You walk into the shed on a cold, clear morning and the roof is dripping onto the ute, the workbench and the hay. It isn’t a leak. It’s condensation, and it’s one of the most common complaints about steel buildings. The good news is it can be designed out. It matters most when the work inside can be damaged by drips: timber furniture making, kitchen manufacturing where MDF or chipboard carcasses swell when wet, or vintage car restoration, where a drip can mark a finished panel.
Why it happens #
On a clear night, a steel roof loses heat to the sky and can get colder than the air around it. Warm, moist air inside the shed touches the cold underside of the roof sheet and drops its moisture as water droplets. Once enough builds up, it drips.
Where the moisture comes from #
- A new concrete slab. Fresh concrete releases a lot of moisture as it dries, often for months.
- The ground, if there’s no vapour barrier under the slab or the floor is bare earth.
- Wet vehicles, boats, aircraft and machinery brought in after rain or a wash.
- Animals, which breathe out moisture constantly.
- Unflued gas heaters, which put water vapour straight into the air.
- Washing down inside the building.
The fixes #
1. Stop moisture at the source. A vapour barrier (damp-proof membrane) under the slab is the cheapest condensation control there is, but it has to go in before the concrete. Avoid unflued gas heaters, and let wet gear drip outside where you can.
2. Put something between the moisture and the cold steel. Options include:
- a roof underlay under the cladding, which catches and holds droplets
- an anti-condensation cladding, a pre-painted sheet with a fleece layer bonded to the underside of the sheet and replaces a separate underlay in suitable buildings
- roof insulation, with the right vapour control layer, which also makes the shed warmer in winter and cooler in summer
3. Ventilate. Moving air carries moisture out before it condenses. Ridge vents, eave gaps and wall vents all help, and open-front farm sheds already ventilate well.
Fixing an existing shed #
Retrofitting isn’t easy. Ventilation can be improved and moisture sources managed, but adding an underlay usually means taking the roofing or cladding off, which is expensive. Condensation control is best discussed when the building is being designed, which is why we ask how you’ll use the building before we design it.
What we recommend #
Tell us if the shed will hold vehicles, animals, hay, tools or anything that hates damp, and whether you’ll spend time working in it. We’ll suggest the right mix of vapour barrier, underlay or anti-condensation cladding, insulation and ventilation. There’s no one standard answer: it depends on how you use the building and how you want to manage it.
Talk to Steelspan #
Planning a shed and want it dry inside? Use our Easy Shed Quote or call 0800 787 726.
