fixing, laps and the rest of the roof
Underlay laps run in the direction the water flows, and the membrane is supported at the eaves so it drains into the gutter and not behind the fascia. The tiles and ridge then follow BS 5534, which Marley points out is not required by law but sets out what an appropriate fixing looks like. Our guide to dry ridge or mortar covers how the ridge is held and ventilated at the top.
On our pitched roofing jobs, the result is a roof space with a named underlay, a measured eaves gap and photographs that show the work.
