Roof tile vents
Do you need tile vents in a pitched roof?
The short answer
Not always. A pitched roof needs some way for air to move through the roof space, and often the gaps at the eaves provide it. Tile vents are the extra: a vent unit that replaces a single tile to let air in or out, serve a pipe or duct, or ventilate a corner the eaves cannot reach. Whether yours needs them turns on the roof's construction and its insulation.
What a tile vent is, when eaves ventilation is enough, what a damp, stale roof space looks like and how vents are fitted.
What a tile vent is for
A tile vent is a moulded unit, shaped like the tiles around it, that sits in the covering and has a grille or slot to let air through. A slim version fits in the same course as the tiles either side. Some vents ventilate the space below the tiles. Others carry a pipe, such as a soil vent or an extractor duct, up through the roof and out to the open air, with the vent body dealing with the weathering around it, so the pipe stays clear of the tiles.
This answer is about the first kind, vents that move air through the roof. Pipe terminals are separate fittings that depend on what the pipe serves, and the plumber or heating engineer usually specifies those. If your question is whether a loft or roof void needs more air, the vents discussed from here on are the ones that matter, and they are the ones a roofer fits.
Why a pitched roof needs air at all
Warm, damp air rises from the house into the roof. Cooking, washing and breathing add moisture to the air, and in a cold loft that moisture finds cold surfaces to settle on. Left alone, it condenses on the felt's lower face and on the timber, and over time that means mould, staining and rot. The remedy is a steady movement of outside air to carry the damp away.
The reference for managing moisture in buildings is BS 5250, which BSI currently lists as under review. For ventilation at the eaves, Marley's guidance on cold roofs with a high-resistance underlay gives a continuous gap of 25mm for pitches of 15 degrees or less, and 10mm for steeper pitches. That gap is usually made with a vent strip or a fascia vent at the bottom of the slope. Our guide to underlay and ventilation covers how the two work together.
When the eaves are enough, and when they are not
On a plain gabled house with a clear loft, air entering at the eaves on both sides can often do the job by itself. The picture changes when the shape of the roof blocks the route. A lean-to or a flat-topped section may have no way for air to leave, a hipped roof can trap air in the corners, and a long run of eaves covered by an extension may have no opening at all. High-level vents near the ridge give that air an exit.
Loft insulation can cause the opposite problem. Packed too tightly into the eaves, it closes the gap that air needs. Anyone adding insulation should keep that opening clear. Where the roof has been re-felted, the type of underlay changes the ventilation requirement, so the choice of felt and the choice of vents belong together. We settle both at survey, from what we see in the loft and on the roof.
Signs your roof space is short of air
The clues are easy to recognise once you know them. Look for black mould spots on the rafters or the underside of the felt, a musty smell in the loft, water droplets on the felt or on nail points, and damp or darkened insulation. In winter you may see a fine frost on the timbers that melts into drips when the weather warms. Ceilings below may show brown rings that look like a leak but happen with the weather dry.
Do not assume condensation every time, since a real leak and a ventilation problem can look alike. A stain after rain points to water coming in, and a stain after a cold, still, dry spell points to condensation. A roofer can tell them apart from the photographs and a look at the pattern. If you want to prepare for the colder months, our guide to tiled roof winter preparation lists the checks.
Fitting vents, and what it involves
A tile vent is fitted by lifting the tile it replaces, cutting the felt to suit, setting the vent unit on the batten and fixing it as the maker directs. It sits in the line of the tiles so it looks like part of the roof, and the surrounding tiles are relaid over it. A good fitter keeps the felt lapped around the opening, so rain that blows through the grille cannot reach the timber.
The number and position of vents come from the roof design and the vent maker's data, not a rule of thumb, so the count is worked out for each roof. They are a small item next to the access, which is where most of the cost sits. Where air is needed shows up in the survey photographs, and the figure you are given afterwards is a single written total. Our pitched roofing page covers the wider work on tiled roofs, and the guide to roof tile types helps match a vent to your tile.
Related questions
Tile vent questions
Will a tile vent let rain into the loft?
A well-made vent keeps the weather out while letting air through, and the felt around it is lapped so that wind-blown rain drains away. Problems come from poor fitting, such as a gap in the felt, not from the vent itself.
Can tile vents cure condensation on their own?
Sometimes, but not alone. Moisture rising from the house, blocked eaves and tight-packed insulation all play a part, so a vent is one piece of the answer. We look at the whole roof space before suggesting where air should enter and leave.