The short answer
A slate vent is a moulded fitting, usually plastic, that is made in the shape and colour of a slate and fitted in place of one or more slates to allow air or a pipe to pass through the roof. Common uses are ventilating the roof space, giving a soil pipe or drain vent an outlet, and terminating a bathroom or kitchen extractor duct. It must be sealed and lapped like a slate so that rain cannot get in.
The jobs a slate vent does
A roof is not a sealed box. It has to breathe, and many services need to reach the outside through it. Slate vents cover three jobs. The first is ventilating the roof space. Warm, moist air from the house rises into the loft, and if it cannot escape it condenses on cold timbers and underlay. A vent in the slope gives it a way out, working with eaves ventilation to move air across the space. The second is releasing air from a drainage stack, usually a soil vent pipe that must terminate above the roof. The third is carrying extractor ducts from a bathroom, kitchen or cooker hood to the outside, where the damp air is released.
Each job uses a different version of the fitting, and they are not interchangeable. A vent made for roof-space airflow does not seal a duct, and an extractor terminal is not designed to carry a soil stack.
How a slate vent is built and fitted
The typical roof-space vent is a flat plastic tile in the form of a slate, with an opening covered by a louvred grille and a lip that tucks under the slates above. Some replace one slate, others a block of several. To fit it, the roofer removes the slate or slates, cuts the underlay and, where needed, a batten, then fixes the vent to the battens so its upper edge sits under the course above and its lower edge overlaps the course below. The slates around are relaid to lap correctly. A pipe terminal works the same way, with a collar that rises around the pipe and a flashing skirt that laps under the slates above and over the slates below.
The cut in the underlay needs a sealed edge or a bonded collar so that any water that gets through the vent does not run onto the insulation. Vents for extractor ducts need an insulated duct, or condensation will run back into the room. Our guide on slate roof underlay and ventilation covers how the whole roof build-up breathes.
Getting the position and colour right
On a slate roof, placement matters for looks as well as function. A vent in the middle of the front slope is far more visible than one low down on the rear, and a cluster of fittings in random spots looks untidy. Where possible, put vents at the back, in line with the courses, and keep pipes and ducts together. Colour should match the slates: black, grey and brown are common, and a well-chosen fitting is hard to spot from the ground. A tile vent fitted in a roof of different size or shape from the surrounding slates will show up, so ask for a profile that suits the slate.
New openings on a street-facing slope of a listed or conservation-area house can attract comment from the council, so find out first what it will accept.
Common faults
The usual faults come from the lap and the seal. A vent set too high leaves a gap under the course above, and wind-driven rain gets in. A vent fixed to the slates rather than the battens can sag and crack. A collar cut too tight round a pipe, or caulked with mastic instead of flashed, will split in winter. Blocked grilles stop airflow, and debris or a bird's nest can build up behind them. You may notice mould on the loft timbers, damp on the ceiling beneath, or a stain on the wall where a duct drips. Our page on slate roof maintenance includes checking fittings in the routine.
Eaves, ridge and the bigger ventilation picture
A single vent in the slope is rarely the whole answer. Roof-space ventilation works best when air enters low, at the eaves, and leaves high, at the ridge or through vents near it, so that it sweeps across the underside of the roof. Slate roofs often have no eaves vents at all if they were built with open boarding and no felt, which is why added underlay can create a damp loft that never had a problem before. Adding vents is a balance, and the openings have to be sized and spread sensibly. A survey from the loft, with a torch and a moisture meter, shows where moisture is collecting and where air is moving.
When we need to add or replace a vent, we show the position on the slope, photograph the existing roof and underlay, and confirm the fitting with you. The price is fixed. Explore our other slate roofing work in Basingstoke for related jobs.
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