The short answer
Often, yes. A pitched garage roof with insulation and a plastic-type underlay needs air moving through the roof space, or damp air turns to water against the chilled underside of the covering. BS 5250, the British Standard for handling moisture in buildings, sets eaves gaps to prevent that. A bare, uninsulated garage is more forgiving, though a re-roof is a good time to check.
Moisture follows the air. A garage roof stays dry when damp air has an exit and no cold surface waits to catch it.
Why a garage roof can drip without leaking
Condensation is water that forms when moist air touches something cold. Cars arrive wet, laundry hangs to dry, a tumble dryer vents into the space, and the air lifts all that moisture towards the roof. If the underside of the covering is cold, the moisture beads and falls back as drips. It looks like a leak, yet the roof itself is intact.
How can I tell condensation from a leak?
Look at the timing. A leak shows after rain, usually in one spot below a defect. Condensation turns up on cold, still mornings and spreads across a wide area, with beads on nail points, damp underlay and musty timber. Our answer on garage roof condensation goes through the symptoms in more depth.
Left alone, repeated wetting rots battens and rafters, rusts fixings and spoils whatever is stored below. It also tempts owners into paying for patch repairs that cannot work, which is why we find the cause before pricing anything. Photos taken inside the garage on a cold morning help us a great deal, so take a few if you can.
What BS 5250 asks of a pitched garage roof
BS 5250:2021 is listed as current, though it is under review. It deals with a cold pitched roof, where insulation lies along the ceiling and the void above stays unheated. The first question is which underlay was laid, since that sets the size of the gaps.
What is the difference between HR and LR underlay?
HR underlay is non-breathable, so moisture cannot pass through it and the void needs a clear opening along the eaves. Marley's guidance gives 25mm for pitches of 15 degrees or less, and 10mm for anything steeper. LR underlay lets vapour through. Where the ceiling below is airtight, it can rely on what Marley calls fortuitous ventilation at the eaves, 3mm of normal construction gap, subject to third-party certification.
Those figures belong to the product, and an older roof may match none of them. At the survey we look along the eaves from a ladder, because openings choked by insulation or by a fascia board fitted tight to the wall are common, and the cure is often small. Read our guide to insulating a garage roof before adding any.
Flat garage roofs and air movement
Flat garage roofs come in two basic builds. A warm roof lays its insulation over the deck, so the timbers sit inside the warm side and no air gap is wanted. A cold roof packs insulation between the joists and depends on a clear air path above it. Each needs different care, and from the pavement you cannot tell which one you own. A look at the roof edge and a small inspection opening usually settles it in a few minutes at the survey.
Can a flat garage roof be cured by adding vents?
Sometimes, but vents are only one part. Air has to cross the whole void from side to side, and gaps in the ceiling must be closed so household moisture is not pushed up into it. If the joists are already stained or soft, a vent cannot rescue them, and the straight answer is a new deck.
When the covering is renewed, Part L lists the waterproof layer of a flat roof under renovation. That is the time to settle insulation and airflow together as one plan. Doing them separately, years apart, means opening the roof twice and paying for the access twice.
Sheeted garages, workshops and common sense
A single-skin sheeted garage, corrugated metal for example, leaks air at every lap, and that draught works as ventilation of a rough kind. Trouble starts when someone adds a lining or insulation beneath without thinking about the cold sheet above. Moisture then condenses on the sheet and runs down behind the lining where nobody sees it.
Should I line my garage roof myself?
Establish the material of the roof before you touch it, and test it if there is doubt. A garage built or refurbished before 2000 should be treated as a possible asbestos site, and screwing a lining to purlins means disturbing sheets that HSE calls always fragile. Our page on asbestos garage roof removal describes a safer route.
A workshop or gym needs more thought, because people breathe out moisture and heaters run for hours. If you plan that kind of use, say so when we visit, and the roof is specified for it. Our garage and asbestos roofs page sets out the full range of garage work. After a survey, you see pictures of the roof and receive a single written total.
More on moisture in garage roofs
Is a garage roof vent the same as an eaves gap?
No. An eaves gap is a continuous opening running the length of the eaves, usually formed with a ventilator strip to keep insects out. A vent tile or roof vent is one outlet nearer the top. BS 5250 looks at the whole airflow path, so one without the other rarely solves the problem.
Will insulating my garage roof cause condensation?
It can, if it blocks the eaves or the ceiling below is left unsealed. Insulation keeps the roof timbers colder, so any moist air reaching them condenses. Done properly, with airflow kept clear or a warm roof design chosen, it does the opposite and keeps the timbers dry. Plan the build-up first.
The full picture: Garage and asbestos roof replacement in Taunton ยท More short answers