How to choose underlay for roofs?

Roof underlay is an important part of the roof structure. It sits under the actual roofing and protects the building from moisture, snow and dirt if any gets past the outer roof, while also acting as wind protection. Choosing the right underlay affects how well the roof performs and how long it lasts.

What to consider when choosing

Several factors affect which underlay is right for a given roof:

• Roof type: a pitched roof and a flat roof call for different underlay solutions.
• Climate: the underlay needs to withstand local conditions, such as heavy rain, snow and wind.
• Material: common options include bitumen felt, plastic film and breathable membranes; pick the one that suits your roof structure and climate.
• Breathability: a breathable underlay lets moisture out without letting it in, which is especially useful in warm, humid conditions.
• Durability: choose an underlay that can withstand weather changes and long-term sun and moisture exposure, since a good-quality underlay extends the life of the whole roof.
• Installation method: underlays are fixed in different ways, for example nailed/screwed or fully bonded; choose one that matches the roof structure and the installer's skills.

Common types of underlay

The most common underlay types are:

• Bitumen felt (bitumikermi): a durable, waterproof underlay well suited to steep roofs.
• Plastic film: light and easy to install, commonly used on flatter roofs.
• Breathable membrane: lets moisture escape from inside the structure while keeping it out from above, a good choice in warm and humid conditions.

Summary

Choosing the right underlay is an important part of planning and building a roof. Weigh up the roof type, climate, material, breathability and installation method, and have it installed carefully, to make sure the roof lasts a long time and works as it should.