Question answered
Do you need tile vents in a pitched roof?
The short answer
High-level tile vents are needed in a pitched roof only in some cases. Marley's guidance, based on BS 5250, adds a continuous 5 mm of ventilation near the ridge when the pitch is over 35 degrees, when the span exceeds 10 metres, or on lean-to and mono-pitch roofs, provided the underlay is the high-resistance type. Other roofs rely on eaves ventilation alone.
A loft that cannot breathe turns damp, and damp rots timber from the inside. Whether a pitched roof needs vents at the top depends on its pitch, its span and the type of underlay.
Why a cold loft needs airflow
An ordinary house loft sits above the insulation and stays cold. Warm, damp air from kitchens, bathrooms and bedrooms leaks up through the ceiling, and where it meets the chilly underside of the roof it condenses. With no way out, that moisture soaks the underlay, battens and rafters, and over the years it produces the decay described in rotten roof battens.
Ventilation breaks the cycle. Cold air enters low, usually through a gap at the eaves, and the damp air leaves higher up or through the covering itself. The design principles come from BS 5250, the code of practice for moisture in buildings, and Marley turns them into figures for its own products.
A tile vent provides the upper half of that route: a small opening built into a course of tiles near the ridge, so air can pass out under cover. Older roofs rarely have one, since ridges were once simply bedded in mortar.
What the tile maker advises for eaves and high level
With a high-resistance (HR) underlay, Marley recommends 25 mm of opening at the eaves on pitches up to 15 degrees, and 10 mm on steeper ones. A further 5 mm of continuous ventilation at high level goes in for pitches above 35 degrees, for any pitch spanning over 10 m, and on any lean-to or mono-pitch roof.
That answers the question in the heading. Most ordinary pitched roofs between those limits depend on air entering at the eaves. A steep gable, a long barn-style span or a single-slope extension roof moves into the high-level group.
These figures belong to one kind of underlay. The product data sheet and the manufacturer's technical advisers have the last word on your roof, so we work from them rather than from rules of thumb passed between trades, and we write down which document we used.
A breathable membrane changes the sum
Not every underlay behaves the same. A low-resistance (LR) membrane lets water vapour pass through. Marley says that with LR underlay, a well-sealed ceiling and a covering that is air-permeable, the natural leakage in normal eaves construction, about 3 mm, can do the job, so long as the certification requirements are all satisfied.
The conditions matter. A sealed ceiling means no gaps around the loft hatch, light fittings and pipes, and many older houses are full of them. A new membrane does nothing to close those gaps.
So choosing a particular underlay does not settle the ventilation question by itself. The two have to suit one another, and the felt we meet on a roof such as the one in torn felt under the tiles may be of a different type from the one that goes back. The survey identifies it before any decision is made.
How roof shape shifts the answer
Form changes the maths. A trussed roof with a long, open span, as set out in cut roof or trussed roof, is likelier to cross the 10 metre line than an old cut roof over a terrace. The covering you choose also bears on the venting, since tile profiles differ in how they seat and how much air passes through them.
Dormers and extensions bring their own difficulties. A flat or shallow-pitched dormer roof often needs separate treatment, which we explore in dormer roofs.
Fitting a vent without checking the layout behind it can do harm as well as good. A badly placed opening can draw air through the loft insulation and chill the ceiling below, or let driven rain reach timber that was dry before. That is why the tiles, the underlay and the insulation are looked at as one system, not as three separate jobs, and why a vent is never added just because it is cheap to do.
Checking your own loft
A torch shows a lot. Look at the eaves: is there a clear gap above the insulation, or has the quilt been pushed hard against the felt? Check nail tips for condensation drops, look for dark staining on the timber and sniff for a damp smell. See whether the area under the ridge is dry.
If insulation was added recently, ask how the eaves were kept open. A thick roll stuffed into the corner can shut off the airflow and cause the damp it was meant to cure. Eaves ventilation trays exist for precisely this reason.
On a survey we photograph the eaves and ridge area and identify the underlay, then say whether vents are needed. Our written quote gives one total, and vents go in only where they have a job to do. The broader range of work is on the pitched roofing page.
Roof airflow: further questions
Will fitting one vent stop condensation by itself?
Rarely. Condensation usually begins with moist air leaking up from the house, so sealing the ceiling and extracting steam at source matter as much as any vent. A vent only gives damp air a way out. If the eaves have no airflow, a single high-level opening will not cure the problem.
Do tile vents let rain into the loft?
A well-made vent is built to shed water, and the tiles around it are laid in the normal way. Trouble comes from poor fitting or damaged tiles. If damp appears near a vent after storms, have it checked, because the cause may be the tile, the underlay beneath it or the flashing.
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.