Water dripping from the underside of a steel roof is alarming, and the usual assumption is a leak. More often than not it is condensation, and the two need completely different remedies. Chasing a leak that does not exist wastes a good deal of time.

Telling condensation from a leak

A leak is local and follows the weather. It appears in the same place each time, it starts during or shortly after rain, and you can normally trace it uphill to a lap, a fixing or a flashing.

Condensation is general and follows the temperature. It forms across a broad area of the underside, often as a fine film of droplets rather than a running trickle, and it is worst on cold clear nights, frequently when it has not rained at all. If you find moisture after a dry frosty night, it is condensation.

Why condensation forms

Warm, moist air inside the building meets the cold underside of the sheet and gives up its moisture. Steel is an efficient conductor, so the underside sits close to the outside air temperature. Any source of moisture feeds the process: livestock, stored timber, a bare earth or unsealed concrete floor, vehicles brought in wet, or simply a poorly ventilated space.

Fixing condensation

• Anti-condensation sheets. Our Dripstop sheets carry a membrane bonded to the underside that holds moisture as it forms and releases it back as the air warms, so it evaporates rather than dripping. This is the most direct fix when you are re-sheeting anyway.
• Insulation. An insulated panel keeps the inner surface closer to the internal air temperature, so the dew point is not reached in the first place.
• Ventilation. Moist air has to be able to leave. Ridge and eaves ventilation costs little and addresses the cause rather than the symptom.
• Deal with the moisture source. Sealing a floor or improving drainage often achieves more than anything done at roof level.

Fixing an actual leak

If the evidence points to a leak, the usual culprits are laps and fixings. Our own guidance on overlapping metal roof sheets sets out the detail, but in short: end laps want around 150mm where the pitch is greater than 10 degrees, and 200mm or more below that, with sealing tape or mastic at the joint. Fixings should be square to the sheet and tightened to compress the washer evenly, no more.

Where sheets have simply reached the end of their service life, talk to us about replacements cut to your existing sizes.