When does anti-condensation fleece help under metal roofing?

Condensation has a different cause from rain entering at a poorly formed lap or flashing. Observe when and where drops appear. If one strip gets wet after rain, investigate a possible leak first. An absorbent lining can conceal a faulty connection. The fact that it absorbs water does not prove the roof is watertight.

The fibrous layer bonded beneath the sheet can absorb some condensate before it forms large drops that fall into the building. Its capacity depends on the particular fleece and the test conditions. There is no single storage capacity for every fleece-backed sheet.

The fleece must dry after a cold period. If moisture keeps arriving and little air moves beneath the roof, the lining may stay wet. Once it reaches capacity, further condensation can drip. Indoor conditions matter as much as the presence of the fleece.

Fleece can be useful in uninsulated shelters, garages and outbuildings if it suits the expected humidity and can dry between wet periods. An open side improves air exchange, but also consider the location and what goes on inside the building.

A dry tool store behaves differently from a garage that regularly receives wet vehicles. Drying materials or working with water increases the moisture load. Tell the roof supplier about these activities. A detail that works next door may behave differently in another indoor environment.

A heated room needs a roof designed for insulation and vapour control. A thin fleece cannot replace thermal insulation, airtightness or the required roof layers. Adding mineral wool beneath the sheets changes moisture conditions and calls for an assessment of the whole build-up.

The same applies to very humid rooms. Where a set temperature must be maintained, compare a complete built-up roof with a suitably specified sandwich panel. Both options require proper junction details and building ventilation. Core type and panel thickness alone cannot resolve every thermal bridge or leak.

The absorbent fleece must not draw water in from outside. Follow the system details at eaves and laps. The lining may need specific edge treatment in these areas. Use the method specified for the product.

Agree the details with the installer and supplier before work starts. Treatment methods and distances can differ between fleeces. Check penetrations, ends and contact with flashings. Protect the sheets in storage too. If a pack gets wet, check how to dry it before leaving it tightly wrapped.

Compare fleece surcharges on the same area basis. Check whether the price is per square metre of total sheet area or installed cover. Include edge treatment and measures that allow drying. Even a small surcharge offers little value if the building needs a different roof assembly.

Once the building is in use, record the conditions in which moisture appears. Converting a garage into a heated workshop calls for a fresh assessment of the roof. Check the effects before closing ventilation openings to reduce draughts. Changes to airflow affect how the lining dries and how the roof performs.

Recheck after changing how goods are stored. Tall stacks can block airflow and alter drying conditions even when the roof itself has not changed.