Where your lead protects your roof from the weather, Geotec felt protects the lead from the roof. It does this in three ways. Firstly, lead laid nakedly on materials like concrete or stone may be at risk of corrosion, while lead on timber may slowly be leached away by moisture. This may start to damage and destroy the lead layer. Geotec felt serves as an intermediate layer that will guard the lead from moisture or corrosion.
Secondly, Geotec felt decreases drag arising from thermal movement. Lead expands and contracts as the temperature changes it. However, often the materials it is laid on do not. In addition, they may be rough or have an uneven surface. This causes damage as the lead strains against the material holding it up. Geotec felt provides a gentle, smooth surface for lead to thrive on, letting it expand and contract safely.
Thirdly, geotec felt is porous and permits airflow. This allows moisture to be dried out and ventilates the underside of the lead and the topside of the materials the lead would otherwise rest on. This reduces damp, and therefore the chance of rotting or mould. Geotec felt can therefore be an important asset in reducing roof condensation.