Roof & loft ventilation
In short
A roof space needs air moving through it or the moisture from the house condenses on the underside of the covering. Ventilation is cheap, quick and fixes the great majority of damp lofts — which are routinely and expensively misdiagnosed as leaking roofs.
Also called: Loft ventilation · Eaves vents · Ridge vents · Tile vents
Signs you need this
- Condensation droplets on the underside of the roof
- Black mould on rafters or the underside of the felt
- Damp, flattened or musty loft insulation
- Insulation stuffed tight into the eaves with no air gap
- An extractor fan ducting into the loft rather than outside
- Damp that appears on cold mornings rather than during rain
The principle is simple. A cold roof space — one insulated at ceiling level, which is nearly all of them — will collect moisture unless air moves through it. Air comes in low at the eaves, warms slightly, rises, and leaves high at the ridge, carrying the moisture with it. Interrupt either end and the loft gets damp.
In practice the low end is what gets blocked, and usually by insulation. Every energy efficiency improvement of the last thirty years has involved putting more insulation in the loft, and it is nearly always pushed hard into the eaves where the roof meets the wall — which is precisely where the air was getting in. The house gets warmer and the loft gets wetter.
The fix at that end is a pair of things: rafter trays, which are simple channels that hold the insulation clear of the roof so a gap remains, and eaves or over-fascia vents to let outside air into that gap. Neither is expensive and both can be done without touching the covering, which is why this is one of the most cost-effective jobs on any roof.
The high end is ridge ventilation. A dry ridge system ventilates as it goes — the ventilated ridge roll under the mechanically fixed units allows air out along the whole ridge line — which is one of the quieter benefits of moving away from mortar bedding. Where the ridge is staying as it is, individual tile vents or slate vents set into the covering near the ridge do the same job.
The other half of the answer is reducing what goes up there. Sealing the loft hatch, sealing around pipe and cable penetrations, and — importantly — checking where the bathroom and kitchen extractors actually discharge. A depressing number of them terminate in the loft rather than outside, which puts the wettest air in the house directly into the roof space every day.
Getting this right is worth real money, and not only in comfort. Condensation rots battens and rafter feet, ruins the performance of the insulation it soaks, and produces staining that homeowners routinely pay to have a roof re-covered over. The ventilation work costs a small fraction of that and it is the thing that should be ruled in or out before anybody quotes for a covering.
What the job covers
- Assessment of the existing air path, low level and high
- Rafter trays fitted to hold insulation clear at the eaves
- Eaves or over-fascia vents installed
- Ridge or tile vents added at high level where needed
- Loft hatch, downlights and penetrations sealed
- Extractor ducting checked and terminated outside
What it costs
We don't publish a price for this yet, because a number pulled out of the air is worse than no number at all — your roofer has to honour it. On roofing it would also be close to meaningless: access is often the biggest single factor, and it varies enormously house to house. You'll get a firm price for your actual job when they ring you back, before any work starts. The quote itself is free and there's no obligation either way.
Common questions
How much ventilation does a loft need?
Enough for a continuous path from eaves to ridge — air in low, out high. The specific amount depends on the roof build-up and whether the underlay is breathable, but the practical test is simpler: if the insulation is blocking the eaves and there is nothing at the ridge, you do not have enough, whatever the calculation says.
Will more ventilation make my house colder?
No. The ventilation is above the insulation, in the unheated roof space, and that space is meant to be at outside temperature. The insulation at ceiling level is what keeps the house warm. What does make a house colder is wet insulation, which performs far worse than dry — so ventilating the loft usually improves things.
I have a breathable membrane. Do I still need vents?
In most cases yes. A breathable membrane substantially reduces condensation risk but it does not remove the need for a ventilation strategy, and it certainly does not make it acceptable to block the eaves with insulation. Any roofer telling you a breathable underlay means no ventilation is needed is overstating it.
Where does my bathroom fan go?
It should terminate outside, through the roof or a wall, with an insulated duct and a proper terminal. A surprising number end a foot inside the loft, which pumps the wettest air in the house straight into the roof space every time somebody showers. It is worth going up and looking.
Need this doing in Macclesfield?
Tell us the job and it goes straight to our local roofer — one call back, usually the same day. Free quote, no obligation.